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Climate Change 2007: The Physical Science Basis Summary for Policymakers: Overview 5/24/2017 1 Climate Change 2007: The Physical Science Basis What is the “Physical Science Basis” report? Summary of existing scientific research on climate change. First part of 2007 IPCC Fourth Assessment Report. Describes progress in understanding the human and natural drivers of climate change, observed climate change, climate processes and attribution, and estimates of projected future climate change. Builds upon past IPCC assessments (including the TAR, Third Assessment Report, 2001) Includes large amounts of new data, more sophisticated data analyses, better understanding of processes and their simulation using models 5/24/2017 2 Climate Change 2007: The Physical Science Basis Section 1: Human and Natural Drivers of Climate Change – Findings 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. TERMS: pre-industrial 5/24/2017 3 Climate Change 2007: The Physical Science Basis 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 TERMS: carbon dioxide, methane, nitrous oxide 5/24/2017 4 Climate Change 2007: The Physical Science Basis The understanding of anthropogenic warming and cooling influences on climate has improved since the TAR, leading to very high confidence that the global average net effect of human activities since 1750 has been one of warming, with a radiative forcing of +1.6 [+0.6 to +2.4] W m-2. TERMS: TAR, anthropogenic, confidence levels, radiative forcing, W m-2 5/24/2017 5 Climate Change 2007: The Physical Science Basis The combined radiative forcing due to increases in carbon dioxide, methane, and nitrous oxide is +2.30 W m-2 Its rate of increase during the industrial era is very likely to have been uprecedented in more than 10,000 years. The carbon dioxide radiative forcing increased by 20% from 1995 to 2005, the largest change for any decade in the at least the last 200 years. 5/24/2017 6 Climate Change 2007: The Physical Science Basis Section 2: Direct Observations of Recent Climate Change – Findings Warming of the climate system is unequivocal, as is now evident from observations of increases in global average air and ocean temperatures, widespread melting of snow and ice, and rising global average sea level. Eleven of the last twelve years (1995-2006) rank among the 12 warmest years in the instrumental record of global surface temperature (since 1850). 5/24/2017 7 Climate Change 2007: The Physical Science Basis Observations since 1961 show that the average temperature of the global ocean has increased to depths of at least 3000 m and that the ocean has been absorbing more than 80% of the heat added to the climate system. Such warming causes seawater to expand, contributing to sea level rise. Mountain glaciers and snow cover have declined on aveage in both hemispheres and have contributed to sea level rise. Losses from the ice sheets of Greenland and Antarctica have very likely contributed to sea level rise over 1993 to 2003. 5/24/2017 8 Climate Change 2007: The Physical Science Basis Global average sea level rose at an average rate of of 1.8 [1.3 to 2.3] mm per year over 1961 to 2003. The rate was faster over 1993 to 2003: about 3.1 [2.4 to 3.8] mm per year. The total 20th century rise is estimated to be 0.17 [0.12 to 0.22] m. 5/24/2017 9 Climate Change 2007: The Physical Science Basis At continental, regional, and ocean basin scales, numerous long-term changes in climate have been observed. These include changes in arctic temperatures and ice, widespread changes in precipitation amounts, ocean salinity, wind patterns and aspects of extreme weather including droughts, heavy precipitation, heat waves, and the intensity of tropical cyclones. 5/24/2017 10 Climate Change 2007: The Physical Science Basis Section 3: A Palaeoclimatic Perspective Palaeoclimatic information supports the interpretation that the warmth of the last half century is unusual in at least the previous 1,300 years. The last time the polar regions were significantly warmer than present for an extended period (about 125,000 years ago), reductions in polar ice volume led to 4 to 6 m of sea level rise. 5/24/2017 11 Climate Change 2007: The Physical Science Basis Section 4: Understanding and Attributing Climate Change Most of the observed increase in global average temperatures since the mid-20th century is very likely due to the observed increase in anthropogenic greenhouse gas concentrations. This is an advance since the TAR’s conclusion that “most of the observed warming over the last 50 years is likely to have been due to the increase in greenhouse gas concentrations.” Discernible human influences now extend to other aspects of climate, including ocean warming, continental-average temperatures, temperature extremes, and wind patterns. 5/24/2017 12 Climate Change 2007: The Physical Science Basis Section 5: Projections of Future Changes in Climate For the next two decades, a warming of about 0.2°C (0.36°F) per decade is projected for a range of SRES emission scenarios. Even if the concentrations of all greenhouse gases and aerosols had been kept constant at year 2000 levels, a further warming of about 0.1°C (0.18°F) per decade would be expected. 5/24/2017 13 Climate Change 2007: The Physical Science Basis Continued greenhouse gas emissions at or above current rates would cause further warming and induce many changes in the global climate system during the 21st century that would very likely be larger than those observed during the 20th century. 5/24/2017 14 Climate Change 2007: The Physical Science Basis Anthropogenic warming and sea level rise would continue for centuries due to the time scales associated with climate processes and feedbacks, even if greenhouse gas concentrations were to be stabilised. 5/24/2017 15