Greenhouse gases and emissions resulting from human activity

Every few years, leading climate scientists at the UN’s Intergovernmental Panel on Climate Change (IPCC) have released major, definitive reports detailing the progress in understanding climate change. From the outset they have recommended that there be emission reductions. This body is comprised of hundreds of climate scientists around the world.
At the beginning of January 2007, the IPCC’s fourth major report summarized that they were even more certain than before of human-induced climate change because of better scientific understanding:
Global atmospheric concentrations of carbon dioxide, methane and nitrous oxide have increased markedly as a result of human activities since 1750 and now far exceed pre-industrial values determined from ice cores spanning many thousands of years. The global increases in carbon dioxide concentration are due primarily to fossil fuel use and land-use change, while those of methane and nitrous oxide are primarily due to agriculture.
… The understanding of anthropogenic warming and cooling influences on climate has improved since the Third Assessment Report (TAR), leading to very high confidence that the globally averaged net effect of human activities since 1750 has been one of warming.
Most of the observed increase in globally averaged temperatures since the mid-20th century is very likely due to the observed increase in anthropogenic greenhouse gas concentrations.
— Climate Change 2007: The Physical Science Basis; Summary for Policymakers , IPCC, February 5th, 2007 [emphasis is original]
Their definition of “very high confidence” and “very likely” is a 90% chance of being correct. (Their 2001 report claimed a 66% certainty.)
This report was produced by some 600 authors from 40 countries. Over 620 expert reviewers and a large number of government reviewers also participated, according to the IPCC’s media advisory.
As Inter Press Service notes, although the IPCC has become the “gold standard” for global scientific collaboration, their reports are inherently conservative:
The IPCC operates under the World Meteorological Organisation (WMO) and the United Nations Environment Programme (UNEP) and does not fund any research itself. It collects, evaluates and synthesises scientific data. Any U.N. country can be a member of the IPCC and can challenge the findings in its reports. And consensus is required for every word in the “Summary for Policy Makers” section included in each report.
It’s an inherently conservative process, with oil-rich countries like Kuwait and Saudi Arabia always trying to tone down the conclusions and emphasise uncertainties and unknowns, said Weaver.