Climate Change and...
- Climate Variability
- Climate Models
Effects of Climate Change
ABSTRACT: We present results from the BIOSOPE cruise in the eastern South Pacific Ocean. In particular, we present estimates of the anthropogenic carbon CantTrOCA distribution in this area using the TrOCA method recently developed by Touratier and Goyet (2004a, b) and Touratier et al. (2007). We study the distribution of this anthropogenic carbon taking into account of the hydrodynamic characteristics of this region. We then compare these results with earlier estimates in nearby areas of the anthropogenic carbon as well as other anthropogenic tracer (CFC-11). The highest concentrations of CantTrOCA are located around 13° S 132° W and 32° S 91° W, and their concentrations are larger than 80μmol kg−1 and 70μmol kg−1 , respectively. The lowest concentrations were observed below 800 m depths (≤2μ mol kg−1 ) and at the Oxygen Minimum Zones (OMZ), mainly around 140° W (<11 μmol kg−1 ). The comparison with earlier work in nearby areas provides a general trend and indicates that the results presented here are in general agreement with previous knowledge. This work further improves our understanding on the penetration of anthropogenic carbon in the eastern Pacific Ocean.
ABSTRACT: Various methods have been proposed for mitigating release of anthropogenic CO2 to the atmosphere, including deep-sea injection of CO2 captured from fossil-fuel fired power plants. Here, we use a schematic model of ocean chemistry and transport to analyze the geochemical consequences of a new method for separating carbon dioxide from a waste gas stream and sequestering it in the ocean. This method involves reacting CO2 -rich power-plant gases with seawater to produce a carbonic acid solution which in turn is reacted on site with carbonate mineral (e.g., limestone) to form Ca2+ and bicarbonate in solution, which can then be released and diluted in the ocean. Such a process is similar to carbonate weathering and dissolution which would have otherwise occurred naturally, but over many millennia. Relative to atmospheric release or direct ocean CO2 injection, this method would greatly expand the capacity of the ocean to store anthropogenic carbon while minimizing environmental impacts of this carbon on ocean biota. This carbonate-dissolution technique may be more cost-effective and less environmentally harmful, and than previously proposed CO2 capture and sequestration techniques.
FIRST PARAGRAPH: Most carbon dioxide released into the atmosphere as a result of the burning of fossil fuels will eventually be absorbed by the ocean1 , with potentially adverse consequences for marine biota2, 3, 4 . Here we quantify the changes in ocean pH that may result from this continued release of CO2 and compare these with pH changes estimated from geological and historical records. We find that oceanic absorption of CO2 from fossil fuels may result in larger pH changes over the next several centuries than any inferred from the geological record of the past 300 million years, with the possible exception of those resulting from rare, extreme events such as bolide impacts or catastrophic methane hydrate degassing.
K. Caldeira, M.E. Wickett (2005). Ocean model predictions of chemistry changes from carbon dioxide emissions to the atmosphere and ocean. Journal of Geophysical Research - Oceans 110 (C09S04): doi:10.1029/2004JC002671
ABSTRACT: We present ocean chemistry calculations based on ocean general circulation model simulations of atmospheric CO2 emission, stabilization of atmospheric CO2 content, and stabilization of atmospheric CO2 achieved in total or in part by injection of CO2 to the deep ocean interior. Our goal is to provide first-order results from various CO2 pathways, allowing correspondence with studies of marine biological effects of added CO2 . Parts of the Southern Ocean become undersaturated with respect to aragonite under the Intergovernmental Panel on Climate Change Special Report on Emissions Scenarios (SRES) A1, A2, B1, and B2 emission pathways and the WRE pathways that stabilize CO2 at 650 ppm or above. Cumulative atmospheric emission of 5000 Pg C produces aragonite undersaturation in most of the surface ocean; 10,000 Pg C also produces calcite undersaturation in most of the surface ocean. Stabilization of atmospheric CO2 at 450 ppm produces both calcite and aragonite undersaturation in most of the deep ocean. The simulated SRES pathways produce global surface pH reductions of ~0.3–0.5 units by year 2100. Approximately this same reduction is produced by WRE650 and WRE1000 stabilization scenarios and by the 1250 Pg C emission scenario by year 2300. Atmospheric emissions of 5000 Pg C and 20,000 Pg C produce global surface pH reductions of 0.8 and 1.4 units, respectively, by year 2300. Simulations of deep ocean CO2 injection as an alternative to atmospheric release show greater chemical impact on the deep ocean as the price for having less impact on the surface ocean and climate. Changes in ocean chemistry of the magnitude shown are likely to be biologically significant.
L. Cao, K. Caldeira, A. K. Jain (2007). Effects of carbon dioxide and climate change on ocean acidification and carbonate mineral saturation. Geophysical Research Letters 34 (L05607): doi:10.1029/2006GL028605
ABSTRACT: We use an earth system model of intermediate complexity to show how consideration of climate change affects predicted changes in ocean pH and calcium carbonate saturation state. Our results indicate that consideration of climate change produces second-order modifications to ocean chemistry predictions made with constant climate; these modifications occur primarily as a result of changes in sea surface temperature, and climate-induced changes in dissolved inorganic carbon concentrations. Under a CO2 emission scenario derived from the WRE1000 CO2 stabilization concentration pathway and a constant climate, we predict a 0.47 unit reduction in surface ocean pH relative to a pre-industrial value of 8.17, and a reduction in the degree of saturation with respect to aragonite from a pre-industrial value of 3.34 to 1.39 by year 2500. With the same CO2 emissions but the consideration of climate change under a climate sensitivity of 2.5°C the reduction in projected global mean surface pH is about 0.48 and the saturation state of aragonite decreases to 1.50. With a climate sensitivity of 4.5°C, these values are 0.51 and 1.62, respectively. Our study therefore suggests that future changes in ocean acidification caused by emissions of CO2 to the atmosphere are largely independent of the amounts of climate change.
ABSTRACT: The uptake of carbon dioxide up by the oceans is an important factor in controlling the carbon dioxide content of the atmosphere. But how has this uptake varied in the past? In his Perspective, Elderfield discusses recent studies that have arrived at very different answers to this question. He concludes that the influence of shifts in atmospheric carbon dioxide--be they manmade or part of glacial climate cycles--on oceanic carbon sequestration remains unclear.
ABSTRACT: Over geological time, photosynthetic carbon fixation in the oceans has exceeded respiratory oxidation of organic carbon. The imbalance between the two processes has resulted in the simultaneous accumulation of oxygen in, and drawdown of carbon dioxide from, the Earth's atmosphere, and the burial of organic carbon in marine sediments1–3 . It is generally assumed that these processes are limited by the availability of phosphorus4,5 , which is supplied by continental weathering and fluvial discharge5–7 . Over the past two million years, decreases in atmospheric carbon dioxide concentrations during glacial periods correlate with increases in the export of organic carbon from surface waters to the marine sediments8–11 , but variations in phosphorus fluxes appear to have been too small to account for these changes12,13 . Consequently, it has been assumed that total oceanic primary productivity remained relatively constant during glacial-to-interglacial transitions, although the fraction of this productivity exported to the sediments somehow increased during glacial periods12,14 . Here I present an analysis of the evolution of biogeochemical cycles which suggests that fixed nitrogen, not phosphorus, limits primary productivity on geological timescales. Small variations in the ratio of nitrogen fixation to denitrification can significantly change atmospheric carbon dioxide concentrations on glacial-to-interglacial timescales. The ratio of these two processes appears to be determined by the oxidation state of the ocean and the supply of trace elements, especially iron.
ABSTRACT: Changes in oceanic primary production, linked to changes in the network of global biogeochemical cycles, have profoundly influenced the geochemistry of Earth for over 3 billion years. In the contemporary ocean, photosynthetic carbon fixation by marine phytoplankton leads to formation of ~45 gigatons of organic carbon per annum, of which 16 gigatons are exported to the ocean interior. Changes in the magnitude of total and export production can strongly influence atmospheric CO2 levels (and hence climate) on geological time scales, as well as set upper bounds for sustainable fisheries harvest. The two fluxes are critically dependent on geophysical processes that determine mixed-layer depth, nutrient fluxes to and within the ocean, and food-web structure. Because the average turnover time of phytoplankton carbon in the ocean is on the order of a week or less, total and export production are extremely sensitive to external forcing and consequently are seldom in steady state. Elucidating the biogeochemical controls and feedbacks on primary production is essential to understanding how oceanic biota responded to and affected natural climatic variability in the geological past, and will respond to anthropogenically influenced changes in coming decades. One of the most crucial feedbacks results from changes in radiative forcing on the hydrological cycle, which influences the aeolian iron flux and, in turn, affects nitrogen fixation and primary production in the oceans.
ABSTRACT: Rising atmospheric carbon dioxide (CO2 ) concentrations over the past two centuries have led to greater CO2 uptake by the oceans. This acidification process has changed the saturation state of the oceans with respect to calcium carbonate (CaCO3 ) particles. Here we estimate the in situ CaCO3 dissolution rates for the global oceans from total alkalinity and chlorofluorocarbon data, and we also discuss the future impacts of anthropogenic CO2 on CaCO3 shell–forming species. CaCO3 dissolution rates, ranging from 0.003 to 1.2 micromoles per kilogram per year, are observed beginning near the aragonite saturation horizon. The total water column CaCO3 dissolution rate for the global oceans is approximately 0.5 ± 0.2 petagrams of CaCO3 -C per year, which is approximately 45 to 65% of the export production of CaCO3 .
ABSTRACT: Climate change is expected to influence the capacities of the land and oceans to act as repositories for anthropogenic CO2 and hence provide a feedback to climate change. A series of experiments with the National Center for Atmospheric Research–Climate System Model 1 coupled carbon–climate model shows that carbon sink strengths vary with the rate of fossil fuel emissions, so that carbon storage capacities of the land and oceans decrease and climate warming accelerates with faster CO2 emissions. Furthermore, there is a positive feedback between the carbon and climate systems, so that climate warming acts to increase the airborne fraction of anthropogenic CO2 and amplify the climate change itself. Globally, the amplification is small at the end of the 21st century in this model because of its low transient climate response and the near-cancellation between large regional changes in the hydrologic and ecosystem responses. Analysis of our results in the context of comparable models suggests that destabilization of the tropical land sink is qualitatively robust, although its degree is uncertain.
J. K. Hill, P. A. Wheeler (2002). Organic carbon and nitrogen in the northern California current system: comparison of offshore, river plume, and coastally upwelled waters. Progress in Oceanography 53 (2-4): 369-387
ABSTRACT: During the first year of the Northeast Pacific GLOBEC program we examined the spatial distributions of dissolved and particulate organic carbon and nitrogen in the surface waters off the Oregon and Washington coasts of North America. Eleven east–west transects were sampled from nearshore waters to 190 km offshore. Hydrographic data and the distribution of inorganic nutrients were used to characterize three distinct water sources: oligotrophic offshore water, the Columbia River plume, and the coastal upwelling region inshore of the California Current. Warm, high salinity offshore water had very low levels of inorganic nutrients, particulate organic carbon (POC) and dissolved organic carbon (DOC). Warm, low salinity water in the Columbia River plume was relatively low in nitrate, but showed a strong negative correlation between salinity and silicate. The river plume water had the highest levels of total organic carbon (TOC) (up to 180μM) and DOC (up to 150 μM) observed anywhere in the sampling area. Cold, high salinity coastal waters had high nutrient levels, moderate to high levels of POC and particulate organic nitrogen (PON), and low to moderate levels of DOC and dissolved organic nitrogen (DON). Each of these regions has characteristic C:N ratios for particulate and dissolved organic material. The results are compared to concentrations and partitioning of particulate and dissolved organic carbon and nitrogen in other regions of the North Pacific and North Atlantic Oceans.
Hirsch, A. I., Michalak, A. M., Bruhwiler, L. M., Peters, W., Dlugokencky, E. J., Tans, P. P. (2006). Inverse modeling estimates of the global nitrous oxide surface flux from 1998–2001. Global Biogeochemical Cycles 20 (GB1008): doi:10.1029/2004GB002443
ABSTRACT: Measurements of nitrous oxide in air samples from 48 sites in the Cooperative Global Air Sampling Network made by NOAA/ESRL GMD CCGG (the Carbon Cycle Greenhouse Gases group in the Global Monitoring Division at the NOAA Earth System Research Laboratory in Boulder, Colorado) and the three-dimensional chemical transport model TM3 were used to infer global nitrous oxide fluxes and their uncertainties from 1998–2001. Results are presented for four semihemispherical regions (90°S–30°S, 30°S to equator, equator to 30°N, 30°N–90°N) and six broad “super regions” (Southern Land, Southern Oceans, Tropical Land, Tropical Oceans, Northern Land, and Northern Oceans). We found that compared to our a priori estimate (from the International Geosphere-Biosphere Programme's Global Emissions Inventory Activity), the a posteriori flux was much lower from 90°S–30°S and substantially higher from equator to 30°N. Consistent with these results, the a posteriori flux from the Southern Oceans region was lower than the a priori estimate, while Tropical Land and Tropical Ocean estimates were higher. The ratio of Northern Hemisphere to Southern Hemisphere fluxes was found to range from 1.9 to 5.2 (depending on the model setup), which is higher than the a priori ratio (1.5) and at the high end of previous estimates. Globally, ocean emissions contributed 26–36% of the total flux (again depending on the model setup), consistent with the a priori estimate (29%), though somewhat higher than some other previous estimates.
ABSTRACT: Organic carbon and organic phosphorus concentrations in marine sediment cores with known sedimentation rates were determined and collected from the literature. With the exception of organic-rich sediments from upwelling areas, organic phosphorus concentrations in marine sediments do not correlate linearly with organic carbon concentrations. The C/P ratios of organic matter buried in marine sediments vary systematically with sedimentation rate, over the range studied (0.0001 to 2.5 cm/y). Low C/P ratios (< 200 atomic) occur in sediments with sedimentation rates both less than 0.002 cm/y and greater than 1 cm/y. Higher C/P ratios (up to 600) are found at intermediate sedimentation rates.
A simple steady state model is developed for the early diagenesis of organic carbon and organic phosphorus in marine sediments. The processes included are sediment burial, pore water diffusion, and bacterial decomposition of organic matter by oxic respiration and sulfate reduction. Preferential regeneration of phosphorus relative to carbon is assumed to take place during oxic respiration. The modeling shows that both organic matter preservation and the degree of elemental fractionation between phosphorus and carbon depend strongly on the overall sedimentation rate. Very high sedimentation rates (coastal-deltaic environment) result in a good preservation of deposited organic matter with an initial C/P ratio close to the Redfield value. At very slow sedimentation rates (pelagic environment), the oxidation of the sedimentary organic matter is nearly complete and produces a low C/P residual organic material. This residual organic matter may represent either refractory detrital organic compounds enriched in phosphorus or in situ synthesized low C/P bacterial remains. The high C/P ratios at intermediate sedimentation rates (slope and shelf environment) reflect the preferential regeneration of phosphorus during the incomplete decomposition of the metabolizable organic matter.
ABSTRACT: The Earth system processes most of the chemical components of its atmosphere and oceans in geologically short periods of time. It does this in a regulated way, one that maintains a remarkably constant surface environment. This we know primarily from the fossil record of uninterrupted, complex life on Earth that extends over the last billion years.
We are only beginning to understand the feedbacks that control the chemistry of the oceans and atmosphere. Numerical models of biogeochemical cycles are most stable when there is feedback between the amount of a chemical component in the ocean or atmosphere and its transfer to or from that reservoir. Coupling of subsystem, especially of those operating on different time scales, enhances stability.
An example of the role of feedback in stabilizing Earth's chemical environment is the mechanism of control of atmospheric oxygen. There appears to be no strong relationship between oxygen level and oxygen consumption. However, oxygen production may be a function of oxygen level; the burial rate of organic carbon (oxygen production) in marine sediments may be sensitive to bottom water oxygenation levels. Also, combustion may be an effective mechanism of transferring nutrients (namely phosphorus) from efficient, terrestrial ecosystems to less efficient, marine ecosystems. When O2 rises, fires become more frequent and P is transferred to the ocean, stimulating marine organic carbon burial but depressing global burial rates. Global O2 production rates decline, as does the O2 level: a negative feedback.
Models of ocean chemical composition are presently incapable of reproducing the temporal constancy indicated by geological observations. These models do not incorporate ion-exchange equilibria as important processes in marine geochemical cycles. When included, these equilibria significantly damp the fluctuations in ion ratios calculated by the extant models.
FIRST PARAGRAPH: The increase of carbon dioxide in the atmosphere due to the combustion of fossil fuels has stimulated (Revelle and Suess, 1957) a broad spectrum of research. One of the key questions concerns the amount that can be absorbed by the oceans. The combination of elementary chemistry with the oceanic volume gives an upper estimate of 5/6 for the storage capacity of the oceans, as pointed out by Arrhenius at the end of the last century, already (quoted after Revelle, 1985). Due to the slowness of the oceanic deep circulation this asymptotic value may be expected after several thousand years only. At present, it is estimated that approximately one third of the carbon dioxide emissions enter the ocean (Houghton et al., 1990). This number represents a compromise between model studies and indirect conclusions from direct observations. Simplified box models were developed already 20 years ago (Oeschger et al., 1975). More recently, three dimensional circulation models were used for such studies, too (Maier-Reimer and Hasselmann, 1987 and Sarmiento et al., 1992). The results confirm more or less the previous estimates.
ABSTRACT: Output from a coupled atmosphere–ocean model forced by the IS92a greenhouse gas scenario was used to investigate the feedback between climate change and the oceanic uptake of CO2 . To improve the climate simulation, we used Gent and co-workers eddy parameterization in the ocean and a prognostic equation for export production from the upper ocean. For the period of 1850 to 2100, the change in the oceanic uptake of CO2 with climate was separated into 3 feedbacks. (i) Climate change warmed the sea-surface temperature which increased the partial pressure of CO2 in the surface ocean and reduced the accumulated ocean uptake by 48 Gt C. (ii) Climate change reduced meridional overturning and convective mixing and increased density stratification in high latitudes which slowed the transport of anthropogenic CO2 into the ocean interior and reduced the cumulative ocean CO2 uptake by 41 Gt C. (iii) Climate change altered "natural" cycling of carbon in the ocean which increased the cumulative ocean CO2 uptake by 33 Gt C. The change in natural carbon cycling with climate change was dominated by 2 opposing factors. First, the supply of nutrients to the upper ocean decreased which reduced the export of organic matter (by 15% by year 2100) and produced a net CO2 flux out of the ocean. However, associated with the reduced nutrient supply was the reduction in the supply of dissolved inorganic carbon to the upper ocean, which produced net CO2 flux into the ocean. For our model, the latter effect dominated. By the year 2100, the combinations of these 3 climate change feedbacks resulted in a decrease in the cumulative oceanic CO2 uptake of 56 Gt C or 14% of the 402 Gt C of oceanic CO2 uptake predicted by a run with no climate change. Our total reduction in oceanic CO2 uptake with climate change for the 1850 to 2100 period was similar to the 58 Gt C reduction in oceanic CO2 uptake predicted by Sarmiento and Le Quéré. However, our consistency with this previous estimate is misleading. By including the Gent and co-workers eddy parameterization in the ocean, we reduced the positive feedback between climate change and the oceanic uptake of CO2 from 169 to 89 Gt C (80 Gt C change). This reduction reflects a decrease in both sea surface warming and anthropogenic forcing feedbacks. By using a prognostic parameterization of export production, we reduced the negative feedback response of the natural carbon cycle to climate change from 111 to 33 Gt C (78 Gt C). These 2 large offsetting changes in the ocean response to climate change produced only a net change of 2 Gt C. This resulted in a net reduction in oceanic uptake of 2 Gt C from the previous study.
ABSTRACT: Many of the reactions and biogeochemical processes that occur in the marine environment are related directly or indirectly to the mineralization of organic matter. Decomposition of organic matter is responsible for the recycling of essential nutrients, for the oxygen balance of the ocean and its sediments and for most early diagenetic processes. The rate at which organic matter is mineralized varies over orders of magnitude and depends on, among other factors, the composition and orgin of the organic material being degraded and on the environmental conditions. However, direct relationship between organic matter decomposition rates and organic matter composition or between mineralization rates and organic matter decomposition pathways (e.g. oxic versus anoxic) are difficult to establish.
Park, G., K. Lee, P. Tischenko (2008). Sudden, considerable reduction in recent uptake of anthropogenic CO2 by the East/Japan Sea. Geophysical Research Letters 35 (35, L23611): doi:10.1029/2008GL036118
ABSTRACT: The East/Japan Sea in the western temperate North Pacific is ventilated from the surface to the bottom over decades. Such short overturning circulation indicates that the anthropogenic CO2 content of the East/Japan Sea is intimately tied to changing surface conditions over similarly short periods. Three surveys in the East/Japan Sea (1992, 1999 and 2007, respectively) have provided a rare opportunity to measure changes in the accumulation rate of anthropogenic CO2 in the East/Japan Sea over the past 15 years in response to changes in surface conditions. We found that the mean uptake rate of anthropogenic CO2 by the East/Japan Sea was 0.3 ± 0.2 mol C m−2 yr−1 for the period 1999–2007, in marked contrast to the rate of 0.6 ± 0.4 mol C m−2 yr−1 for the period 1992–1999. The striking feature is that nearly all anthropogenic CO2 taken up in the more recent period was confined to waters less than 300 m in depth (mean winter mixed layer depth). The rapid and substantial reduction in accumulation in the more recent period is surprising, and is attributed to considerable weakening of overturning circulation, which is responsible for transporting anthropogenic CO2 from the surface to the interior of the East/Japan Sea. This previously undocumented finding may be an indicator of future changes in the global ocean during the coming period of global warming.
ABSTRACT: We review one of the most ancient of all the global biogeochemical cycles and one which reflects the profound geochemical and biological changes that have occurred as the Earth system has evolved through time—that of calcium carbonate (CaCO3 ). In particular, we highlight a Mid-Mesozoic Revolution in the nature and location of carbonate deposition in the marine environment, driven by the ecological success of calcareous plankton. This drove the creation of a responsive deep-sea sedimentary sink of CaCO3 . The result is that biologically driven carbonate deposition provides a significant buffering of ocean chemistry and of atmospheric CO2 in the modern system. However, the same calcifying organisms that under-pin the deep-sea carbonate sink are now threatened by the continued atmospheric release of fossil fuel CO2 and increasing acidity of the surface ocean. We are not yet in a position to predict what the impact on CaCO3 production will be, or how the uptake of fossil fuel CO2 by the ocean will be affected. This uncertainty in the future trajectory of atmospheric CO2 that comes from incomplete understanding of the marine carbonate cycle is cause for concern.
C. L. Sabine, R. A. Feely, N. Gruber, R. M. Key, K. Lee, J. L. Bullister, R. Wanninkhof, C. S. Wong, D. W. R. Wallace, B. Tilbrook, F. J. Millero, T. Peng, A. Kozyr, T. Ono, A. F. Rios (2004). The oceanic sink for anthropogenic CO2 . Science 305 (5682): 367-371
ABSTRACT: Using inorganic carbon measurements from an international survey effort in the 1990s and a tracer-based separation technique, we estimate a global oceanic anthropogenic carbon dioxide (CO2 ) sink for the period from 1800 to 1994 of 118 ± 19 petagrams of carbon. The oceanic sink accounts for 48% of the total fossil-fuel and cement-manufacturing emissions, implying that the terrestrial biosphere was a net source of CO2 to the atmosphere of about 39 ± 28 petagrams of carbon for this period. The current fraction of total anthropogenic CO2 emissions stored in the ocean appears to be about one-third of the long-term potential.
ABSTRACT: The ocean is a significant sink for anthropogenic carbon dioxide, taking up about a third of the emissions arising from fossil-fuel use and tropical deforestation. Increases in the atmospheric carbon dioxide concentration account for most of the remaining emissions, but there still appears to be a 'missing sink' which may be located in the terrestrial biosphere.
ABSTRACT: Dissolved organic matter in the oceans represents one of the biosphere's principal stores of organic carbon. A large proportion of this matter is drained from the continents — particularly from northern peatlands, which contain 20% of the global soil carbon1 . Freeman et al.2 have suggested that rising temperatures may enhance this transport of dissolved organic carbon (DOC) from peatlands to the oceans. We argue here that warming can affect DOC export in different ways, depending on whether it is accompanied by increased or decreased precipitation. An alteration in the rate of relocation of organic carbon from the continents to the oceans cannot therefore be predicted on the basis of temperature change alone.