TS.1 Scope, approach and method of the Working Group II assessment
The decision to produce a Fourth Assessment Report (AR4) was taken by the 19th Session of the Intergovernmental Panel on Climate Change (IPCC) in April 2002.
The Working Group II Report has twenty chapters. The core chapters (3 – 16) address the future impacts of climate change on sectors and regions, the potential for adaptation and the implications for sustainability. Chapter 1 looks at observed changes and Chapter 2 assesses new methodologies and the characterisation of future conditions. Chapters 17 – 20 assess responses to impacts through adaptation (17), the inter-relationships between adaptation and mitigation (18), key vulnerabilities and risks (19) and, finally, perspectives on climate change and sustainability (20).
The Working Group II Fourth Assessment, in common with all IPCC reports, has been produced through an open and peer-reviewed process. It builds upon past assessments and IPCC Special Reports, and incorporates the results of the past 5 years of climate change impacts, adaptation and vulnerability research. Each chapter presents a balanced assessment of the literature which has appeared since the Third Assessment Report (TAR), including non-English language and, where appropriate, ‘grey’ literature.
This Assessment aims to describe current knowledge of climate-change impacts, adaptation and vulnerability. Specifically it addresses five questions:
- What is the current knowledge about impacts of climate change which are observable now? (addressed in Section TS.2 of the Technical Summary)
- What new scenarios and research methods have led to improvements in knowledge since the Third Assessment? (addressed in Section TS.3)
- What is the current knowledge about future effects of climate change on different sectors and regions? (addressed in Section TS.4)
- What is the current knowledge about adaptation, the interaction between adaptation and mitigation, key vulnerabilities, and the role of sustainable development in the context of climate change? (addressed in Section TS.5)
- What gaps exist in current knowledge and how best can these be filled? (addressed in Section TS.6).
Each of the twenty chapters of the Working Group II Fourth Assessment had a minimum of two Coordinating Lead Authors, six Lead Authors and two Review Editors. The writing team and review editors were appointed by the IPCC Bureau on the recommendation of the Working Group II Co-Chairs and Vice-Chairs. They were selected from the pool of nominated experts, in consultation with the international community of scientists active in the field, and taking into consideration expertise and experience. In total, the Working Group II Fourth Assessment involved 48 Coordinating Lead Authors, 125 Lead Authors and 45 Review Editors, drawn from 70 countries. In addition there were 183 Contributing Authors and 910 Expert Reviewers.
This Technical Summary is intended to capture the most important scientific aspects of the full Working Group II Assessment. Reducing the information from 800 pages to 50 requires much condensing; consequently every statement in the Summary appears with its source in the Assessment, enabling the reader to pursue more detail. Sourcing information is provided in square brackets in the text (see Box TS.1). Uncertainty information is provided in parentheses (see Box TS.2 for definitions of uncertainty). Key terms are defined in Box TS.3.
Box TS.1. Sourcing in the Technical Summary
For example, source [3.3.2] refers to Chapter 3, Section 3, Sub-section 2. In the sourcing, F = Figure, T = Table, B = Box, ES = Executive Summary.
References to the Working Group I Fourth Assessment are shown as, for example, [WGI AR4 SPM] which refers to the Working Group I Fourth Assessment Summary for Policymakers, [WGI AR4 10.3.2] which refers to Chapter 10 Section 10.3.2, and [WGI AR4 Chapter 10] when the whole chapter is referred to. Where a source refers to both the WGI and WGII Fourth Assessments, these are separated by a semi-colon, for example [WGI AR4 10.2.1; 2.1.4]. References to Working Group III are treated in the same way.
Box TS.2. Communication of uncertainty in the Working Group II Fourth Assessment
A set of terms to describe uncertainties in current knowledge is common to all parts of the IPCC Fourth Assessment, based on the Guidance Notes for Lead Authors of the IPCC Fourth Assessment Report on Addressing Uncertainties, produced by the IPCC in July 2005.
Description of confidence
On the basis of a comprehensive reading of the literature and their expert judgement, authors have assigned a confidence level to the major statements in the Technical Summary on the basis of their assessment of current knowledge, as follows:
| | Terminology | Degree of confidence in being correct |
| | Very high confidence | At least 9 out of 10 chance of being correct |
| | High confidence | About 8 out of 10 chance |
| | Medium confidence | About 5 out of 10 chance |
| | Low confidence | About 2 out of 10 chance |
| | Very low confidence | Less than a 1 out of 10 chance |
Description of likelihood
Likelihood refers to a probabilistic assessment of some well-defined outcome having occurred or occurring in the future, and may be based on quantitative analysis or an elicitation of expert views. In the Technical Summary, when authors evaluate the likelihood of certain outcomes, the associated meanings are:
| | Virtually certain | >99% probability of occurrence |
| | Very likely | 90 to 99% probability |
| | Likely | 66 to 90% probability |
| | About as likely as not | 33 to 66% probability |
| | Unlikely | 10 to 33% probability |
| | Very unlikely | 1 to 10% probability |
| | Exceptionally unlikely | <1% probability |
Box TS.3. Definitions of key terms
Climate change in IPCC usage refers to any change in climate over time, whether due to natural variability or as a result of human activity. This usage differs from that in the Framework Convention on Climate Change, where climate change refers to a change of climate that is attributed directly or indirectly to human activity that alters the composition of the global atmosphere and that is in addition to natural climate variability observed over comparable time periods.
Adaptation is the adjustment in natural or human systems in response to actual or expected climatic stimuli or their effects, which moderates harm or exploits beneficial opportunities.
Vulnerability is the degree to which a system is susceptible to, and unable to cope with, adverse effects of climate change, including climate variability and extremes. Vulnerability is a function of the character, magnitude and rate of climate change and the variation to which a system is exposed, its sensitivity and its adaptive capacity.