Introduction

The Earth sciences form core disciplines contributing to the interdisciplinary assessment of human-induced climate change. Assessment exploits understanding gained from the huge, ongoing scientific endeavor to better understand the climate system as well as from interdisciplinary research to better understand how the climate system interacts with human activities. The behavior and response of the Earth system defines the links between human activities influencing the climate system, and climate system influences on society. Papers in this section provide examples of research and review of the analysis and modeling of the Earth system, and the application of such models to provide a framework to address questions associated with the following three sections of this book: impacts and adaptation, mitigation of Greenhouse gases, and policy design and decisionmaking under uncertainty.

This section examines key issues relevant to our ability to forecast future climate, construct and test models of the Earth system for use in integrated assessment, characterize uncertainty in forecasts, and analyze illustrative cases of interactions of human activities with the climate system. While the specific topics addressed in this section are hardly comprehensive, they do give examples of how interdisciplinary studies have not only drawn from fundamental understanding generated by the Earth sciences, but have also contributed to better understanding of the research needs to address societal questions and have begun to carry out this research in conjunction with specialists. Such interaction is leading to the continued and rapid advance of integrated assessment research.

Introduction to approaches to estimating future climate change

An important approach to assessment of the risks of climate change relies on estimates of future climate based on a variety of methods including: simulation of the climate system; analysis of the sensitivity of climate system simulations to model parameters, parameterizations and models of climate system; and model-based statistical estimation constrained by a variety of historical data. The results of each of these methods are contingent on assumptions. Increasingly sophisticated approaches are being applied, and the implications, or even enumeration, of assumptions is becoming increasingly complex. This chapter gives an overview of the progression of methods used to estimate change in the future climate system and the climate sensitivity parameter. Model-based statistical estimation has the potential of synthesizing information from emerging climate data with models of the climate system to arrive at probabilistic estimates, provided all important uncertain factors can be addressed. A catalog of uncertain factors is proposed including consideration of their importance in affecting climate change estimates. Addressing and accounting for factors in the catalog, beginning with the most important and tractable, is suggested as an orderly way of improving estimates of future climate change.

A variety of methods have been used to generate projections or estimates. The simulations of state-of-the-art models discussed in Section 4.2 are often used as scenarios (plausible representations) of how climate might change in the future (Mearns et al., 2001).