This Report, the third IPCC Working Group I Assessment Report since 1990, assesses the state of scientific understanding of the climate system and its variability and change, in particular human-induced climate change. This section provides a ‘road map’ to the 14 chapters of this report and the major issues they are designed to address. Each chapter provides an initial summary of the Working Group I Second Assessment Report (IPCC, 1996) and then goes on to emphasise the progress made since then. The chapters can be viewed as covering the following three broad areas: past changes and the factors that can force climate change (Chapters 2 to 6), our present understanding and ability to model the climate system (Chapters 7, 8 and 14) and possible future climate change (Chapters 9 to 13).

In order to understand, assess and quantify the possible human influence on climate, an analysis of past climate variability and change is required (Chapter 2). The chapter tackles such questions as: how much is the world warming and is the recent global warming unusual? It looks in detail at trends and variability during the recent instrumental period (the last 100 years or so) and draws on palaeo-data to put them into the context of climate over much longer periods.

There are many factors that are known to influence climate, both natural and human-induced. The increase in concentrations of greenhouse gases and aerosols through human activity is of particular concern. Chapters 3 to 5 examine how well the three most important human contributions to the changing composition of the atmosphere; carbon dioxide, other greenhouse gases and aerosols, are understood, including the physical, chemical and biological processes which determine the atmospheric concentrations of these components. The next step, taken in Chapter 6, is to evaluate how this change in atmospheric composition has affected radiative forcing within the context of other factors such as land-use change, volcanic eruptions and solar variations.

Understanding the climate response to these various radiative forcings and projecting how they could affect future climate requires an understanding of the physical processes and feedbacks in the climate system and an ability to model them (Chapter 7). The only tools available for such projections of future climate are numerical models of the climate system of various complexity. An evaluation of such models against observations of the present and past climate and model intercomparisons provide the basis for confidence in such tools (Chapter 8).

Climate models together with scenarios of future emissions of radiatively active atmospheric components, as for example the SRES scenarios (Nakic´enovic´ et al., 2000), recently developed by IPCC specifically for this purpose, are used to project future climate change. State-of-the-art projections for the next 100 years are assessed in Chapter 9, mainly at a global level, but also including large-scale patterns, their spatial and temporal variability and extreme events. Partly in response to the need for more details of climate change at a regional level, research in this area has been particularly active over the last 5 years. A new chapter, compared to previous assessments, has been included which examines the various techniques available to derive regional climate projections and, as far as is currently possible, assesses regional climate change information (Chapter 10). Chapter 11 assesses the current state of knowledge of the rate of change of global average and regional sea level in response to climate change.

A key conclusion from the SAR was that “the balance of evidence suggests that there is a discernible human influence on global climate”. Chapter 12 assesses research over the last 5 years on the detection and attribution of climate change drawing on the developments in observational research (Chapters 2 to 6) and modelling (Chapters 7 to 10) to consider how this conclusion has changed.

Data derived directly from projections with climate models are often inappropriate for assessing the impacts of climate change which can require detailed, regional or local information as well as observational data describing current (or baseline) climate. Climate change scenarios are plausible representations of future climate constructed explicitly for impact assessment and form a key link between IPCC Working Groups I and II. For the first time, Working Group I have included a chapter dedicated to climate scenarios (Chapter 13) - this is intended to provide an assessment of scenario generation techniques, rather than to present scenarios themselves.

All chapters of the report highlight areas of certainty and uncertainty, and gaps in current knowledge. Chapter 14 draws together this information to present key areas that need to be addressed to advance understanding and reduce uncertainty in the science of climate change.

A comprehensive and integrated summary of all results of this assessment report may be found in the Technical Summary in this volume. A brief summary highlighting points of particular policy relevance is presented in the Summary for Policymakers.

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