Understanding and Attributing Climate Change

• Climate change detection and attribution
• Detection - determinating that observed change highly unlikely to have been caused by internal climate variability
• Attribution - establishing the most likely causes of detected change
• External forcing mechanisms, both natural and anthropogenic, considered as potential causes of climate change
• Natural
• Solar variability
• Volcanic activity
• Anthropogenic
• Greenhouse gas increases
• Tropospheric and stratospheric ozone changes
• Sulphate aerosol changes
• Other aerosol changes
• Land cover changes
• Estimates of internal variability usually obtained from long coupled model control runs
• Relative importance of various forcing mechanisms estimated by differences in spatial and temporal patterns (fingerprints)
• Substantial differences in latitudinal/vertical patterns of temperature change due to different forcings
• Impacts of fossil fuel black carbon + organic matter + biomass burning concentrated in Northern Hemisphere
• Volcanic eruptions increase planetary albedo and stratospheric absorption
• Differences in temporal trends in the two hemispheres could indicate varying importance of aerosol and greenhouse gases
• Understanding past climate change
• Pre-industrial climate change
• Cooler conditions during last glacial maximum (21 kyr) related to increased albedo and decreased carbon dioxide
• Warmer, wetter conditions during mid-Holocene optimum (8 kyr) related to higher amplitude annual cycle
• Climate variations over last 1,000 years of pre-industrial period probably related to volcanic activity and solar variability
• Twentieth Century climate change
• Very unlikely that observed increase in global average temperature due to natural forcing
• Spatial patterns of warming trend also unlikely to be due to natural forcing
• Large positive change from greenhouse gases partially offset by negative impacts of other anthropogenic forcings
• Anthropogenic forcing leads to increased predictability in late 20th century
• Some ability to detect anthropogenic warming signal on sub-continental scales
• Summer 2003 European heat wave
• As much as 5 standard deviations above mean
• Anthropogenic warming increases the risk by 4 times
• Increasing trend in tropopause height can't be explained by natural forcings
• Detection ability improves for larger areas or tropical regions with low natural variability
• Estimates of climate sensitivity
• Equilibrium climate sensitivity
• Equilibrium global temperature change in response to CO₂ doubling
• Modes of pdfs generally range from about 1.2^oC to 4^oC
• Median around 3^oC - 3.5^oC
• Very likely (>95%) to be above 1^oC, but upper end not well determined
• Transient climate response
• Response of global average temperature to CO₂ increasing at rate of 1%/yr at the time of doubling
• Model results show 5%-95% range of 1.5^oC to 2.8^oC
• Median around 2^oC
• Overall evidence of anthropogenic climate change
• Good confidence for surface and atmospheric temperature
• Continued
• Pretty good for ocean and cryosphere, less so for hydrologic cycle
• Continued