Climate Change and...
Abrupt Climate Change
- Climate Variability
- Climate Models
Effects of Climate Change
Alley, R.B., J. Marotzke, W.D. Nordhaus, J.T. Overpeck, D.M. Peteet, R.A. Pielke, Jr., R.T. Pierrehumbert, P.B. Rhines, T.F. Stocker, L.D. Talley, J.M. Wallace (2003). Abrupt climate change. Science 299: 2005-2010
ABSTRACT: Large, abrupt, and widespread climate changes with major impacts have occurred repeatedly in the past, when the Earth system was forced across thresholds. Although abrupt climate changes can occur for many reasons, it is conceivable that human forcing of climate change is increasing the probability of large, abrupt events. Were such an event to recur, the economic and ecological impacts could be large and potentially serious. Unpredictability exhibited near climate thresholds in simple models shows that some uncertainty will always be associated with projections. In light of these uncertainties, policy-makers should consider expanding research into abrupt climate change, improving monitoring systems, and taking actions designed to enhance the adaptability and resilience of ecosystems and economies.
INTRODUCTION: What is the natural variability of our climate? This simple question is, in fact, very hard to answer, from either the theoretical or observational point of view. However, it is of utmost importance if we really want to detect and predict the consequences of human activities that have altered several components of the climate system, the most noticeable change being a considerable increase in atmospheric carbon dioxide from the burning of fossil fuels. The climate system is complex because it is made up of several components (such as the atmosphere, oceans, and ice sheets), each of which has its own response times and thermodynamic properties. Those components not only interact nonlinearly with each other, but are connected to other complex systems such as the carbon cycle, which regulates greenhouse gas concentrations in the atmosphere. The climate can be affected by various types of so-called external forcings or influences (such as changes in insolation) that have different spatial and temporal scales of propagation in the system. A further problem is that internal rearrangements and resonances make it difficult to determine a true equilibrium state. Indeed, the steady state is characterized by a significant noise level and oscillations that are not always easy to distinguish from real transient changes of the global climate.
ABSTRACT: Two hypotheses have been put forward to explain the large and abrupt climate changes that punctuated glacial time. One attributes such changes to reorganizations of the ocean's thermohaline circulation and the other to changes in tropical atmosphere-ocean dynamics. In an attempt to distinguish between these hypotheses, two lines of evidence are examined. The first involves the timing of the freshwater injections to the northern Atlantic that have been suggested as triggers for the global impacts associated with the Younger Dryas and Heinrich events. The second has to do with evidence for precursory events associated with the Heinrich ice-rafted debris layers in the northern Atlantic and with the abrupt Dansgaard-Oeschger warmings recorded in the Santa Barbara Basin,
ABSTRACT: The possibility of a reduced Atlantic thermohaline circulation in response to increases in greenhouse-gas concentrations has been demonstrated in a number of simulations with general circulation models of the coupled ocean–atmosphere system. But it remains difficult to assess the likelihood of future changes in the thermohaline circulation, mainly owing to poorly constrained model parameterizations and uncertainties in the response of the climate system to greenhouse warming. Analyses of past abrupt climate changes help to solve these problems. Data and models both suggest that abrupt climate change during the last glaciation originated through changes in the Atlantic thermohaline circulation in response to small changes in the hydrological cycle. Atmospheric and oceanic responses to these changes were then transmitted globally through a number of feedbacks. The palaeoclimate data and the model results also indicate that the stability of the thermohaline circulation depends on the mean climate state.
ABSTRACT: About 8200 years ago, the climate of much of the Northern Hemisphere cooled abruptly for a period of about 200 years. In their Perspective, Clarke et al. examine the most likely culprit for this cooling: an outburst of fresh water from a vast, ice-dammed glacial lake in North America. The superlake had formed when the kilometers-thick ice sheet covering much of North America disintegrated. When the ice dam became unstable, fresh water flooded from the lake into the North Atlantic. It remains unclear how this fresh water affected ocean circulation or whether the outburst occurred in more than one stage, but the timing points strongly to the outburst flood as the trigger of the 8200-year climate event.
ABSTRACT: Paleoclimatic data are increasingly showing that abrupt change is present in wide regions of the globe. Here a mechanism for abrupt climate change with global implications is presented. Results from a tropical coupled ocean–atmosphere model show that, under certain orbital configurations of the past, variability associated with El Niño–Southern Oscillation (ENSO) physics can abruptly lock to the seasonal cycle for several centuries, producing a mean sea surface temperature (SST) change in the tropical Pacific that resembles a La Niña. It is suggested that this change in SST would have a global impact and that abrupt events such as the Younger Dryas may be the outcome of orbitally driven changes in the tropical Pacific.
D. L. Hartmann, J. M. Wallace, V. Limpasuvan, D. W. J. Thompson, J. R. Holton (2000). Can ozone depletion and global warming interact to produce rapid climate change?. Proceedings of the National Academy of Sciences 97 (4): 1412-1417
ABSTRACT: The atmosphere displays modes of variability whose structures exhibit a strong longitudinally symmetric (annular) component that extends from the surface to the stratosphere in middle and high latitudes of both hemispheres. In the past 30 years, these modes have exhibited trends that seem larger than their natural background variability, and may be related to human influences on stratospheric ozone and/or atmospheric greenhouse gas concentrations. The pattern of climate trends during the past few decades is marked by rapid cooling and ozone depletion in the polar lower stratosphere of both hemispheres, coupled with an increasing strength of the wintertime westerly polar vortex and a poleward shift of the westerly wind belt at the earth's surface. Annular modes of variability are fundamentally a result of internal dynamical feedbacks within the climate system, and as such can show a large response to rather modest external forcing. The dynamics and thermodynamics of these modes are such that strong synergistic interactions between stratospheric ozone depletion and greenhouse warming are possible. These interactions may be responsible for the pronounced changes in tropospheric and stratospheric climate observed during the past few decades. If these trends continue, they could have important implications for the climate of the 21st century.
ABSTRACT: The prospect of rapid dynamic changes in the environment is a pressing concern that has profound management and public policy implications. Worries over sudden climate change and irreversible changes in ecosystems are rooted in the potential that nonlinear systems have for complex and 'pathological' behaviours. Nonlinear behaviours have been shown in model systems and in some natural systems, but their occurrence in large-scale marine environments remains controversial. Here we show that time series observations of key physical variables for the North Pacific Ocean that seem to show these behaviours are not deterministically nonlinear, and are best described as linear stochastic. In contrast, we find that time series for biological variables having similar properties exhibit a low-dimensional nonlinear signature. To our knowledge, this is the first direct test for nonlinearity in large-scale physical and biological data for the marine environment. These results address a continuing debate over the origin of rapid shifts in certain key marine observations as coming from essentially stochastic processes or from dominant nonlinear mechanisms. Our measurements suggest that large-scale marine ecosystems are dynamically nonlinear, and as such have the capacity for dramatic change in response to stochastic fluctuations in basin-scale physical states.
L. D. Keigwin, G. A. Jones, S. J. Lehman, E. A. Boyle (1991). Deglacial meltwater discharge, North Atlantic deep circulation, and abrupt climate change. Journal of Geophysical Research 96 (C9): 16,811–16,826
ABSTRACT: High-resolution paleogeochemical data from the North Atlantic Ocean indicate that in the interval 15,000 to 10,00014 C years before present (B.P.) North Atlantic Deep Water (NADW) production was decreased or eliminated four times: at about 14,500 (and probably older), 13,500, 12,000 and 10,500 years B.P. Each of these changes occurred at the same time as abrupt events of meltwater discharge to the surface ocean (inferred from oxygen isotope studies of planktonic foraminifera and from glacial geological studies on land). In addition, each of these times may be associated with brief episodes of cooler climate in the North Atlantic region, the best example of which is the Younger Dryas cooling of 10,500 years ago. These results support models linking meltwater discharge, decreased NADW production, and decreased North Atlantic heat flux.
ABSTRACT: The ocean's thermohaline circulation has long been recognized as potentially unstable and has consequently been invoked as a potential cause of abrupt climate change on all timescales of decades and longer. However, fundamental aspects of thermohaline circulation changes remain poorly understood.
FIRST PARAGRAPH: Recent scientific evidence shows that major and widespread climate changes have occurred with startling speed. For example, roughly half the north Atlantic warming since the last ice age was achieved in only a decade, and it was accompanied by significant climatic changes across most of the globe. Similar events, including local warmings as large as 16°C, occurred repeatedly during the slide into and climb out of the last ice age. Human civilizations arose after those extreme, global ice-age climate jumps. Severe droughts and other regional climate events during the current warm period have shown similar tendencies of abrupt onset and great persistence, often with adverse effects on societies.
ABSTRACT: Many aspects of Earth’s climate system have changed abruptly in the past and are likely to change abruptly in the future. Although abrupt shifts in temperature are most dramatic in glacial climates, abrupt changes, resulting in an altered probability of drought, large floods, tropical storm landfall, and monsoon rainfall, are all important concerns even in the absence of significant anthropogenic climate forcing. Continued climate change will likely increase the probability of these types of abrupt change and also make abrupt changes in ocean circulation and sea level more likely. Although global warming may have already triggered abrupt change, current understanding and modeling capability is not sufficient to specify details of future abrupt climate change. Improved adaptation strategies arewarranted, as well as efforts to avoid crossing climate change thresholds beyond which large abrupt changes in sea level, ocean circulation, and methane-clathrate release could greatly amplify the impacts of climate change.
ABSTRACT: Successful prediction of future global climate is critically dependent on understanding its complex history, some of which is displayed in paleoclimate time series extracted from deep-sea sediment and ice cores. These recordings exhibit frequent episodes of abrupt climate change believed to be the result of nonlinear response of the climate system to internal or external forcing, yet, neither the physical mechanisms nor the nature of the nonlinearities involved are well understood. At the orbital (104 –105 years) and millennial scales, abrupt climate change appears as sudden, rapid warming events, each followed by periods of slow cooling. The sequence often forms a distinctive saw-tooth shaped time series, epitomized by the deep-sea records of the last million years and the Dansgaard–Oeschger (D/O) oscillations of the last glacial. Here I introduce a simplified mathematical model consisting of a novel arrangement of coupled nonlinear differential equations that appears to capture some important physics of climate change at Milankovitch and millennial scales, closely reproducing the saw-tooth shape of the deep-sea sediment and ice core time series, the relatively abrupt mid-Pleistocene climate switch, and the intriguing D/O oscillations. Named LODE for its use of the logistic-delayed differential equation, the model combines simplicity in the formulation (two equations, small number of adjustable parameters) and sufficient complexity in the dynamics (infinite-dimensional nonlinear delay differential equation) to accurately simulate details of climate change other simplified models cannot. Close agreement with available data suggests that the D/O oscillations are frequency modulated by the third harmonic of the precession forcing, and by the precession itself, but the entrained response is intermittent, mixed with intervals of noise, which corresponds well with the idea that the climate operates at the edge between chaos and order. LODE also predicts a persistent ~1.5 ky oscillation that results from the frequency modulated regional climate oscillation.
ABSTRACT: The oxygen isotope record from the Greenland Ice Sheet Project 2 (GISP2) ice core was reanalyzed in the frequency and time domains. The prominent 1470-year spectral peak, which has been associated with the occurrence of Dansgaard-Oeschger interstadial events, is solely caused by Dansgaard-Oeschger events 5, 6, and 7. This result emphasizes the nonstationary character of the oxygen isotope time series. Nevertheless, a fundamental pacing period of ~1470 years seems to control the timing of the onset of the Dansgaard-Oeschger events. A trapezoidal time series model is introduced which provides a template for the pacing of the Dansgaard-Oeschger events. Statistical analysis indicates only a =3% probability that the number of matches between observed and template-derived onsets of Dansgaard-Oeschger events between 13 and 46 kyr B.P. resulted by chance. During this interval the spacing of the Dansgaard-Oeschger onsets varied by ±20% around the fundamental 1470-year period and multiples thereof. The pacing seems unaffected by variations in the strength of North Atlantic Deep Water formation, suggesting that the thermohaline circulation was not the primary controlling factor of the pacing period.
ABSTRACT: Much to the surprise of investigators, evidence is mounting that major changes in the earth's climate can take place in a very short time. Data from ice cores and ocean sediments suggest, for example, that 11,650 years ago the climate in Greenland switched from ice-age conditions to the current relatively warm conditions (a warming of 5 to 10 degrees Celsius on average) in only 40 years. The author describes the oceanic currents that influence climate and establish its stability, as well as "triggers" that may perturb changes—including the possibility that "greenhouse" warming could invoke a rapid switch.