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WIREs Clim Change
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Analyzing abrupt and nonlinear climate changes and their impacts

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Abstract The Earth system shows the tendency to change in nonlinear and sometimes abrupt ways; small changes in external forcing can lead to large and perhaps irreversible changes in outcome. The prospect of crossing important ‘tipping points’ and realizing their impacts poses unique challenges to decision makers within society, hoping to avoid damaging anthropogenic influence on Earth systems. Abrupt and nonlinear changes are by their very nature highly uncertain and difficult to predict, and so hard to avoid or adapt to. After briefly introducing key concepts in nonlinear dynamics, we summarize paleoclimate evidence for past abrupt and nonlinear changes in major Earth systems such as, ocean and atmospheric circulation patterns, sea ice and terrestrial ice sheets, atmospheric composition, and the terrestrial biosphere. For each of these systems we then review observational, theoretical, and modeling evidence for potential future abrupt changes, and associated impacts. We outline the extra challenges that are faced in predicting abrupt or nonlinear as opposed to more gradual climate change, and in providing a risk analysis for their impacts on Earth and societal systems. We examine the potential for early warning systems of abrupt change, and discuss differences in attitude to risk which may dictate societal response to low probability–high impact events. Finally, we outline the promising directions of research needed to better quantify the risk of abrupt and nonlinear climate change. WIREs Clim Change 2011 2 663–686 DOI: 10.1002/wcc.130 This article is categorized under: Assessing Impacts of Climate Change > Evaluating Future Impacts of Climate Change

The equilibrium response of a system to a small perturbation in forcing determines its ability to undergo an abrupt change. A ball in a potential well represents an Earth system. Initially (furthest from the viewer), a small perturbation to the state of the system allows the ball to return to the stable equilibrium state at the bottom of the potential well. The path of the ball within the changing shape of the potential well represents the response of the system to increasing (red) and decreasing (black) external forcing. The system is capable of ‘catastrophic’ changes, due to the emergence of a second equilibrium solution with increasing forcing. A small perturbation can now lead to an abrupt change of state, with the ball falling quickly to the other stable equilibrium. A system close to this bifurcation point may be forced across the threshold by internal variability. The system shows some irreversibility, or hysteresis; once the catastrophic change has occurred, forcing must reduce far beyond the bifurcation point (the ball must be pushed far from the viewer) to return to the original equilibrium state.

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Climate–carbon cycle interactions in the C4MIP models, under the A2 emissions scenario. (a) Effect of climate–carbon cycle feedbacks on the rate of rise of atmospheric CO2—each line shows, for each model, the difference in CO2 projected with and without climate–carbon cycle feedbacks. (b) Land–atmosphere CO2 fluxes—each curve shows the simulated flux in a different model (Reprinted with permission from Ref 184. Copyright 2006 American Meteorological Society)

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Trends in the airborne fraction of CO2 emissions (fossil fuel and deforestation) from 1960 to 2006, estimated from observations by Canadell et al.189 (black, with a similar estimate to that given in Ref 186) and Knorr187 (green), compared with the airborne fraction trend simulated by the C4MIP models with climate–carbon cycle feedbacks (red) and without climate–carbon cycle feedbacks (blue) (Reprinted with permission from Ref 188. Copyright 2011 The Royal Society)

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Surface air temperature change in the first decade after MOC shutdown in a global warming scenario (IS92a) (Reprinted with permission from Ref 172. Copyright 2008 Reidel Pub. Co.)

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The hysteresis curves of 11 Earth System Models of Intermediate Complexity (EMICS) in a study comparing the response of North Atlantic Deep Water (NADW) flux to quasi‐equilibrium changes in freshwater forcing in the North Atlantic. The upper panel shows models with simplified oceans and the lower panel models with three‐dimensional coupled oceans. Circles represent the present day climate state of each model (Reprinted with permission from Ref 168. Copyright 2005 American Geophysical Union)

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Central Greenland temperature in the last 49,000 years, from the GISP2 ice core record.15 Abrupt changes in the climate of the North Atlantic region were common during the last glacial period. The transition into the Bølling‐Allerød (warm period) and out of the following Younger Dryas (cold period), which together make up the last of the warming–cooling cycles of the last glacial period,16–19 appear to be intimately associated with changes in the ocean's thermohaline circulation.20 The start of the Bølling‐Allerød, ∼14,500 BP, saw Greenland temperatures jump by ∼ 10°C within several decades,21 followed by a global reorganization in the terrestrial biosphere over the next ∼100 years.22 Warm temperatures were experienced across much of the northern hemisphere,23 before cooling back to glacial conditions into the Younger Dryas, concomitant with an apparent large reduction in the rate of the ocean's overturning circulation.24 The Younger Dryas then terminated abruptly ∼11,500 BP with a warming of ∼ 15°C,25 half of which is believed to have occurred over as little as 15 years.26 Accompanying these abrupt jumps in temperature were rapid shifts within diverse components of the Earth system, ranging from East African aridity27 to Western European storminess.28

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