First, scientists assess the state of knowledge about climate change and summarize it. Then government representatives from countries all over the world work with the scientists to simplify and condense the material so that it can be useful for policymakers. This is how all the countries in the world come to a shared understanding of the climate problem that is based on scientific knowledge.
The IPCC’s reports form the basis for negotiations on international agreements such as the UNFCCC (the United Nations Framework Convention on Climate Change) and the Kyoto Protocol (seenext unit).
Roughly every five years the IPCC releases an assessment report with contributions from several different working groups. The third and as of this date most recent report, the Third Assessment Report (TAR), was published in 2001. TAR puts a great deal of emphasis on explaining how certain each of its conclusions is, and whether scientists agree or disagree. Most of the conclusions are ranked on a scale of how certain they are. The scale goes from “virtually certain” to “uncertain.”
The IPCC concludes that the global average surface temperature has increased by 0.6 °C over the last hundred years, and that it is “likely” that people have caused most of the warming over the last 30-40 years by emitting greenhouse gases. They conclude that we are now more certain than ever that human activity contributes significantly to global warming.