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amplify (or reduce) the response to a given climate forcing. Uncertainty in the sensitivity of Earth’s climate system also makes it difficult to precisely quantify the effectiveness of actions or strategies that might be taken to limit the magnitude of future climate change.

There may be tipping points or thresholds that, once crossed, lead to irreversible events. Some of the physical and biological feedbacks triggered by climate change can become irreversible when they pass a certain threshold or tipping point. For example, there is general scientific consensus that the Arctic, which is systematically losing summer sea ice thickness and extent on an annual basis, is expected to become permanently ice-free during summers by the middle of the 21st century, regardless of how future emissions change. This change to an ice-free summer Arctic is expected, in part, because of the positive feedback between warming and sea ice melting (see Chapter 6). A number of other possible tipping points and irreversible changes have been identified in the Earth system, and human systems can also experience tipping points, such as the collapse of an economy or political system. Because of the possibility of crossing such thresholds, simple extrapolations of recent trends may underestimate future climate change impacts. Given the complexity of coupled human-environment systems, it is difficult to forecast when a tipping point might be approaching, but the probability of crossing one increases as the climate system moves outside the range of natural variability.

Analyses of impacts resulting from higher levels of climate change are limited. Most scientific analyses of climate change have focused on the impacts associated with a global temperature change of between 3.6°F to 5.4°F (2°C to 3°C) by the end of the 21st century, relative to preindustrial conditions. Yet model-based projections of future global temperature change range from 2°F to more than 11°F, and even larger changes are possible. For comparison, the higher end of the expected range of future temperature change is comparable to the estimated temperature difference between the present climate and the climate at the height of the last ice age, when glaciers covered the sites presently occupied by New York, Chicago, and Seattle and ecosystems around the world were radically different. Although there have been some recent efforts to estimate the impacts that might be associated with global temperature changes of greater than 9°F or 10°F (5°C or 6°C) over the next century (see, for example, University of Oxford, Tyndall Centre, and Hadley Centre Met Office, 2009), relatively little scientific information is available regarding the potential risks posed by such extreme changes in global climate.

Climate change does not act in isolation. As noted in several parts of Chapter 2 and in many of the chapters in Part II, climate change is just one of many stressors affecting

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