This scenario provides the basis for chapter 5 in this volume, by Leon Fuerth, on severe consequences of climate change for national and international security over the next thirty years. It assumes that the AR4 projections of both warming and attendant impacts are systematically biased low. Multiple lines of evidence support this assumption, and it is therefore important to consider from a risk perspective.9 For instance, the models used to project future warming either omit or do not account for uncertainty in potentially important positive feedbacks that could amplify warming (for example, release of greenhouse gases from thawing permafrost, reduced ocean and terrestrial CO2 removal from the atmosphere), and there is some evidence that such feedbacks may already be occurring in response to the present warming trend.10 Hence, climate models may underestimate the degree of warming from a given amount of greenhouse gases emitted to the atmosphere by human activities alone. Additionally, recent observations of climate system responses to warming (for example, changes in global ice cover, sea level rise, tropical storm activity, average annual precipitation) suggest that IPCC models underestimate the responsiveness of some aspects of the climate system to a given amount of warming.11 On these premises, the second scenario assumes that omitted positive feedbacks occur quickly and amplify warming to double that projected for emissions scenario SRES A1B, and that the climate system components respond more strongly to warming than predicted (see table 3-1).
According to our current understanding of physical inertia in the climate system, warming of 2.6°C (4.5°F) seems highly unlikely on the thirty-year time scale. Bearing in mind, however, that the IPCC projections show only average change with a smooth evolution over time and have tended to underestimate climate system response to warming already realized, a combination of underestimated change and abrupt episodes could plausibly result in an unexpectedly large and rapid warming with larger than expected impacts in a matter of a few decades. Moreover, a recent study aimed at quantifying the uncertainty surrounding model projections of future temperature found a greater than one-in-twenty chance that by 2040, warming could exceed 2°C (3.6°F) relative to 1990 for the highest SRES emissions scenario in the absence of strong positive feedbacks and abrupt change.12
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