National and international science academies and scientific societies have assessed current scientific opinion on climate change. These assessments are generally consistent with the conclusions of the Intergovernmental Panel on Climate Change (IPCC), summarized below: 1. Warming of the climate system is unequivocal, as evidenced by increases in global average air and ocean temperatures, the widespread melting of snow and ice, and rising global average sea level. 2. Most of the global warming since the mid-20th century is very likely due to human activities. 3. No scientific body of national or international standing maintains a formal opinion dissenting from any of these main points.