Past societies have reacted when they understood that their own activities were causing deleterious environmental change by controlling or modifying the offending activities. The scientific evidence has now become overwhelming that human activities, especially the combustion of fossil fuels, are influencing the climate in ways that threaten the well-being and continued development of human society. If humanity is to learn from history and to limit these threats, the time has come for stronger control of the human activities that are changing the fundamental conditions for life on Earth.

To decide on effective control measures, an understanding of how human activities are changing the climate, and of the implications of unchecked climate change, needs to be widespread among world and national leaders, as well as in the public. The purpose of this report is to provide, for a broad range of audiences, an update of the newest understanding of climate change caused by human activities, the social and environmental implications of this change, and the options available for society to respond to the challenges posed by climate change.

This understanding is communicated through six key messages. 

The United Nations Framework Convention on Climate Change (UNFCCC) meeting to be held in Copenhagen in December 2009 (the 15th Conference of the Parties, COP-15) will be a critical step in developing a global response to the threat of climate change caused by human activities. The primary scientific input to those negotiations is the Fourth Assessment Report of the Intergovernmental Panel on Climate Change (IPCC), published in 2007. The IPCC report has already been instrumental in increasing both public and political awareness of the societal risks associated with unchecked emission of greenhouse gases.
Since the production of the IPCC report, new knowledge has emerged that furthers understanding of the impacts of human influence on the climate and the response options and approaches that are available to tackle this complex issue. To bring this new knowledge together, the International Alliance of Research Universitiesi organised an international scientific congress on climate change, Climate Change: Global Risks, Challenges and Decisions, which was held in Copenhagen from 10-12 March 2009. Participation in the Congress was open to all. Most of the approximately
2500 people attending the Congress were researchers, many of whom have also been contributors to the IPCC reports. Participants came from nearly 80 different countries and contributed with more than 1400
scientific presentations. Abstracts for all of the scientific presentations made can be found at www.iop.org/EJ/volume/1755-1315/6, and a transcript of the closing plenary session can be found at
environmentalresearchweb.org/cws/article/opinion/39126.

This synthesis report presents an up-to-date overview of a broad range of research relevant to climate change - including fundamental climate science, the impacts of a changing climate on society and environment, and the many tools and approaches available to deal effectively with the challenge of climate change. The report has been produced by a writing team comprised of members of the Scientific Steering Committee for the IARU Congress and individuals invited to give the writing team academic and geographic breadth. It is based on the 16 plenary talks given at the Congress as well as input from over 80 chairs and cochairs of the 58 parallel sessions held at the Congress. The names of the plenary speakers and the chairs and co-chairs of the parallel sessions can be found on the inside cover of this volume. The writing team has, in addition to presentations at the Congress, drawn upon recent publications in the scientific literature to create this synthesis. This report has been critically reviewed by representatives of the Earth System Science Partnership (ESSP), by the parallel session chairs and co-chairs, and by up to four independent researchers from each IARU university. This extensive review process has been implemented to ensure that the messages contained in the report are solidly and accurately based on the new research produced since the last IPCC Report, and that they faithfully reflect the most recent work of the international climate change research community.