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.
We propose measuring vulnerability of selected outcome variables of concern (e.g. agricultural yield) to identified stressors (e.g. climate change) as a function of the state of the variables of concern relative to a threshold of damage, the sensitivity of the variables to the stressors, and the magnitude and frequency of the stressors to which the system is exposed. In addition, we provide a framework for assessing the extent adaptive capacity can reduce vulnerable conditions. We illustrate the utility of this approach by evaluating the vulnerability of wheat yields to climate change and market fluctuations in the Yaqui Valley, Mexico.