BOX 2.1
Climate Change and Risk

As used here, the term risk applies to undesired events that may occur in the future but are not certain to occur. Analysts typically quantify risks along two dimensions—the probability that an event will occur, and the magnitude or consequence(s) of the event—and multiply the two to get a risk estimate (probability times consequence).a Consequences, however, have many dimensions. They vary in terms of which human values and concerns they affect (lives, livelihoods, community integrity, nonhuman species, etc.), where they are likely to occur, whom they may affect, and when they are likely to cause harm. Consequences also vary in terms of perception and significance to those who face the risks—for example, in the degree to which the consequence is understood or evokes dread (unknown risks can sometimes concern people more than other risks), and even in the trust in the organizations that manage such risks (mistrust in the managing organizations tends to increase perceived risk).b

The risks posed by climate change are thus complex. As this chapter discusses, climate change drives a variety of biophysical processes, which leads to a variety of potential consequences for many things that people value. Risks will change over time, and consequences will be highly variable across different locations and population groups. Scientific analyses can improve understanding of the risks associated with climate change, including how different human reactions might change those risks and at what cost. One way this is done, for instance, is through scenario analyses that illustrate a range of possible future conditions and that can be used to test out the performance of different response strategies. But regardless of how much supporting scientific information is available, making choices about how to act in the face of uncertainty can prove contentious if people disagree about the nature of the risks they face or about which elements of these risks are most important.


a C. Jaeger, O. Renn, E. A. Rosa, and T. Webler, Risk, Uncertainly and Rational Action (London: Earthscan, 2001).

b P. Slovic, B. Fischhoff, and S. Lichtenstein, “Facts and fears: Understanding perceived risk,” in Societal Risk Assessment: How Safe Is Safe Enough?(R. C. Schwing and W. A. Albers, Jr., eds. New York: Plenum, 1980); O. Renn, Risk Governance: Coping with Uncertainty in a Complex World (London: Earthscan, 2008).

  • The concentration of CO2 in the atmosphere has increased markedly over the past 150 years (see Figure 2.2) and is now higher than at any time in at least 800,000 years.9
  • The long-term rise in CO2 concentrations can be attributed primarily to the growth in human CO2 emissions from fossil fuel burning (Figure 2.2), with deforestation and other land use and land cover changes also contributing.10
  • Concentrations of other GHGs, including methane, nitrous oxide, and certain halogenated gases, have also increased as a result of human activities.11

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