involve questions of values, e.g., regarding how much risk to people or to nature might be considered too much. Some climate changes could be beneficial for some people or regions, while being damaging to others. For example, with global warming, fewer people may die in cold waves while more people die in heat waves; similarly, crops may be more productive in parts of Canada while less productive in the United States, raising issues of international food trade and transfer (see Section 5.1). Treatment of such effects as cancelling one another would generally be a value judgment and is not deemed to be appropriate here. Due to these considerations, we do not comprehensively cover possible benefits of climate change that could accrue to some people or regions. The study does not seek to evaluate the financial costs or feasibility of achieving any given stabilization target, nor to identify possible mitigation strategies to attain the targets.

The primary challenge for this study is to quantify insofar as possible the expected outcomes of different stabilization targets using analyses and information drawn from the scientific literature. Data available from publicly available archives were used in analyses carried out for this study, including, e.g., the CMIP3 climate model intercomparison project, the new representative concentration pathway (RCP) scenarios, etc. Some analyses and runs were also carried out, using published models and methods. The report covers emissions, concentrations, changes in the physical climate (such as temperature, rainfall, sea level, etc.) and their time scales, as well as associated impacts (such as food production, flooding, ecosystem damage, etc). In evaluating impacts, we seek to identify a baseline that includes expectations regarding adaptation to climate change where appropriate, but we also identify instances where adaptation is possible but where its feasibility, likelihood, or effectiveness is presently unknown. The report represents a brief summary of a vast scientific literature and seeks to be illustrative and representative rather than comprehensive.

Warming is the frame of reference for evaluation of impacts used in this report for both conceptual and practical reasons. Many key future climate impacts are dependent upon the amount of global warming. Indeed, available data and modeling suggest that the magnitudes of several key impacts can be evaluated with relatively good accuracy for given amounts of global warming. Much of the available literature and analysis of climate impacts can be tied to specific warming levels but not readily linked to CO2-equivalent concentrations (due for example to lack of specification of aerosol forcing between studies). Indicated warming levels are related here to the corresponding best estimates and uncertainties in CO2-equivalent concentrations as well as to cumulative carbon emissions.

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