This chapter draws on the results of the entire TAR to assess the state of knowledge concerning Article 2 of the UNFCCC. Article 2 of the UNFCCC states that:

" ...the ultimate objective of this Convention…is to achieve…stabilization of greenhouse gas concentrations in the atmosphere at a level that would prevent dangerous anthropogenic interference with the climate system. Such a level should be achieved with a time frame sufficient to allow ecosystems to adapt naturally to climate change, to ensure that food production is not threatened and to enable economic development to proceed in a sustainable manner." (UNEP/WMO, 1992).

The ultimate goal for stabilizing GHG concentrations is to avoid "dangerous anthropogenic interference with the climate system." The question of what is dangerous is one that the authors of this chapter cannot answer. Danger is a function of the degree to which effects are negative and the degree to which those effects are unacceptable. The latter is a value judgment. The TAR's task is to define what is known about the effects of climate change—to identify their character and their implications and whether they are negative or positive. It is not about determining whether these effects are acceptable.

The preceding chapters review the literature about vulnerability to climate change in regions and sectors. The goal of this chapter is to draw on very disparate reasons for concern regarding climate change impacts in a manner that will enable readers to evaluate the relationship between increases in global mean temperature and impacts (for an explanation of why change in global mean temperature is used as an indicator, see Section 19.1.2). It attempts to enable readers to understand the risks of higher magnitudes of increased global mean temperature.

To provide information to readers in a manner that will enable them to make judgments about what level of climate change may be dangerous, this chapter addresses "reasons for concern," which represent a way for readers to think about the seriousness of climate change impacts. These reasons for concern are taken from debates and literature about the risks of climate change. The authors of this chapter make no judgment regarding whether one or several reasons for concern are more important than others. Nor do we attempt to combine the reasons for concern to produce a single "bottom line."

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: Some unique and threatened systems may be irreparably harmed by changes in climate beyond certain thresholds.
   
2. The relationship between global mean temperature increase and the distribution of impacts: Some regions, countries, islands, and cultures may be adversely affected by climate change, whereas others could benefit, at least up to a point. For example, in some sectors, adverse effects may be experienced in some parts of the world while other parts may have net gains. Within countries, some regions or groups of people could be harmed while others benefit or experience less harm.
   
3. The relationship between global mean temperature increase and global aggregated impacts: Using a consistent method of measurement and aggregation of climate change impacts, we address how aggregate impacts change as global mean temperature increases, whether aggregate impacts are positive at some levels of temperature increase and negative at others, whether change will occur smoothly or in a more complex dynamic pattern, and whether aggregate impacts mask unequal distribution of impacts.
   
4. The relationship between global mean temperature increase and the probability of extreme weather events: As mean climate changes, so too will the probability of extreme weather events such as days with very high or very low temperatures, extreme floods, droughts, tropical cyclones, and storms. This chapter addresses how the probability and consequences of such events may change as global mean temperature increases.
   
5. The relationship between global mean temperature increase and the probability of large-scale singular events, such as collapse of the West Antarctic ice sheet (WAIS) or shutdown of the North Atlantic thermohaline circulation (THC): This chapter addresses what is known about how the probabilities of such events change as the magnitude of climate change increases.

In addition, this chapter addresses whether changes in climate during the 20th century have resulted in observed impacts. The IPCC has documented these changes, and an important question is whether these changes have resulted in measurable impacts on nature or society. Important questions include the following:

• Are the observed effects of climate change consistent with model predictions, particularly those that estimate more serious impacts at larger GHG concentrations?
  
• Even if it is not clear whether observed effects are caused primarily by climate change, do these effects give us information about the potential vulnerability of systems to climate change?

Observations are not a reason for concern. Instead, they help us determine whether impacts that are relevant to any of the five reasons for concern have occurred.

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