IPCC Fourth Assessment Report: Climate Change 2007
Climate Change 2007: Working Group III: Mitigation of Climate Change

Decision-making, risk and uncertainty

Mitigation policies are developed in response to concerns about the risk of climate change impacts. However, deciding on a proper reaction to these concerns means dealing with uncertainties. Risk refers to cases for which the probability of outcomes and its consequences can be ascertained through well-established theories with reliable, complete data, while uncertainty refers to situations in which the appropriate data may be fragmentary or unavailable. Causes of uncertainty include insufficient or contradictory evidence as well as human behaviour. The human dimensions of uncertainty, especially coordination and strategic behaviour issues, constitute a major part of the uncertainties related to climate change mitigation (high agreement, much evidence) [2.3.3; 2.3.4].

Decision-support analysis can assist decision makers, especially if there is no optimum policy that everybody can agree on. For this, a number of analytical approaches are available, each with their own strengths and weaknesses, which help to keep the information content of the climate change problem within the cognitive limits of the large number of decision makers and support a more informed and effective dialogue among the many parties involved. There are, however, significant problems in identifying, measuring and quantifying the many variables that are important inputs to any decision-support analysis framework – particularly impacts on natural systems and human health that do not have a market value, and for which all approaches are simplifications of the reality (high agreement, much evidence) [2.3.7].

When many decision makers with different value systems are involved in a decision, it is helpful to be as clear as possible about the value judgments underpinning any analytic outcomes they are expected to draw on. This can be particularly difficult and subtle where analysis aims to illuminate choices associated with high levels of uncertainty and risk (medium agreement, medium evidence) [2.3.2; 2.3.7].

Integrated assessments can inform decision makers of the relationship between geophysical climate change, climate-impact predictions, adaptation potentials and the costs of emission reductions and the benefits of avoided climate change damage. These assessments have frameworks to deal with incomplete or imprecise data.

To communicate the uncertainties involved, this report uses the terms in Table TS.1 to describe the relative levels of expert agreement on the respective statements in the light of the underlying literature (in rows) and the number and quality of independent sources qualifying under IPCC rules[4] upon which a finding is based (in columns). The other approaches of ‘likelihood’ and ‘confidence’ are not used in this report as human choices are concerned, and none of the other approaches used provides sufficient characterization of the uncertainties involved in mitigation (high agreement, much evidence) [2.4].

Table TS.1: Qualitative definition of uncertainty [Table 2.2].

Table TS.1

Note: This table is based on two dimensions of uncertainty: the amount of evidence[5] and the level of agreement. The amount of evidence available about a given technology is assessed by examining the number and quality of independent sources of information. The level of agreement expresses the subjective probability of the results being in a certain realm.

  1. ^  IPCC rules permit the use of both peer-reviewed literature and non-peer-reviewed literature that the authors deem to be of equivalent quality.
  2. ^  ‘Evidence’ in this report is defined as: Information or signs indicating whether a belief or proposition is true or valid. See Glossary.