Global Environmental Change

Abstract

Abstract Human activity is altering the composition of Earth's atmosphere through the addition of greenhouse gases and particulates. Anthropogenic changes in the properties of the atmosphere can be thought of as external forcing factors on the climate system. There is a fair degree of confidence in results on greenhouse gas forcing and the climate's response to that forcing. However, knowledge of the forcing and response due to aerosols remains highly uncertain. There are also natural external forcing factors that influence climate, such as changes in orbital geometry and changes in solar irradiance and volcanic activity. The climate system, even when not perturbed by external factors, produces substantial amounts of natural variability. Thus detection and attribution of the effects of external forcing is a statistical signal‐in‐noise problem. The detection part of this problem is the process of demonstrating that an observed change is not likely to have been entirely the result of natural internal variability. The attribution aspects of the problem are more difficult because it is not possible to conduct controlled experiments with the climate system. The practical approach that has been taken in the climate research community involves statistical analysis and the assessment of multiple lines of evidence to (a) demonstrate that observed changes are consistent with forcing of the climate by a combination of anthropogenic and natural external factors, and (b) demonstrate that the changes are inconsistent with alternative, physically plausible explanations.