The chapter is organized into the following sections:

• Section 19.2 addresses the insights we can gain by examining observed effects of climate change. Are we seeing impacts of climate change on nature and society?
  
• Section 19.3 addresses what changes in global mean temperature may cause harm to threatened and unique systems. For example, are threatened and unique systems at risk from even low levels of increase in global mean temperature? Are some societies at particular risk at low levels of temperature increase?
  
• Section 19.4 addresses the evidence regarding the relationship between change in global mean temperature and distribution of impacts. Are adverse or positive impacts from climate change distributed equally around the world and within countries? Are some regions harmed at certain levels of climate change while others benefit? Are some subgroups or cultures at greater risk than the population as a whole?
  
• Section 19.5 addresses what insights we gain from aggregate or comprehensive approaches to measuring impacts. What do approaches such as monetization or looking at the number of people who are harmed or benefited tell us about the relationship between aggregate impacts and higher temperatures? This section also addresses insights gained from integrated assessment models (IAMs).
  
• Section 19.6 addresses the potential for increases in extreme climate events and large-scale singular effects. As temperatures increase or the rate of temperature rise increases, does the potential for extreme climate events and singular effects such as a change in ocean circulation patterns or the collapse of ice sheets increase? Can thresholds of change in terms of magnitudes or rates of change be identified?
  
• Section 19.7 addresses the limitations of the information used in this chapter to address observations and the reasons for concern. It also addresses future research that is needed to narrow these uncertainties.
  
• Section 19.8 summarizes the findings on observations and the reasons for concern.

Sections 19.2 and 19.3 draw most heavily on the TAR. Examples can be found in the region and sector chapters of this report; the sections in this chapter do not introduce new information. Instead, they synthesize that information in ways that the other chapters are unable to because they do not examine all regions and sectors. Sections 19.4, 19.5, and 19.6 draw on information that is not found in the regional and sectoral chapters. They do so because they address issues that those chapters cannot:

• Comparison of impacts across regions (Section 19.4). The sectoral chapters do this for each sector, but this can be done comprehensively only in this chapter.
  
• Aggregation of impacts (Section 19.5). This requires use of common metrics to aggregate impacts across sectors and regions. None of the other chapters can do this.
  
• Examination of changes in extreme events and large-scale discontinuities (Section 19.6). This generally is not addressed in the region and sector chapters because the climate change scenarios that are used most commonly in impacts studies examine only changes in average conditions, not changes in extreme events or large-scale discontinuities (see Chapter 4).

Thus, this chapter contains much new information in a framework that can help readers judge what may considered to be a dangerous level of climate change.

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