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This chapter reviews projected climate change impacts over the next thirty to one hundred years and outlines three climate change scenarios, of three grades of severity, that cover a plausible range of impact severity. These scenarios , based on current scientific understanding and uncertainty regarding past and future climate change, guide assessments in later chapters of potential security consequences of climate change impacts. The general approach is to settle on three different levels of global average temperature change for each scenario, and then extract relevant projected impacts from the Fourth Assessment Report (AR4) of the Intergovernmental Panel on Climate Change (IPCC) and other peer-reviewed scientific sources. We focus particularly on changes in freshwater resources, crop production, extreme weather events, sea level rise, and the meridional overturning circulation (MOC) of the North Atlantic Ocean. Because the purpose of this project is to assess potential security risks of future climate change, the primary criterion for the climate impacts scenarios outlined here is plausibility rather than probability. Rather than asking, What is the most likely climate-driven outcome? we ask, What potential climate-driven outcomes are plausible, given current scientific understanding and uncertainties about the future climate? Recent observations indicate that 49 three Three Plausible Scenarios of Future Climate Change JAY GULLEDGE The scenarios outlined in this section are not predictions of future conditions and should not be read or cited as such. 11100-03_CH03.qxd 5/8/08 12:43 PM Page 49 projections from climate models have been too conservative; the effects of climate change are unfolding faster and more dramatically than expected. Given the uncertainty in calculating climate change, and the fact that existing estimates may be biased low at this time, plausibility is an important measure of future impacts. Under this umbrella of plausibility, potential changes that the IPCC or other assessments may characterize as improbable are considered plausible here if significant uncertainty persists regarding their probability; collapse of the North Atlantic MOC is an example. Because projections of sea level rise remain particularly uncertain, direct consultation with experts and the author’s professional judgment inform the sea level rise scenarios outlined here. Accordingly, the impacts scenarios presented here are not predictions, but are potential outcomes either supported by or not ruled out by current scientific understanding, and therefore deserving of consideration when potential risks are assessed. These impacts scenarios are meaningful only in the context of the security risk assessments offered in this volume and should not be misconstrued as predictions. Physically deterministic predictions of future climate are currently impossible, and perfect foresight would obviate in any case the need for scenario-based assessments. This is the unavoidable backdrop of uncertainty against which analysts must assess the implications of global climate change. Scenario-Based Approach According to the IPCC, a scenario is “a coherent, internally consistent and plausible description of a possible future state of the world. Scenarios are not predictions or forecasts but are alternative images without ascribed likelihoods of how the future might unfold.”1 In this volume we develop a group of three impacts scenarios: expected, severe, and catastrophic. Although guided in general by the IPCC AR4 and other authoritative scientific sources, these impacts scenarios are unique to this study and were created specifically for its purposes. The IPCC uses independent scenarios of man-made greenhouse gas emissions called SRES (Special Report on Emissions Scenarios) scenarios in its assessment process.2 The SRES scenarios make assumptions about future population growth, economic and infrastructure development, and energy policy that result in plausible, alternative pathways of future greenhouse gas emissions in the absence of policies to mitigate climate change. In the IPCC assessments and other studies, multiple SRES emissions scenarios are used 50 JAY GULLEDGE 11100-03_CH03.qxd 5/8/08 12:43 PM Page 50 as plausible alternatives to drive climate models, thus producing multiple plausible projections of future climate conditions. As described later, the SRES A1B emissions scenario is used in our study solely to derive levels of temperature change for each of our three impacts scenarios. We then extract impacts from published studies (primarily the AR4) based on those levels of temperature change, regardless of which emissions scenarios were used to drive climate models in those studies. A caveat of this approach is that different SRES emissions scenarios assume different demographic trends, such as total population, population living near coastlines, and level of economic and technological development in developing countries. These differences alter estimates of population sizes affected by climate impacts, particularly sea level...


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