Climate Change 2001:
Working Group II: Impacts, Adaptation and Vulnerability
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EXECUTIVE SUMMARY

This chapter synthesizes the results of Work Group II of the Third Assessment Report (TAR) and assesses the state of knowledge concerning Article 2 of the United Nations Framework Convention on Climate Change (UNFCCC). The TAR's task is to define what is known about the effects of climate change: how sensitive systems are, what adaptive capacity they have, and what their vulnerability is. It is not the goal of this assessment to determine whether these effects are tolerable or are considered dangerous.

The goal of this chapter is to synthesize information on climate change impacts in a manner that will enable readers to evaluate the relationship between increases in global mean temperature and impacts. The chapter focuses on certain "reasons for concern" that may aid readers in making their own determination about what is a "dangerous" climate change. Each reason for concern is consistent with a paradigm that can be used by itself or in combination with other paradigms to help determine what level of climate change is dangerous. The reasons for concern are:

  1. The relationship between global mean temperature increase and damage to or irreparable loss of unique and threatened systems
  2. The relationship between global mean temperature increase and the distribution of impacts
  3. The relationship between global mean temperature increase and global aggregate damages
  4. The relationship between global mean temperature increase and the probability of extreme weather events
  5. The relationship between global mean temperature increase and the probability of large-scale singular events such as the breakup of the West Antarctic Ice Sheet or the collapse of the North Atlantic thermohaline circulation.

In addition, we examine what observed effects of climate change tell us with regard to Article 2 of the UNFCCC. Increase in global mean temperature since 1900 (i.e., mean global warming) is used as the common metric against which impacts are measured. This metric is closely related to greenhouse gas (GHG) concentrations but is more relevant for impact assessments.

Some general caveats apply to all of the reasons for concern:

It does not appear to be possible to combine the different reasons for concern into a unified reason for concern that has meaning and is credible. However, we can review the relationship between impacts and temperature for each reason for concern and draw some preliminary conclusions about the potential severity and risk of impacts for the individual reasons for concern. Note that the following findings do not incorporate the costs of limiting GHG emissions to levels that are sufficient to avoid changes that may be considered dangerous. Also note that there is substantial uncertainty regarding the impacts of climate change at the temperatures mentioned. These temperatures should be taken as approximate indications of impacts, not as absolute thresholds. In addition, change in global mean temperature does not describe all relevant aspects of climate change impacts, such as rate and pattern of change and changes in precipitation, extreme climate events, or lagged (or latent) effects such as rising sea levels. For simplification, we group different levels of temperature increase into "small," "medium," and "large." "Small" denotes a global mean temperature increase of as much as approximately 2°C; "medium" denotes a global mean temperature increase of approximately 2-3°C; and "large" denotes a global mean temperature increase of more than approximately 3°C.

Based on a review of the literature of observations of climate change impacts, as reflected in other chapters in the TAR, we conclude the following:



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