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:

• In spite of many studies on climate change impacts, there still is substantial uncertainty about how effective adaptation will be (and could be) in ameliorating negative effects of climate change and taking advantage of positive effects.
  
• The effect of changes in baseline conditions, such as population and economic growth and development of new technologies that could change vulnerability, has not been adequately considered in most impact studies.
  
• Most impact studies assess the effects of a stable climate, so our understanding of what rates of change may be dangerous is limited.

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:

• Observations: Statistically significant associations between trends in regional climate and impacts have been documented in ~100 physical processes and ~450 biological species or communities in terrestrial and polar environments. Although the presence of multiple factors (e.g., land-use change, pollution, biotic invasion) makes attribution of observed impacts to regional climate change difficult, more than 90% (~99% physical, ~80% biophysical) of the changes documented worldwide are consistent with how physical and biological processes are known to respond to climate. Based on expert judgment, we have high confidence that the overall patterns and processes of observations reveal a widespread and coherent impact of 20th-century climate changes on many physical and biological systems. Signals of regional climate change impacts may be clearer in physical and biological systems than in socioeconomic systems, which also are simultaneously undergoing many complex changes that are not related to climate change, such as population growth and urbanization. Socioeconomic systems have complex and varying mechanisms for adapting to climate change. There are preliminary indications that some social and economic systems have been affected in part by 20th-century regional climate changes (e.g., increased damages from flooding and droughts in some locations). It generally is difficult to separate climate change effects from coincident or alternative explanations for such observed regional impacts.
  
• Unique and Threatened Systems: Tropical glaciers, coral reefs, mangroves, ecotones, and biodiversity "hot spots" are examples of unique and threatened entities that are confined to narrow geographical ranges and are very sensitive to climate change. However, their degradation or loss could affect regions outside their range. There is medium confidence that several of these systems will be affected by a small temperature increase; for example, coral reefs will bleach and glaciers will recede. At higher magnitudes of temperature increase, other and more numerous unique and threatened systems would be adversely affected.
  
• Distribution of Impacts: The impacts of climate change will not be evenly distributed among the peoples of the world. There is high confidence that developing countries will be more vulnerable to climate change than developed countries, and there is medium confidence that climate change would exacerbate income inequalities between and within countries. There also is medium confidence that a small temperature increase would have net negative impacts on market sectors in many developing countries and net positive impacts on market sectors in many developed countries. However, there is high confidence that with medium to high increases in temperature, net positive impacts would start to decline and eventually would turn negative, and negative impacts would be exacerbated. Estimates of distributional effects are uncertain because of aggregation and comparison methods, assumptions about climate variability, adaptation, levels of development, and other factors.
  
• Aggregate Impacts: With a small temperature increase, there is medium confidence that aggregate market sector impacts would amount to plus or minus a few percent of world gross domestic product (GDP), and there is low confidence that aggregate nonmarket impacts would be negative. Most people in the world would be negatively affected by a small to medium temperature increase. Most studies of aggregate impacts find that there are net damages at the global scale beyond a medium temperature increase and that damages increase from there with further temperature increases. The important qualifications raised with regard to distributional analysis (previous bullet item) also apply to aggregate analysis. By its nature, aggregate analysis masks potentially serious equity differences. Estimates of aggregate impacts are controversial because they treat gains for some as canceling out losses for others and because the weights that are used to aggregate over individuals are necessarily subjective.
  
• Extreme Climate Effects: The frequency and magnitude of many extreme climate events increase even with a small temperature increase and will become greater at higher temperatures (high confidence). Extreme events include, for example, floods, soil moisture deficits, tropical and other storms, anomalous temperatures, and fires. The impacts of extreme events often are large locally and could strongly affect specific sectors and regions. Increases in extreme events can cause critical design or natural thresholds to be exceeded, beyond which the magnitude of impacts increases rapidly (high confidence).
  
• Large-Scale Singularities: Large-scale singularities in the response of the climate system to external forcing, such as shutdown of the North Atlantic thermohaline circulation or collapse of the West Antarctic ice sheet, have occurred in the past as a result of complex forcings. Similar events in the future could have substantial impacts on natural and socioeconomic systems, but the implications have not been well studied. Determining the timing and probability of occurrence of large-scale singularities is difficult because these events are triggered by complex interactions between components of the climate system. The actual impact could lag the climate change cause (involving the magnitude and the rate of climate change) by decades to millenia. There is low to medium confidence that rapid and large temperature increases would exceed thresholds that would lead to large-scale singularities in the climate system.

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