This chapter focuses on certain reasons for concern with regard to what might be considered a "dangerous" climate change (reported as increases in global mean temperature; see Section 19.1.2). Each reason for concern can be used by itself or in combination with other reasons for concern to examine different aspects of vulnerability to climate change. We offer no judgment about how to use some or all of these reasons for concern to determine what is a dangerous level of climate change. The reasons for concern are as follows:

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 globally aggregated impacts
4. The relationship between global mean temperature increase and the probability of extreme weather events
5. The relationship between greenhouse concentrations and the probability of large-scale singular events.

In addition, we address what observed effects of climate change tell us with regard to Article 2 of the UNFCCC. We review the state of knowledge with regard to what observations and each reason for concern tell us about climate change impacts.

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

• Statistically significant associations between trends in regional climate and impacts have been documented in ~10 physical processes and ~450 biological species, in terrestrial and marine environments on all continents. 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, such as population growth and urbanization. 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.

There is preliminary evidence that unique and threatened systems are beginning to be affected by regional climate change and that some systems have been affected by recent increases in extreme climate events in some areas. Many high-latitude and high-altitude systems are displaying the effects of regional climate change. It is difficult to define observed impacts at aggregate levels, and evidence of large-scale singular events occurring as a result of recent climate change is lacking.

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