Climate Change and...
- Climate Variability
- Climate Models
Effects of Climate Change
ABSTRACT: Observational evidence suggests that river inflows to the Arctic Ocean have increased over the last 30 years. Continued increases have the potential to alter the freshwater balance in the Arctic and North Atlantic Oceans and hence the thermohaline circulation. Simulations with a macroscale hydrological model and climate change scenarios derived from six climate models and two emissions scenarios suggest increases of up to 31% in river inflows to the Arctic by the 2080s under high emissions and up to 24% under lower emissions, although there are large differences between climate models. Uncertainty analysis suggests low sensitivity to model form and parameterization but higher sensitivity to the input data used to drive the model. The addition of up to 0.048 sverdrup (Sv, 106 m3 s-1 ) is a large proportion of the 0.06–0.15 Sv of additional freshwater that may trigger thermohaline collapse. Changes in the spatial distribution of inflows to the Arctic Ocean may influence circulation patterns within the ocean.
N. W. Arnell, M. G. R. Cannell, M. Hulme, R. S. Kovats, J. F. B. Mitchell, R. J. Nicholls, M. L. Parry, M. T. J. Livermore, A. White (2002). The consequences of CO2 stabilisation for the impacts of climate change. Climatic Change 53 (4): 413-446
ABSTRACT: This paper reports the main results of an assessment of the global-scale implications of the stabilisation of atmospheric CO2 concentrations at 750 ppm (by 2250) and 550 ppm (by 2150), in relationto a scenario of unmitigated emissions. The climate change scenarios were derived from simulation experiments conducted with the HadCM2 global climate model and forced with the IPCC IS92a, S750 and S550 emissions scenarios. The simulated changes in climate were applied to an observed global baseline climatology, and applied with impacts models to estimate impacts on natural vegetation, water resources, coastal flood risk and wetland loss, crop yield and food security, and malaria. The studies used a single set of population and socio-economic scenarios about the future that are similar to those adopted in the IS92a emissions scenario.An emissions pathway which stabilises CO2 concentrations at 750 ppmby the 2230s delays the 2050 temperature increase under unmitigated emissions by around 50 years. The loss of tropical forest and grassland which occurs by the 2050s under unmitigated emissions is delayed to the 22nd century, and the switch from carbon sink to carbon source is delayed from the 2050s to the 2170s. Coastal wetland loss is slowed. Stabilisation at 750 ppm generally has relatively little effect on the impacts of climate change on water resource stress, and populations at risk of hunger or falciparum malaria until the 2080s.A pathway which stabilises CO2 concentrations at 550 ppm by the 2170s delays the 2050 temperature increase under unmitigated emissions by around 100 years. There is no substantial loss of tropical forest or grassland, even by the 2230s, although the terrestrial carbon store ceases to act as a net carbon sink by around 2170 (this time because the vegetation has reached a new equilibrium with the atmosphere). Coastal wetland loss is slowed considerably, and the increase in coastal flood risk is considerably lower than under unmitigated emissions. CO2 stabilisation at 550 ppm reduces substantially water resource stress, relative to unmitigated emissions, but has relatively little impact on populations at risk of falciparum malaria, and may even cause more people to be at risk of hunger. While this study shows that mitigation avoids many impacts, particularly in the longer-term (beyond the 2080s), stabilisation at 550 ppm appears to be necessary to avoid or significantly reduce most of the projected impacts in the unmitigated case.
M. Beniston (2004). he 2003 heat wave in Europe: A shape of things to come? An analysis based on Swiss climatological data and model simulations. Geophysical Research Letters 31 (L02202): doi:10.1029/2003GL018857
ABSTRACT: The 2003 heat wave that affected much of Europe from June to September bears a close resemblance to what many regional climate models are projecting for summers in the latter part of the 21st century. Model results suggest that under enhanced atmospheric greenhouse-gas concentrations, summer temperatures are likely to increase by over 4°C on average, with a corresponding increase in the frequency of severe heat waves. Statistical features of the 2003 heat wave for the Swiss site of Basel are investigated and compared to both past, 20th century events and possible future extreme temperatures based on model simulations of climatic change. For many purposes, the 2003 event can be used as an analog of future summers in coming decades in climate impacts and policy studies.
ABSTRACT: Today's comparatively warm climate has been the exception more than the rule during the last 500,000 years or more. If recent warm periods (or interglacials) are a guide, then we may soon slip into another glacial period. But Berger and Loutre argue in their Perspective that with or without human perturbations, the current warm climate may last another 50,000 years. The reason is a minimum in the eccentricity of Earth's orbit around the Sun.
ABSTRACT: Long-term changes in evaporation and potential evapotranspiration can have profound implications for hydrologic processes as well as for agricultural crop performance. This paper analyses evaporation time series data for different stations in India, and for the country as a whole, for different seasons on both a short-term (15 years) and long-term (32 years) basis for pan evaporation and on a short-term basis alone for potential evapotranspiration. The analysis shows that both pan evaporation and potential evapotranspiration have decreased during recent years in India. The likely causative meteorological parameters for such changes are identified. Future scenarios of potential evapotranspiration, and its component energy and aerodynamic terms, for India based on results from six global climate model climate change experiments are also calculated and intercompared. Future warming seems likely to lead in general to increased potential evapotranspiration over India, although this increase will be unequal between regions and seasons. Such changes could have marked implications for economic and environmental welfare in the country, especially if the increases in evaporation are not compensated by adequate increases in rainfall.
ABSTRACT: By using a climate system model of intermediate complexity, we have simulated long-term natural climate changes occurring over the last 9000 years. The paleo-simulations in which the model is driven by orbital forcing only, i.e., by changes in insolation caused by changes in the Earth's orbit, are compared with sensitivity simulations in which various scenarios of increasing atmospheric CO2 concentration are prescribed. Focussing on climate and vegetation change in northern Africa, we recapture the strong greening of the Sahara in the early and mid-Holocene (some 9000–6000 years ago), and we show that some expansion of grasslandinto the Sahara is theoretically possible, if the atmospheric CO2 concentration increases well above pre-industrial values and if vegetation growth is not disturbed. Depending on the rate of CO2 increase, vegetation migration into the Sahara can be rapid, up to 1/10th of the Saharan area per decade, but could not exceed a coverage of 45%. In ourmodel, vegetation expansion into today's Sahara is triggered by an increase in summer precipitation which is amplified by a positive feedback between vegetation and precipitation. This is valid for simulations with orbital forcing and greenhouse-gas forcing. However, we argue that the mid-Holocene climate optimum some 9000 to 6000 years ago with its marked reduction of deserts in northern Africa is not a direct analogue for future greenhouse-gas induced climate change, as previously hypothesized. Not only does the global pattern of climate change differ between the mid-Holocene model experiments and the greenhouse-gas sensitivity experiments, but the relative role of mechanisms which lead to a reduction of the Sahara also changes. Moreover, the amplitude of simulated vegetation cover changes in northern Africa is less than is estimated for mid-Holocene climate.
Conway, D., Hulme, M. (1996). The impacts of climate variability and future climate change in the Nile basin on water resources in Egypt. International Journal of Water Resources Development 12 (3): 277-296
ABSTRACT: This paper describes the application of hydrologic models of the Blue Nile and Lake Victoria sub-basins to assess the magnitude of potential impacts of climate change on Main Nile discharge. The models are calibrated to simulate historical observed runoff and then driven with the temperature and precipitation changes from three general circulation model (GCM) climate scenarios. The differences in the resulting magnitude and direction of changes in runoff highlight the inter-model differences in future climate change scenarios. A 'wet' case, 'dry' case and composite case produced +15 (+12), -9 (-9) and + 1(+7) per cent changes in mean annual Blue Nile (Lake Victoria) runoff for 2025, respectively. These figures are used to estimate changes in the availability of Nile water in Egypt by making assumptions about the runoff response in the other Nile sub-basins and the continued use of the Nile Waters Agreement. Comparison of these availability scenarios with demand projections for Egypt show a slight surplus of water in 2025 with and without climate change. If, however, water demand for desert reclamation is taken into account then water deficits occur for the present-day situation and also 2025 with ('dry' case GCM only) and without climate change. A revision of Egypt's allocation of Nile water based on the recent low-flow decade-mean flows of the Nile (1981-90) shows that during this period Egypt's water use actually exceeded availability. The magnitude of 'natural' fluctuations in discharge therefore has very important consequences for water resource management regardless of future climate change.
ABSTRACT: A long-term, homogeneous set of daily maximum and minimum temperature data representing a subset of daily U.S. Historical Climatology Network stations is used to analyze trends in extreme temperature occurrence across the contiguous United States. Time series of various lengths are analyzed, with the longest spanning the period 1900–96. Trends in the annual occurrence of extreme maximum and minimum temperatures (e.g., values greater than the 90th, 95th, or 99th percentile) are strongly influenced by high exceedence counts during drought periods in the 1930s and 1950s. Peaks in exceedences during these years result in predominantly decreasing warm exceedence trends across the country during the 1930–96 period. This is uncharacteristic of recent years (1960–96) in which a large majority of stations show increases in warm extreme temperature exceedences. Significant increases in warm minimum temperature exceedences are found at nearly one-third of the stations during this period. Multiday warm temperature exceedence runs also show strong increases during this more recent period. The most rapid increases in high maximum and minimum temperature extremes occur at stations classified as urban, by satellite land use information.
Trends in the annual occurrence of extremely cold maximum and minimum temperatures display an analogous decrease during the 1960–96 period. Here again, there is a distinct shift in the number of decreasing trends between the 1950–96 and 1960–96 periods. Based on starting decades prior to 1960, there is not a strong tendency for either increasing or decreasing trends. The period 1910–96 is an exception, with almost all stations exhibiting decreasing cold extreme occurrence trends. The extreme cold exceedence trends during the 1960–96 period are also influenced by urbanization, but to a lesser degree than the warm extremes.
M. D. Dettinger, D. R. Cayan, M. K. Meyer, A. E. Jeton (2004). Simulated hydrologic responses to climate variations and change in the Merced, Carson, and American River basins, Sierra Nevada, California, 1900–2099. Climatic Change 62 (1-3): 283-317
ABSTRACT: Hydrologic responses of river basins in the Sierra Nevada of California to historical and future climate variations and changes are assessed by simulating daily streamflow and water-balance responses to simulated climate variations over a continuous 200-yr period. The coupled atmosphere-ocean-ice-land Parallel Climate Model provides the simulated climate histories, and existing hydrologic models of the Merced, Carson, and American Rivers are used to simulate the basin responses. The historical simulations yield stationary climate and hydrologic variations through the first part of the 20th century until about 1975 when temperatures begin to warm noticeably and when snowmelt and streamflow peaks begin to occur progressively earlier within the seasonal cycle. A future climate simulated with business-as-usual increases in greenhouse-gas and aerosol radiative forcings continues those recent trends through the 21st century with an attendant +2.5 °C warming and a hastening of snowmelt and streamflow within the seasonal cycle by almost a month. The various projected trends in the business-as-usual simulations become readily visible despite realistic simulated natural climatic and hydrologic variability by about 2025. In contrast to these changes that are mostly associated with streamflow timing, long-term average totals of streamflow and other hydrologic fluxes remain similar to the historical mean in all three simulations. A control simulation in which radiative forcings are held constant at 1995 levels for the 50 years following 1995 yields climate and streamflow timing conditions much like the 1980s and 1990s throughout its duration. The availability of continuous climate-change projection outputs and careful design of initial conditions and control experiments, like those utilized here, promise to improve the quality and usability of future climate-change impact assessments.
P. B. Duffy, R. W. Arritt, J. Coquard, W. Gutowski, J. Han, J. Iorio, J. Kim, L.R. Leung, J. Roads, E. Zeledon (2006). Simulations of present and future climates in the western United States with four nested regional climate models. Journal of Climate 19 (6): 873-895
ABSTRACT: In this paper, the authors analyze simulations of present and future climates in the western United States performed with four regional climate models (RCMs) nested within two global ocean–atmosphere climate models. The primary goal here is to assess the range of regional climate responses to increased greenhouse gases in available RCM simulations. The four RCMs used different geographical domains, different increased greenhouse gas scenarios for future-climate simulations, and (in some cases) different lateral boundary conditions. For simulations of the present climate, RCM results are compared to observations and to results of the GCM that provided lateral boundary conditions to the RCM. For future-climate (increased greenhouse gas) simulations, RCM results are compared to each other and to results of the driving GCMs. When results are spatially averaged over the western United States, it is found that the results of each RCM closely follow those of the driving GCM in the same region in both present and future climates. This is true even though the study area is in some cases a small fraction of the RCM domain. Precipitation responses predicted by the RCMs in many regions are not statistically significant compared to interannual variability. Where the predicted precipitation responses are statistically significant, they are positive. The models agree that near-surface temperatures will increase, but do not agree on the spatial pattern of this increase. The four RCMs produce very different estimates of water content of snow in the present climate, and of the change in this water content in response to increased greenhouse gases.
ABSTRACT: Recent research has shown that decadal-to-multidecadal (D2M) climate variability is associated with environmental changes that have important consequences for human activities, such as public health, water availability, frequency of hurricanes, and so forth. As scientists, how do we convert these relationships into decision support products useful to water managers, insurance actuaries, and others, whose principal interest lies in knowing when future climate regime shifts will likely occur that affect long-horizon decisions? Unfortunately, numerical models are far from being able to make deterministic predictions for future D2M climate shifts. However, the recent development of paleoclimate reconstructions of the Atlantic Multidecadal Oscillation (AMO) (Gray et al., ) and Pacific Decadal Oscillation (PDO); (MacDonald and Case, ) give us a viable alternative: to estimate probability distribution functions from long climate index series that allow us to calculate the probability of future D2M regime shifts. In this paper, we show how probabilistic projections can be developed for a specific climate mode - the AMO as represented by the Gray et al. () tree-ring reconstruction. The methods are robust and can be applied to any D2M climate mode for which a sufficiently long index series exists, as well as to the growing body of paleo-proxy reconstructions that have become available. The target index need not be a paleo-proxy calibrated against a climate index; it may profitably be calibrated against a specific resource of interest, such as stream flow or lake levels.
ABSTRACT: We present a review of climate change projections over the Mediterranean region based on the most recent and comprehensive ensembles of global and regional climate change simulations completed as part of international collaborative projects. A robust and consistent picture of climate change over the Mediterranean emerges, consisting of a pronounced decrease in precipitation, especially in the warm season, except for the northern Mediterranean areas (e.g. the Alps) in winter. This drying is due to increased anticyclonic circulation that yields increasingly stable conditions and is associated with a northward shift of the Atlantic storm track. A pronounced warming is also projected, maximum in the summer season. Inter-annual variability is projected to mostly increase especially in summer, which, along with the mean warming, would lead to a greater occurrence of extremely high temperature events. The projections by the global and regional model simulations are generally consistent with each other at the broad scale. However, the precipitation change signal produced by the regional models shows substantial orographically-induced fine scale structure absent in the global models. Overall, these change signals are robust across forcing scenarios and future time periods, with the magnitude of the signal increasing with the intensity of the forcing. The intensity and robustness of the climate change signals produced by a range of global and regional climate models suggest that the Mediterranean might be an especially vulnerable region to global change.
Hamlet, A. F., P. W. Mote, M. Clark, D. P. Lettenmaier (2005). Effects of temperature and precipitation variability on snowpack trends in the western United States. Journal of Climate 18 (21): 4545-4561
ABSTRACT: Recent studies have shown substantial declines in snow water equivalent (SWE) over much of the western US in the last half century, as well as trends towards earlier spring snowmelt and peak spring streamflows. These trends are influenced both by interannual and decadal scale climate variability, and also by temperature trends at longer time scales that are generally consistent with observations of global warming over the 20th century.
In this study we examine linear trends in April 1 snow water equivalent (SWE) over the western US as simulated by the Variable Infiltration Capacity hydrologic model implemented at 1/8 degree latitude-longitude spatial resolution, and driven by a carefully quality controlled gridded daily precipitation and temperature data set for the period 1915-2003. The long simulations of snowpack are used as surrogates for observations, and are the basis for an analysis of regional trends in snowpack over the western U.S. and southern British Columbia.
By isolating the trends due to temperature and precipitation in separate simulations, the influence of temperature and precipitation variability on the overall trends in SWE is evaluated. Downward trends in April 1 SWE over the western U.S. from 1916 to 2003, 1947-2003, and for a time series constructed using two warm Pacific Decadal Oscillation (PDO) epochs concatenated together, are shown to be primarily due to widespread warming. These temperature-related trends are not well explained by decadal climate variability associated with the PDO. Trends in SWE associated with precipitation trends, however, are very different in different time periods and are apparently largely controlled by decadal variability rather than longer term trends in climate.
Hayhoe, K., Bradbury, J., Wake, C., Anderson, B., Liang, X.-Z., DeGaetano, A.T., Maurer, E. P., Stoner, A. M., Wuebbles, D., Zhu, J. (2008). Regional climate change projections for the Northeast USA. Mitigation and Adaptation Strategies for Global Change 13 (5-6): 425-436
ABSTRACT: Climate projections at relevant temporal and spatial scales are essential to assess potential future climate change impacts on climatologically diverse regions such as the northeast United States. Here, we show how both statistical and dynamical downscaling methods applied to relatively coarse-scale atmosphere-ocean general circulation model output are able to improve simulation of spatial and temporal variability in temperature and precipitation across the region. We then develop high-resolution projections of future climate change across the northeast USA, using IPCC SRES emission scenarios combined with these downscaling methods. The projections show increases in temperature that are larger at higher latitudes and inland, as well as the potential for changing precipitation patterns, particularly along the coast. While the absolute magnitude of change expected over the coming century depends on the sensitivity of the climate system to human forcing, significantly higher increases in temperature and in winter precipitation are expected under a higher as compared to lower scenario of future emissions from human activities.
INTRODUCTION: To understand the impact of a possibly warmer future climate, geologists are searching the past for warmer-than-present interglacial intervals — examples of what we may expect in a world with more greenhouse gases than ours1. A common tactic is to look at the last interglacial (around 120,000 years ago), but a period 423,000 to 362,000 years ago may fit the bill better, because the Earth's orbital geometry has been similar during the Holocene (the present interglacial period) to what it was then. This period is known to palaeoclimatologists as stage 11 or 'MIS 11', according to a numbering system for glacial advances and retreats marked in the marine oxygen isotope record (Fig. 1). At a recent symposium*, evidence emerged of extreme climatic variation and peculiar interplays between ocean temperature, thermohaline circulation, plankton ecology, sea level and reef growth during MIS 11, all of which may provide insight into the response of the natural carbon cycle to future climate change.
ABSTRACT: This paper reviews observed (1900–2000) and possible future (2000–2100) continentwide changes in temperature and rainfall for Africa. For the historic period we draw upon a new observed global climate data set which allows us to explore aspects of regional climate change related to diurnal temperature range and rainfall variability. The latter includes an investigation of regions where seasonal rainfall is sensitive to El Niño climate variability. This review of past climate change provides the context for our scenarios of future greenhouse gas-induced climate change in Africa. These scenarios draw upon the draft emissions scenarios prepared for the Intergovernmental Panel on Climate Change’s Third Assessment Report, a suite of recent global climate model experiments, and a simple climate model to link these 2 sets of analyses. We present a range of 4 climate futures for Africa, focusing on changes in both continental and regional seasonal-mean temperature and rainfall. Estimates of associated changes in global CO2 concentration and global-mean sea-level change are also supplied. These scenarios draw upon some of the most recent climate modelling work. We also identify some fundamental limitations to knowledge with regard to future African climate. These include the often poor representation of El Niño climate variability in global climate models, and the absence in these models of any representation of regional changes in land cover and dust and biomass aerosol loadings. These omitted processes may well have important consequences for future African climates, especially at regional scales. We conclude by discussing the value of the sort of climate change scenarios presented here and how best they should be used in national and regional vulnerability and adaptation assessments.
ABSTRACT: The signing of the UN Framework Convention on Climate Change in Rio de Janeiro in June 1992 by 160 nations has firmly identified global climate change due to human pollution as a pressing global environmental concern. Among the responsibilities that the nations which ratify the Convention will have are the drawing up of inventories of greenhouse gas sources and sinks and the formulation of national strategies to respond to climate change through adaptive and or preventive measures. One requirement for identifying appropriate response strategies will be the undertaking of regional assessments of climate change and its associated impacts.
This paper is concerned with climate change in the East Asian region, both over the last 100 years (using instrumental data) and also for the next 100 years (using results from climate model experiments). The juxtaposing of these two analyses, historical and future, enables a better interpretation of the significance of regional climate change to be made. Instrumental temperature and precipitation data for the East Asian region are analysed and compared with the observed global-scale trends in these two variables. Although the region has undoubtedly warmed over the last century, understanding the exact causes of the complex seasonal, diurnal, and spatial dimensions of this warming is difficult. We examine the role of increasing urbanization in inducing rising temperatures and suggest that, although substantial, urban warming cannot account for all of the observed temperature change. The paper also illustrates a flexible composite-model approach to regional climate change scenario construction that avoids the need for multiple transient GCM experiments, which can explicitly incorporate the effects of intermodel uncertainty, and is flexible enough to incorporate new scientific findings and results from new GCM experiments. The scenario presented here suggests that by 2050, mean conditions are expected to be warmer than the extremely warm seasonal anomalies that occurred during the most recent decade in East Asia. Precipitation is estimated to rise over most of the region in all seasons, although the uncertainty range attached to this estimate is much wider than for temperature.
Independent Scientific Advisory Board, (2007). Climate change impacts on Columbia River Basin fish and wildlife. Independent Scientific Advisory Board for the Northwest Power Planning Council, the Columbia River Basin Indian Tribes, and the National Marine Fisheries Service: 146 p.
EXECUTIVE SUMMARY (partial): Warming of the global climate is unequivocal. Evidence includes increases in global average air and ocean temperatures, widespread melting of snow and ice, and rising global mean sea level. Eleven of the last twelve years (1995 -2006) rank among the 12 warmest years in the instrumental record of global surface temperature (since 1850). The linear warming trend over the last 50 years (0.13 +/- 0.03°C per decade) is nearly twice that for the last 100 years. The total global average temperature increase from 1850 – 1899 to 2001 – 2005 is 0.76 +/- 0.19°C.
Climate records show that the Pacific Northwest has warmed about 1.0 ºC since 1900, or about 50% more than the global average warming over the same period. The warming rate for the Pacific Northwest over the next century is projected to be in the range of 0.1-0.6° C/decade. Projected precipitation changes for the region are relatively modest and unlikely to be distinguishable from natural variability until late in the 21st century. Most models project long-term increases in winter precipitation and decreases in summer precipitation. The changes in temperature and precipitation will alter the snow pack, stream flow, and water quality in the Columbia Basin:
-Warmer temperatures will result in more precipitation falling as rain rather than snow
-Snow pack will diminish, and stream flow timing will be altered
-Peak river flows will likely increase
-Water temperatures will continue to rise
These changes will have a variety of impacts on aquatic and terrestrial habitats in the Columbia Basin.
NOTES: Prepared by the Governor's Advisory Group on Global Warming and partially based on the June 15, 2004 proceedings of a symposium entitled "Impacts of Climate Change on the Pacific Northwest" in Corvallis, Oregon. The document is signed by 49 Ph.D.-level scientists with expertise on the impacts of climate change.
Recent climate changes include: since the beginning of the 20th century, a 10% increase in annual precipitation across the region with increases of 30-40% in eastern Washington and northern Idaho. Sea level increase of 1.5-2 mm per year; a decline in April 1 snowpack and 50% decline in snow water equivalent (SWE) during the period 1950-2000. Timing of the peak snowpack is earlier in the year, with increase in March streamflow and decrease in June streamflow.
Expected changes include: an increase in average annual temperature of 2.7 degrees F by 2030 and 5.4 degrees F by 2050. This is likely to result in higher elevation treeline and longer fire season, among other changes. Oregon is expected to remain a winter-dominant precipitation regime, but with less snowfall and more rain, especially at lower elevations. Changes in precipitation (increase or decrease) are less certain. Peak hydropower capacity may shift more to winter months; summer stream temperatures will likely increase. Deep ocean circulation willcontinue to change. Since 1920, nearly every temperature monitoring station in the Pacific Northwest shows a warming trend (Mote 2003).
IPCC, J.T. Houghton, Y. Ding, D.J. Griggs, M. Noguer, P.J. van der Linden, X. Dai, K. Maskell, C.A. Johnson (2001). Chapter 9. Projections of Future Climate. Intergovernmental Panel on Climate Change, Cambridge University Press: 58 p.
EXECUTIVE SUMMARY: The results presented in this chapter are based on simulations made with global climate models and apply to spacial scales of hundreds of kilometres and larger. Chapter 10 presents results for regional models which operate on smaller spatial scales. Climate change simulations are assessed for the period 1990 to 2100 and are based on a range of scenarios for projected changes in greenhouse gas concentrations and sulphate aerosol loadings (direct effect). A few Atmosphere-Ocean General Circulation Model (AOGCM) simulations include the effects of ozone and/or indirect effects of aerosols (see Table 9.1 for details). Most integrations1 do not include the less dominant or less well understood forcings such as land-use changes, mineral dust, black carbon, etc. (see Chapter 6). No AOGCM simulations include estimates of future changes in solar forcing or in volcanic aerosol concentrations.
There are many more AOGCM projections of future climate available than was the case for the IPCC Second Assessment Report (IPCC, 1996) (hereafter SAR). We concentrate on the IS92a and draft SRES A2 and B2 scenarios. Some indication of uncertainty in the projections can be obtained by comparing the responses among models. The range and ensemble standard deviation are used as a measure of uncertainty in modelled response. The simulations are a combination of a forced climate change component together with internally generated natural variability. A number of modelling groups have produced ensembles of simulations where the projected forcing is the same but where variations in initial conditions result in different evolutions of the natural variability. Averaging these integrations preserves the forced climate change signal while averaging out the natural variability noise, and so gives a better estimate of the models' projected climate change.
For the AOGCM experiments, the mean change and the range in global average surface air temperature (SAT) for the 1961 to 1990 average to the mid-21st century (2021 to 2050) for IS92a is +1.3°C with a range from +0.8 to +1.7°C for greenhouse gas plus sulphates (GS) as opposed to +1.6°C with a range from +1.0 to +2.1°C for greenhouse gas only (G). For SRES A2 the mean is +1.1°C with a range from +0.5 to +1.4°C, and for B2, the mean is +1.2°C with a range from +0.5 to +1.7°C. For the end of the 21st century (2071 to 2100), for the draft SRES marker scenario A2, the global average SAT change from AOGCMs compared with 1961 to 1990 is +3.0°C and the range is +1.3 to +4.5°C, and for B2 the mean SAT change is +2.2°C and the range is +0.9 to +3.4°C.
AOGCMs can only be integrated for a limited number of scenarios due to computational expense. Therefore, a simple climate model is used here for the projections of climate change for the next century. The simple model is tuned to simulate the response found in several of the AOGCMs used here. The forcings for the simple model are based on the radiative forcing estimates from Chapter 6, and are slightly different to the forcings used by the AOGCMs. The indirect aerosol forcing is scaled assuming a value of -0.8 Wm-2 for 1990. Using the IS92 scenarios, the SAR gives a range for the global mean temperature change for 2100, relative to 1990, of +1 to +3.5°C. The estimated range for the six final illustrative SRES scenarios using updated methods is +1.4 to +5.6°C. The range for the full set of SRES scenarios is +1.4 to +5.8°C.
These estimates are larger than in the SAR, partly as a result of increases in the radiative forcing, especially the reduced estimated effects of sulphate aerosols in the second half of the 21st century. By construction, the new range of temperature responses given above includes the climate model response uncertainty and the uncertainty of the various future scenarios, but not the uncertainty associated with the radiative forcings, particularly aerosol. Note the AOGCM ranges above are 30-year averages for a period ending at the year 2100 compared to the average for the period 1961 to 1990, while the results for the simple model are for temperature changes at the year 2100 compared with the year 1990.
A traditional measure of climate response is equilibrium climate sensitivity derived from 2xCO2 experiments with mixed-layer models, i.e., Atmospheric General Circulation Models (AGCMs) coupled to non-dynamic slab oceans, run to equilibrium. It has been cited historically to provide a calibration for models used in climate change experiments. The mean and standard deviation of this quantity from seventeen mixed-layer models used in the SAR are +3.8 and +0.8°C, respectively. The same quantities from fifteen models in active use are +3.5 and +0.9°C, not significantly different from the values in the SAR. These quantities are model dependent, and the previous estimated range for this quantity, widely cited as +1.5 to +4.5°C, still encompasses the more recent model sensitivity estimates.
A more relevant measure of transient climate change is the transient climate response (TCR). It is defined as the globally averaged surface air temperature change for AOGCMs at the time of CO2 doubling in 1%/yr CO2 increase experiments. The TCR combines elements of model sensitivity and factors that affect response (e.g., ocean heat uptake). It provides a useful measure for understanding climate system response and allows direct comparison of global coupled models. The range of TCR for current AOGCMs is +1.1 to +3.1°C with an average of 1.8°C. The 1%/yr CO2 increase represents the changes in radiative forcing due to all greenhouse gases, hence this is a higher rate than is projected for CO2 alone. This increase of radiative forcing lies on the high side of the SRES scenarios (note also that CO2 doubles around mid-21st century in most of the scenarios). However these experiments are valuable for promoting the understanding of differences in the model responses.
ABSTRACT: A framework is presented to quantify observed changes in climate within the contiguous United States through the development and analysis of two indices of climate change, a Climate Extremes Index (CEI) and a U.S. Greenhouse Climate Response Index (GCRI). The CEI is based on an aggregate set of conventional climate extreme indicators, and the GCRI is composed of indicators that measure changes in the climate of the United States that have been projected to occur as a result of increased emissions of greenhouse gases.
The CEI supports the notion that the climate of the United States has become more extreme in recent decades, yet the magnitude and persistence of the changes are not large enough at this point to conclude that the increase in extremes reflects a nonstationary climate. Nonetheless, if impacts due to extreme events rise exponentially with the index, then the increase may be quite significant in a practical sense. Similarly, the positive trend of the U.S. GCRI during the twentieth century is consistent with an enhanced greenhouse effect. The increase is unlikely to have arisen due to chance alone (there is about a 5% chance). Still, the increase of the GCRI is not large enough to unequivocally reject the possibility that the increase in the GCRI may be the result of other factors, including natural climate variability, and the similarity between the change in the GCRI and anticipated changes says little about the sensitivity of the climate system to the greenhouse effect. Both indices increased rather abruptly during the 1970s, a time of major circulation changes over the Pacific Ocean and North America.
ABSTRACT: Extreme events act as a catalyst for concern about whether the climate is changing. Statistical theory for extremes is used to demonstrate that the frequency of such events is relatively more dependent on any changes in the variability (more generally, the scale parameter) than in the mean (more generally, the location parameter) of climate. Moreover, this sensitivity is relatively greater the more extreme the event. These results provide additional support for the conclusions that experiments using climate models need to be designed to detect changes in climate variability, and that policy analysis should not rely on scenarios of future climate involving only changes in means.
ABSTRACT: A set of six regional climate model experiments is investigated for future changes in daily temperature and precipitation in Europe. Changes in the probability distributions for these variables are studied. It is found that the asymmetry of these distributions change differently depending on location and season. Large summertime changes in extremely high temperatures in central, eastern and southern Europe are followed by higher than average temperature increases on warm days in general. Likewise, temperatures on cold days increase much more than the average temperature increase during winter in eastern and northern Europe. A comparison with historical data on wintertime temperature shows that the model simulated and observed daily variability are similar. In particular, the much stronger increase in temperatures on cold days, compared to the average temperature increase as observed in warm compared to cold historical periods, is simulated also by the model. The contribution from heavy precipitation events is simulated to increase over most parts of Europe in all seasons.
ABSTRACT: To study the impacts of climate change on water resources in the western U.S., global climate simulations were produced using the National Center for Atmospheric Research/Department of Energy (NCAR/DOE) Parallel Climate Model (PCM). The Penn State/NCAR Mesoscale Model (MM5) was used to downscale the PCM control (20 years) and three future (2040–2060) climate simulations to yield ensemble regional climate simulations at 40 km spatial resolution for the western U.S. This paper describes the regional simulations and focuses on the hydroclimate conditions in the Columbia River Basin (CRB) and Sacramento-San Joaquin River (SSJ) Basin. Results based on global and regional simulations show that by mid-century, the average regional warming of 1 to 2.5 °C strongly affects snowpack in the western U.S. Along coastal mountains, reduction in annual snowpack was about 70% as indicated by the regional simulations. Besides changes in mean temperature, precipitation, and snowpack, cold season extreme daily precipitation increased by 5 to 15 mm/day (15–20%) along the Cascades and the Sierra. The warming resulted in increased rainfall at the expense of reduced snowfall, and reduced snow accumulation (or earlier snowmelt) during the cold season. In the CRB, these changes were accompanied by more frequent rain-on-snow events. Overall, they induced higher likelihood of wintertime flooding and reduced runoff and soil moisture in the summer. Changes in surface water and energy budgets in the CRB and SSJ basin were affected mainly by changes in surface temperature, which were statistically significant at the 0.95 confidence level. Changes in precipitation, while spatially incoherent, were not statistically significant except for the drying trend during summer. Because snow and runoff are highly sensitive to spatial distributions of temperature and precipitation, this study shows that (1) downscaling provides more realistic estimates of hydrologic impacts in mountainous regions such as the western U.S., and (2) despite relatively small changes in temperature and precipitation, changes in snowpack and runoff can be much larger on monthly to seasonal time scales because the effects of temperature and precipitation are integrated over time and space through various surface hydrological and land-atmosphere feedback processes. Although the results reported in this study were derived from an ensemble of regional climate simulations driven by a global climate model that displays low climate sensitivity compared with most other models, climate change was found to significantly affect water resources in the western U.S. by the mid twenty-first century.
ABSTRACT: This paper presents examples of environmental changes in the Canadian Rockies in the context of a 1.5°C increase in mean annual temperatures over the last 100 years. During this period increases in winter temperatures have been more than twice as large as those during spring and summer. Glacier cover has decreased by at least 25% during the 20th century and glacier fronts have receded to positions last occupied ca. 3000 years ago. These two lines of evidence suggest that the climate of the late 20th century is exceptional in the context of the last 1000 to 3000 years. Detailed studies in three closely located upper treeline sites document variable responses of vegetation to climate change that reflect species differences as well as local differences in microclimate and site conditions. Treeline has advanced upslope in response to climate warming, but site and species differences control the rate and nature of the advance. Human impacts on the environment compound the changes due to climate warming. Historic photographs indicate significant changes in the type and density of forest cover due to the absence of significant forest fires within these National Parks during the last 70–80 years. The visual impact of these changes, which partially reflects a policy of fire suppression, is far greater than the impact of changes associated with more direct tourist-related impacts. It is therefore important that monitoring programs examine vegetation changes over the entire landscape rather than focussing exclusively on supposedly climate-sensitive sites.
ABSTRACT: In this paper we present a comprehensive set of interpolated climate data for western Canada, including monthly data for the last century (1901–2006), future projections from general circulation models (68 scenario implementations from 5 GCMs), as well as decadal averages and multiple climate normals for the last century. For each of these time periods, we provide a large set of basic and derived biologically relevant climate variables, such as growing and chilling degree days, growing season length descriptors, frost free days, extreme minimum temperatures, etc. To balance file size versus accuracy for these approximately 15,000 climate surfaces, we provide a stand-alone software solution that adds or subtracts historical data and future projections as medium resolution anomalies (deviations) from the high resolution 1961–1990 baseline normal dataset. For a relative quality comparison between the original normal data generated with the Parameter Regression of Independent Slopes Model (PRISM) and derived historical data, we calculated the amount of variance explained (R2 ) in original weather station data for each year and month from 1901 to 2006. R2 values remained very high for most of the time period covered for most variables. Reduction in data quality was found for individual months (as opposed to annual, decadal or 30-year climate averages) and for the early decades of the last century. We discuss the limitations of the database and provide an overview of recent climate trends for western Canada.
W. S. Merritt, Younes Alila, M. Barton, B. Taylor, S. Cohen, D. Neilsen (2006). Hydrologic response to scenarios of climate change in subwatersheds of the Okanagan basin, British Columbia. Journal of Hydrology 326 (1-4): 79-108
ABSTRACT: Scenarios of climate change were generated for the Okanagan Basin, a snow-driven semi-arid basin located in the southern interior region of British Columbia. Three global climate models (GCMs) were used to generate high (A2) and low (B2) emission scenarios; the Canadian global coupled model (CGCM2), the Australian developed CSIROMk2, and the HadCM3 model developed at the Hadley Centre in the United Kingdom. The three time periods simulated were 2010–2039 (2020s), 2040–2069 (2050s) and 2070–2099 (2080s). An increase in winter temperature of 1.5–4.0 °C and a precipitation increase of the order of 5-20% is predicted by the 2050s. Modelled summer precipitation is more variable with predicted change ranging from zero to a 35% decrease depending on the GCM and emission scenario. Summer temperatures were simulated to increase by approximately 2–4 °C. The UBC Watershed Model was used to model the hydrologic response of gauged sub watersheds in the basin under the altered climates. All scenarios consistently predicted an early onset of the spring snowmelt, a tendency towards a more rainfall dominated hydrograph and considerable reductions in the annual and spring flow volumes in the 2050s and 2080s. Of the three climate models, the CGCM2 model provided the most conservative predictions of the impacts of climate change in Okanagan Basin. Simulations based on the CSIROMk2 climate model suggested greatly reduced snowpack and flow volumes despite a sizeable increase in the winter precipitation. The scenarios raise questions over the availability of future water resources in the Okanagan Basin, particularly as extended periods of low flows into upland reservoirs are likely to coincide with increased demand from agricultural and domestic water users.
ABSTRACT: Temperature projections for the 21st century made in the Third Assessment Report (TAR) of the United Nations Intergovernmental Panel on Climate Change (IPCC) indicate a rise of 1.4 to 5.8°C for 1990–2100. However, several independent lines of evidence suggest that the projections at the upper end of this range are not well supported. Since the publication of the TAR, several findings have appeared in the scientific literature that challenge many of the assumptions that generated the TAR temperature range. Incorporating new findings on the radiative forcing of black carbon (BC) aerosols, the magnitude of the climate sensitivity, and the strength of the climate/carbon cycle feedbacks into a simple upwelling diffusion/energy balance model similar to the one that was used in the TAR, we find that the range of projected warming for the 1990–2100 period is reduced to 1.1–2.8°C. When we adjust the TAR emissions scenarios to include an atmospheric CO2 pathway that is based upon observed CO2 increases during the past 25 yr, we find a warming range of 1.5–2.6°C prior to the adjustments for the new findings. Factoring in these findings along with the adjusted CO2 pathway reduces the range to 1.0–1.6°C. And thirdly, a simple empirical adjustment to the average of a large family of models, based upon observed changes in temperature, yields a warming range of 1.3–3.0°C, with a central value of 1.9°C. The constancy of these somewhat independent results encourages us to conclude that 21st century warming will be modest and near the low end of the IPCC TAR projections.
ABSTRACT: An analysis of the historic flows and water temperatures of the Fraser River system has detected trends in both the annual flow profile and the summer temperatures. This study was undertaken to determine if these trends are likely to continue under the conditions predicted by various global circulation models. To do this, existing flow and temperature models were run with weather data that were derived from actual weather observations, but modified using changes predicted by the global circulation models.
The validity of the flow model results is supported by very close agreement with the historical record. The differences between model output and the historical record for mean flow, mean peak flow, mean minimum flow and peak flow day were not statistically significant; furthermore, there was only a 3–4 day shift in the occurrence of cumulative flow milestones. The temperature model's mean water temperature was only 0.2 °C higher than the historical record.
For the period 2070–2099, the flow model predicted a modest 5% (150 m3 /s) average flow increase but a decrease in the average peak flow of about 18% (1600 m3 /s). These peaks would occur, on average, 24 days earlier in the year even though for 13% of the years the peak flow occurred much later as a result of summer or fall rain, instead of the currently normal spring freshet. In the same period, the summer mean water temperature is predicted to increase by 1.9 °C. The potential exposure of salmon to water temperatures above 20 °C, which may degrade their spawning success, is predicted to increase by a factor of 10.
Trends in both flow and temperature in this study closely match the trends in the historical record, 1961–1990, which suggests that the historical trends may already be related to climate change. While the mean flow of 2726 m3 /s does not show a statistically significant trend, the hydrological profile has been changing.
ABSTRACT: Documenting long-term trends or persistent shifts in temperature and precipitation is important for understanding present and future changes in flora and fauna. Carefully adjusted datasets for climate records in the USA and Canada are combined and used here to describe the spatial and seasonal variation in trends in the maritime, central, and Rocky Mountain climatic zones of the Pacific Northwest. Trends during the 20th century in annually averaged temperature (0.7 degrees C - 0.9 degrees C) and precipitation (13%-38%) exceed the global averages. Largest warming rates occurred in the maritime zone and in winter and at lower elevations in all zones, and smallest warming rates occurred in autumn and in the Rockies. Largest increases in precipitation (upwards of 60% per century) were observed in the dry areas in northeast Washington and south central British Columbia. Increases in precipitation were largest in spring, but were also large in summer in the central and Rocky Mountain climatic zones. These trends have already had profound impacts on streamflow and on certain plant species in the region (Cayan et al. 2001), and other important impacts remain to be discovered. The warming observed in winter and spring can be attributed partially to climatic variations over the Pacific Ocean, and the buildup of greenhouse gases probably also plays an important role.
P. W. Mote, E. A. Parson, A. F. Hamlet, W. S. Keeton, D. Lettenmaier, N. Mantua, E. L. Miles, D. W. Peterson, D. L. Peterson, R. Slaughter, A. K. Snover (2003). Preparing for climatic change: the water, salmon, and forests of the Pacific Northwest. Climatic Change 61 (1-2): 45-88
ABSTRACT: The impacts of year-to-year and decade-to-decade climatic variations on some of the Pacific Northwest's key natural resources can be quantified to estimate sensitivity to regional climatic changes expected as part of anthropogenic global climatic change. Warmer, drier years, often associated with El Niño events and/or the warm phase of the Pacific Decadal Oscillation, tend to be associated with below-average snowpack, streamflow, and flood risk, below-average salmon survival, below-average forest growth, and above-average risk of forest fire. During the 20th century, the region experienced a warming of 0.8 °C. Using output from eight climate models, we project a further warming of 0.5–2.5 °C (central estimate 1.5 °C) by the 2020s, 1.5–3.2 °C (2.3 °C) by the 2040s, and an increase in precipitation except in summer. The foremost impact of a warming climate will be the reduction of regional snowpack, which presently supplies water for ecosystems and human uses during the dry summers. Our understanding of past climate also illustrates the responses of human management systems to climatic stresses, and suggests that a warming of the rate projected would pose significant challenges to the management of natural resources. Resource managers and planners currently have few plans for adapting to or mitigating the ecological and economic effects of climatic change.
ABSTRACT: Analysis of policies to achieve the long-term objective of the United Nations Framework Convention on Climate Change, stabilizing concentrations of greenhouse gases at levels that avoid “dangerous” climate changes, must discriminate among the infinite number of emission and concentration trajectories that yield the same final concentration. Considerable attention has been devoted to path-dependent mitigation costs, generally for CO2 alone, but not to the differential climate change impacts implied by alternative trajectories. Here, we derive pathways leading to stabilization of equivalent CO2 concentration (including radiative forcing effects of all significant trace gases and aerosols) with a range of transient behavior before stabilization, including temporary overshoot of the final value. We compare resulting climate changes to the sensitivity of representative geophysical and ecological systems. Based on the limited available information, some physical and ecological systems appear to be quite sensitive to the details of the approach to stabilization. The likelihood of occurrence of impacts that might be considered dangerous increases under trajectories that delay emissions reduction or overshoot the final concentration.
J.T. Overpeck, B. L. Otto-Bliesner, G. H. Miller, D. R. Muhs, R. B. Alley, J. T. Kiehl (2006). Paleoclimatic evidence for future ice-sheet instability and rapid sea-level rise. Science 311 (5768): 1747-1750
ABSTRACT: Sea-level rise from melting of polar ice sheets is one of the largest potential threats of future climate change. Polar warming by the year 2100 may reach levels similar to those of 130,000 to 127,000 years ago that were associated with sea levels several meters above modern levels; both the Greenland Ice Sheet and portions of the Antarctic Ice Sheet may be vulnerable. The record of past ice-sheet melting indicates that the rate of future melting and related sea-level rise could be faster than widely thought.
E. Sánchez, C. Gallardo, M.A. Gaertner, A. Arribas, M. Castro (2004). Future climate extreme events in the Mediterranean simulated by a regional climate model: a first approach. Global and Planetary Change 44 (1-4): 163-180
ABSTRACT: Within the frame of PRUDENCE (Prediction of Regional scenarios and Uncertainties for Defining EuropeaN Climate change risks and Effects, EVK2-CT2001-00132), 5th Framework European programme project (2002–2005) European research project, two 30-year time-slice simulations with a regional climate model (PROMES-RCM) nested in the Hadley Centre global model have been performed: present-day climate (1961–1990) and one of the IPCC greenhouse gases emission future scenario (A2 IPCC-SRES) for 2071–2100. Model domain is centered in the Mediterranean basin, considered one of the most sensitive areas regarding to global warming and future climate extreme conditions. This study is based on objective indices to describe extreme climate events of maximum and minimum temperature and precipitation. The statistical frequency and persistence of cold spells, heat waves and intense rain days simulated in the current climate run are compared against the ones resulting from future scenario numerical experiment. Description of extreme processes in both intensity and frequency give a different and complementary overview of extreme events changes in future climate conditions for any of the magnitudes analyzed. In fact, a common feature obtained from the results is the absence of correlation between both magnitudes, as much as for temperatures as for precipitation. Results also point to the usefulness of very high-resolution models (RCM) to study extreme events, due to the great spatial variability obtained in any of the variables studied.
W. H. Schlesinger, J. F. Reynolds, G. L. Cunningham, L. F. Huenneke, W. M. Jarrell, R. A. Virginia, W. G. Whitford (1990). Biological feedbacks in global desertification. Science 247 (4946): 1043-1048
ABSTRACT: Studies of ecosystem processes on the Jornada Experimental Range in southern New Mexico suggest that longterm grazing of semiarid grasslands leads to an increase in the spatial and temporal heterogeneity of water, nitrogen, and other soil resources. Heterogeneity of soil resources promotes invasion by desert shrubs, which leads to a further localization of soil resources under shrub canopies. In the barren area between shrubs, soil fertility is lost by erosion and gaseous emissions. This positive feedback leads to the desertification of formerly productive land in southern New Mexico and in other regions, such as the Sahel. Future desertification is likely to be exacerbated by global climate warming and to cause significant changes in global biogeochemical cycles.
M. V. K. Sivakumar, H. P. Das, O. Brunini (2005). Impacts of present and future climate variability and change on agriculture and forestry in the arid and semi-arid tropics. Climatic Change 70 (1-2): 31-72
ABSTRACT: The arid and semi-arid regions account for approximately 30% of the world total area and are inhabited by approximately 20% of the total world population. Issues of present and future climate variability and change on agriculture and forestry in the arid and semi-arid tropics of the world were examined and discussion under each of these issues had been presented separately for Asia, Africa and Latin America. Several countries in tropical Asia have reported increasing surface temperature trends in recent decades. Although, there is no definite trend discernible in the long-term mean for precipitation for the tropical Asian region, many countries have shown a decreasing trend in rainfall in the past three decades. African rainfall has changed substantially over the last 60 yr and a number of theoretical, modelling and empirical analyses have suggested that noticeable changes in the frequency and intensity of extreme events, including floods may occur when there are only small changes in climate. Climate in Latin America is affected by the El Niño-southern oscillation (ENSO) phases and there is a close relationship between the increase and decrease of rainfall depending upon the warm or cold phases of the phenomenon.
Over land regions of Asia, the projected area-averaged annual mean warming is likely to be 1.6 ± 0.2 °C in the 2020s, 3.1 ± 0.3 °C in the 2050s, and 4.6 ± 0.4 °C in the 2080s and the models show high uncertainty in projections of future winter and summer precipitation. Future annual warming across Africa is projected to range from 0.2 °C per decade to more than 0.5 °C per decade, while future changes in mean seasonal rainfall in Africa are less well defined. In Latin America, projections indicate a slight increase in temperature and changes in precipitation. Impacts of climate variability and changes are discussed with suitable examples. Agricultural productivity in tropical Asia is sensitive not only to temperature increases, but also to changes in the nature and characteristics of monsoon. Simulations of the impacts of climate change using crop simulation models show that crop yield decreases due to climate change could have serious impacts on food security in tropical Asia. Climate change is likely to cause environmental and social stress in many of Asia’s rangelands and drylands. In the arid and semi-arid tropics of Africa, which are already having difficulty coping with environmental stress, climate change resulting in increased frequencies of drought poses the greatest risk to agriculture. Impacts were described as those related to projected temperature increases, the possible consequences to water balance of the combination of enhanced temperatures and changes in precipitation and sensitivity of different crops/cropping systems to projected changes. In Latin America, agriculture and water resources are most affected through the impact of extreme temperatures (excessive heat, frost) and the changes in rainfall (droughts, flooding). Adaptation potential in the arid and semi-arid tropics of Asia, Africa and Latin America was described using suitable examples. It is emphasized that approaches need to be prescriptive and dynamic, rather than descriptive and static.
Stouffer, R.J., J. Yin, J.M. Gregory, K.W. Dixon, M.J. Spelman, W. Hurlin, A.J. Weaver, M. Eby, G.M. Flato, H. Hasumi, A. Hu, J.H. Jungclaus, I.V. Kamenkovich, A. Levermann, M. Montoya, S. Murakami, S. Nawrath, A. Oka, W.R. Peltier, D.Y. Robitaille, A. Sokolov, G. Vettoretti, S.L. Weber (2006). Investigating the causes of the response of the thermohaline circulation to past and future climate changes. Journal of Climate 19 (8): 1365-1387
ABSTRACT: The Atlantic thermohaline circulation (THC) is an important part of the earth's climate system. Previous research has shown large uncertainties in simulating future changes in this critical system. The simulated THC response to idealized freshwater perturbations and the associated climate changes have been intercompared as an activity of World Climate Research Program (WCRP) Coupled Model Intercomparison Project/Paleo-Modeling Intercomparison Project (CMIP/PMIP) committees. This intercomparison among models ranging from the earth system models of intermediate complexity (EMICs) to the fully coupled atmosphere–ocean general circulation models (AOGCMs) seeks to document and improve understanding of the causes of the wide variations in the modeled THC response. The robustness of particular simulation features has been evaluated across the model results. In response to 0.1-Sv (1 Sv 106 m3 s−1 ) freshwater input in the northern North Atlantic, the multimodel ensemble mean THC weakens by 30% after 100 yr. All models simulate some weakening of the THC, but no model simulates a complete shutdown of the THC. The multimodel ensemble indicates that the surface air temperature could present a complex anomaly pattern with cooling south of Greenland and warming over the Barents and Nordic Seas. The Atlantic ITCZ tends to shift southward. In response to 1.0-Sv freshwater input, the THC switches off rapidly in all model simulations. A large cooling occurs over the North Atlantic. The annual mean Atlantic ITCZ moves into the Southern Hemisphere. Models disagree in terms of the reversibility of the THC after its shutdown. In general, the EMICs and AOGCMs obtain similar THC responses and climate changes with more pronounced and sharper patterns in the AOGCMs.
J. Turner, S. R. Colwell, G. J. Marshall, T. A. Lachlan-Cope, A. M. Carleton, P. D. Jones, V. Lagun, P. A. Reid, S. Iagovkina (2005). Antarctic climate change during the last 50 years. International Journal of Climatology 25 (3): 279-294
ABSTRACT: The Reference Antarctic Data for Environmental Research (READER) project data set of monthly mean Antarctic near-surface temperature, mean sea-level pressure (MSLP) and wind speed has been used to investigate trends in these quantities over the last 50 years for 19 stations with long records. Eleven of these had warming trends and seven had cooling trends in their annual data (one station had too little data to allow an annual trend to be computed), indicating the spatial complexity of change that has occurred across the Antarctic in recent decades. The Antarctic Peninsula has experienced a major warming over the last 50 years, with temperatures at Faraday/Vernadsky station having increased at a rate of 0.56 °C decade-1 over the year and 1.09 °C decade-1 during the winter; both figures are statistically significant at less than the 5% level. Overlapping 30 year trends of annual mean temperatures indicate that, at all but two of the 10 coastal stations for which trends could be computed back to 1961, the warming trend was greater (or the cooling trend less) during the 1961-90 period compared with 1971-2000. All the continental stations for which MSLP data were available show negative trends in the annual mean pressures over the full length of their records, which we attribute to the trend in recent decades towards the Southern Hemisphere annular mode (SAM) being in its high-index state. Except for Halley, where the trends are constant, the MSLP trends for all stations on the Antarctic continent for 1971-2000 were more negative than for 1961-90. All but two of the coastal stations have recorded increasing mean wind speeds over recent decades, which is also consistent with the change in the nature of the SAM.
U.S. Global Change Research Group, (2000). Climate change impacts on the United States: The potential consequences of climate variability and change. National Assessment Synthesis Team, U.S. Global Change Research Program
ABOUT THIS DOCUMENT: What is this Assessment? The National Assessment of the Potential Consequences of Climate Variability and Change is a landmark in the major ongoing effort to understand what climate change means for the US. Climate science is developing rapidly and scientists are increasingly able to project some changes at the regional scale, identifying regional vulnerabilities, and assessing potential regional impacts. Science increasingly indicates that the Earth's climate has changed in the past and continues to change, and that even greater climate change is very likely in the 21st century. This Assessment has begun a national process of research, analysis, and dialogue about the coming changes in climate, their impacts, and what Americans can do to adapt to an uncertain and continuously changing climate. This Assessment is built on a solid foundation of science conducted as part of the United States Global Change Research Program (USGCRP).
What is this document and who is the NAST? This document is the Assessment Overview, written by the National Assessment Synthesis Team (NAST). The NAST is a committee of experts drawn from governments, universities, industry, and non-governmental organizations. It has been responsible for broad over-sight of the Assessment, with the Federal agencies of the USGCRP. This Overview is based on a longer, referenced "Foundation" report, written by the NAST in cooperation with independent regional and sector assessment teams. These two national-level, peer-reviewed documents synthesize results from studies conducted by regional and sector teams, and from the broader scientific literature.
Why was this Assessment undertaken? The Assessment was called for by a 1990 law, and has been con-ducted under the USGCRP in response to a request from the President's Science Advisor. The NAST developed the Assessment's plan, which was then approved by the National Science and Technology Council, the cabinet-level body of agencies responsible for scientific research, including global change research, in the US government.
ABSTRACT: Global surface temperatures have increased by 0.6 ± 0.2°C in the last century, but this warming has not been evenly distributed across the globe. Some regions, such as the Antarctic Peninsula, have seen a higher than average warming. In their Perspective, Vaughan et al. show that the recent warming in the Antarctic Peninsula has likely been exceptional for 1900 years. Yet global circulation models are unable to reproduce this warming. They conclude that properly targeted national adaptation planning requires a better understanding of regionally specific climate processes.
D. P. Van Vuuren, M. Meinshausen, G.-K. Plattner, F. Joos, K. M. Strassmann, S. J. Smith, T. M. L. Wigley, S. C. B. Raper, K. Riahi, F. de la Chesnaye, M. G. J. den Elzen, J. Fujino, K. Jiang, N. Nakicenovic, S. Paltsev, J. M. Reilly (2008). Temperature increase of 21st century mitigation scenarios. Proceedings of the National Academy of Sciences 105 (40): 15258-15262
ABSTRACT: Estimates of 21st Century global-mean surface temperature increase have generally been based on scenarios that do not include climate policies. Newly developed multigas mitigation scenarios, based on a wide range of modeling approaches and socioeconomic assumptions, now allow the assessment of possible impacts of climate policies on projected warming ranges. This article assesses the atmospheric CO2 concentrations, radiative forcing, and temperature increase for these new scenarios using two reduced-complexity climate models. These scenarios result in temperature increase of 0.5–4.4°C over 1990 levels or 0.3–3.4°C less than the no-policy cases. The range results from differences in the assumed stringency of climate policy and uncertainty in our understanding of the climate system. Notably, an average minimum warming of ≈1.4°C (with a full range of 0.5–2.8°C) remains for even the most stringent stabilization scenarios analyzed here. This value is substantially above previously estimated committed warming based on climate system inertia alone. The results show that, although ambitious mitigation efforts can significantly reduce global warming, adaptation measures will be needed in addition to mitigation to reduce the impact of the residual warming.
ABSTRACT: Key risks associated with projected climate trends for the 21st century include the prospects of future climate states with no current analog and the disappearance of some extant climates. Because climate is a primary control on species distributions and ecosystem processes, novel 21st-century climates may promote formation of novel species associations and other ecological surprises, whereas the disappearance of some extant climates increases risk of extinction for species with narrow geographic or climatic distributions and disruption of existing communities. Here we analyze multimodel ensembles for the A2 and B1 emission scenarios produced for the fourth assessment report of the Intergovernmental Panel on Climate Change, with the goal of identifying regions projected to experience (i) high magnitudes of local climate change, (ii) development of novel 21st-century climates, and/or (iii) the disappearance of extant climates. Novel climates are projected to develop primarily in the tropics and subtropics, whereas disappearing climates are concentrated in tropical montane regions and the poleward portions of continents. Under the high-end A2 scenario, 12-39% and 10-48% of the Earth's terrestrial surface may respectively experience novel and disappearing climates by 2100 AD. Corresponding projections for the low-end B1 scenario are 4-20% and 4-20%. Dispersal limitations increase the risk that species will experience the loss of extant climates or the occurrence of novel climates. There is a close correspondence between regions with globally disappearing climates and previously identified biodiversity hotspots; for these regions, standard conservation solutions (e.g., assisted migration and networked reserves) may be insufficient to preserve biodiversity.
ABSTRACT: We examined climate-carbon cycle feedback by performing a global warming experiment using MIROC-based coupled climate-carbon cycle model. The model showed that by the end of the 21st century, warming leads to a further increase in carbon dioxide (CO2 ) level of 123 ppm by volume (ppmv). This positive feedback can mostly be attributed to land-based soil-carbon dynamics. On a regional scale, Siberia experienced intense positive feedback, because the acceleration of microbial respiration due to warming causes a decrease in the soil carbon level. Amazonia also had positive feedback resulting from accelerated microbial respiration. On the other hand, some regions, such as western and central North America and South Australia, experienced negative feedback, because enhanced litterfall surpassed the increased respiration in soil carbon. The oceanic contribution to the feedback was much weaker than the land contribution on global scale, but the positive feedback in the northern North Atlantic was as strong as those in Amazonia and Siberia in our model. In the northern North Atlantic, the weakening of winter mixing caused a reduction of CO2 absorption at the surface. Moreover, weakening of the formation of North Atlantic Deep Water caused reduced CO2 subduction to the deep water. Understanding such regional-scale differences may help to explain disparities in coupled climate-carbon cycle model results.
ABSTRACT: This study presents trends computed for the past 30-50 years for 11 hydroclimatic variables obtained from the recently created Canadian Reference Hydrometric Basin Network database. It was found that annual mean streamflow has generally decreased during the periods, with significant decreases detected in the southern part of the country. Monthly mean streamflow for most months also decreased, with the greatest decreases occurring in August and September. The exceptions are March and April, when significant increases in streamflow were observed. Significant increases were identified in lower percentiles of the daily streamflow frequency distribution over northern British Columbia and the Yukon Territory. In southern Canada, significant decreases were observed in all percentiles of the daily streamflow distribution. Breakup of river ice and the ensuing spring freshet occur significantly earlier, especially in British Columbia. There is also evidence to suggest earlier freeze-up of rivers, particularly in eastern Canada. The trends observed in hydroclimatic variables are entirely consistent with those identified in climatic variables in other Canadian studies.
Y. Zhang, Y. Xu, W. Dong, L. Cao, M. Sparrow (2006). A future climate scenario of regional changes in extreme climate events over China using the PRECIS climate model. Geophysical Research Letters 33 (L24702): doi:10.1029/2006GL027229
ABSTRACT: Based on the PRECIS climate model system, we simulate the distribution of the present (1961~1990) and future (2071~2100) extreme climate events in China under the IPCC SRES B2 scenario. The results show that for the present case PRECIS simulates well the spatial distribution of extreme climate events when compared with observations. In the future the occurrence of hot events is projected to be more frequent and the growing season will lengthen, while the occurrence of cold events is likely to be much rarer. A warming environment will also give rise to changes in extreme precipitation events. There would be an overall increasing trend in extreme precipitation events over most of China. The southeast coastal zone, the middle and lower reaches of the Yangtze River and North China are projected to experience more extreme precipitation than the present.