eral millennia. Indeed, some effects of 21st century human choices would contribute to climate change for more than 100,000 years. {2.1, 3.2}3

One way of informing these choices is to consider the projected climate changes and impacts that would occur if greenhouse gases increase to particular concentration levels and then stabilize, as highlighted in the Statement of Task (see Appendix A). Alternative futures then can be represented by a broad range of atmospheric concentration “target” levels (hereafter referred to as stabilization targets). The committee was charged to evaluate different stabilization targets with particular emphasis on the avoidance of serious or irreversible impacts on Earth’s climate system. This report does not evaluate the plausibility of any stabilization target, nor does it make any recommendations regarding desirable or “safe” targets.

It should be emphasized that choosing among different targets is a policy issue rather than strictly a scientific one, because such choices 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.

The primary challenge for this study is to quantify, insofar as possible, the outcomes of different stabilization targets using analyses and information drawn from the scientific literature. Expected changes based on broad scientific understanding are discussed, as well as projected values based upon models. Where there is sufficient understanding to be quantitative, numerical values for projected climate change and impacts are provided as a function of stabilization target. A number of important aspects of climate change that are currently understood in a qualitative manner, or for which the time horizon of the response is poorly constrained, are also reviewed. The report represents a brief summary of a vast scientific literature and seeks to be illustrative and representative rather than comprehensive. Special emphasis is placed on climate changes and impacts in North America and the United States.

The report focuses on human forcing of the climate system from carbon dioxide emissions and rising atmospheric concentrations because of the dominant role and unique influences of carbon dioxide on long-term climate change. The role of other anthropogenic greenhouse gases, such as methane, nitrous oxide and halocarbon, and aerosols are also briefly discussed. For many purposes, the total radiative forcing of the suite of anthropogenic greenhouse gases and aerosols can be cast in terms of an

3

Throughout this summary and the technical overview presented in the next section, numbers in curly brackets refer to sections of the main report where details and references are to be found.



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