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Evidence for climate Change The Working Group I Report of the Intergovernmental Panel on Climate Change Fourth Assessment Report Nathan Bindoff and others ACECRC, IASOS, CSIRO MAR University of Tasmania TPAC Royal Society of Tasmania Why the concern about climate change? 0°C Food Water Global temperature change (relative to pre-industrial) 1°C 2°C 3°C 4°C 5°C Projected impacts of climate change Falling crop yields in many areas, particularly developing regions Falling yields in many Possible rising yields in developed regions some high latitude regions Small mountain glaciers disappear – water supplies threatened in several areas Significant decreases in water availability in many areas, including Mediterranean and Southern Africa Sea level rise threatens major cities Ecosystems Extensive Damage to Coral Reefs Rising number of species face extinction Extreme Rising intensity of storms, forest fires, droughts, flooding and heat waves Weather Events Risk of Abrupt and Increasing risk of dangerous feedbacks and Major Irreversible abrupt, large-scale shifts in the climate system Changes Stern report (2006) Changing Atmosphere Radiative change 1750-2005 • Role of aerosols Global mean temperatures are rising faster with time Warmest 12 years: 1998,2005,2003,2002,2004,2006, 2001,1997,1995,1999,1990,2000 SPM-3a Sea level is rising in 20th century SPM-3b Rates of sea level rise: •1.8 + 0.5 mm yr-1, 1961-2003 •1.7 + 0.5 mm yr-1, 20th Century Other evidence from observations •Oceans have warmed •Oceans becoming more acidic •Patterns of rainfall/evaporation are changing •Evidence over both land and oceans •Droughts are more frequent •Extremes events are changing •More warm nights •More storm surges •Strengthening westerlies • Reduced snow, shrinking Arctic Sea-Ice •Melting Glaciers, melting Greenland ice sheet, mass loss from Antarctica “……evidence for climate change is unequivocal….” Climate models, essential to hypothesis testing Observations 1980-2000 Mean Model 1980-2000 Attribution to man • What is attribution? • Anthropogenic greenhouse gas increases very likely caused most of the observed warming since mid-20th century • extremely unlikely due to natural variation All forcing = GHG + Aerosols + solar + volcanic Observations Solar + volcanic TS-23 Continental warming SPM-4 Observations All forcing natural forcing likely shows a significant anthropogenic contribution over the past 50 years Scenarios of future change Projections of Future Changes in Climate Best estimate for low scenario (B1) is 1.8°C (likely range is 1.1°C to 2.9°C), and for high scenario (A1FI) is 4.0°C (likely range is 2.4°C to 6.4°C). Broadly consistent with span quoted for SRES in TAR, but not directly comparable Projections of Future Changes in Climate Low Emissions High Emissions • Spatial patterns: greater warming over land, greater warming at high latitudes • Albedo changes in high latitudes, less snow and sea-ice. Figure SPM-5, TS-28, 10.8, 10.28 Projections of Future Changes in Climate Figure SPM-6, TS-30, 10.9 • Precipitation increases are very likely in high latitudes in 2090-2099 • Decreases are likely in most subtropical land regions in 2090-2099 Scenarios for sea-ice North. Hem. Summer South. Hem. Winter Future Climate: Greenland Ice Sheets 1900 2170 =1.4m Ice Sheets: a key risk for future climate 2610 3030 3660 Projections of Future Climate: Ice Sheets Post 2100 changes, Greenland: • “…..and that the surface mass balance becomes negative at a global average warming (relative to 1961-1990) in excess of 1.2 to 3.9°C. If a negative surface mass balance were sustained for millennia, that would lead to virtually complete elimination of the Greenland ice sheet and a resulting contribution to sea level rise of about 7 m.” Almost all marker scenarios exceed 1.2 to 3.9 °C tipping points. • “.. If radiative forcing were to be stabilized in 2100 at A1B levels11, thermal expansion alone would lead to 0.3 to 0.8 m of sea level rise by 2300 (relative to 1980–1999). “ Implication, while not stated, is that there will be large sea level changes beyond 2100 (eg by 2300 something like 1.5 to 3.5m) Scenarios for Extremes- frost, heat waves, growth The IPCC WGI “Headlines” • “The balance of evidence suggests a discernible human influence on global climate.” (SAR, 1995) • “There is new and stronger evidence that most of the warming observed over the last 50 years is attributable to human activities.” (TAR, 2001) • “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.” (AR4, 2007) • “Discernible human influences now extend to other aspects of climate, including ocean warming, continental-average temperatures, temperature extremes and wind patterns.” (AR4, 2007) Causes for optimism • Past successes – Montreal Protocol (1988) – Pollution controls for Sulphur emissions (acid rain) • There are alternatives – Problem is the number of choices and which is best? • Important to act sooner than later – Already locking in future change – Harder to mitigate or adapt to dangerous change – Important to act gradually – Its cheap Global mean temperatures compared with past (NH) Very likely that last 50 years was warmer than any period in last 500 years Emission Scenarios Medium High Low •High – fossil fuel intensive future •Low – greater alternative energy sources •Mix of economics, technology change. •Do not include mitigation •Aerosols Scenarios for sea-ice 1980-2000 Arctic Summer Antarctic Winter 2080-2100 Ch. 10, Fig. 10.15 Very likely that the Atlantic meridional overturning circulation (MOC) will slow down over the course of the 21st century. Very unlikely that the MOC will undergo a large abrupt transition during the 21st century. Longer-term changes in the MOC cannot be assessed with confidence Studies with additional fresh water from melting of the Greenland Ice Sheet suggest that this will not lead to a complete MOC shutdown in the 21st century. Ice sheet contributions to sea level rise Mass loss of Greenland: • 0.05 ± 0.12 mm yr-1 SLE, 1961-2003 • 0.21 ± 0.07 mm yr-1 SLE, 1991-2003 Antarctic ice sheet loses mass mostly through increased glacier flow Greenland mass loss is increasing Loss: glacier discharge, melting Mass loss of Antarctica: • 0.14 ± 0.41 mm yr-1 SLE, 1961-2003 • 0.21 ± 0.35 mm yr-1 SLE, 1991-2003 A paleoclimate perspective 125,000 years ago, higher Arctic temperatures likely resulted in sea level 4-6m above present - contributions may have come from both Arctic Ice Fields (especially Greenland) and Antarctica Simulated and observed Arctic warming at 125,000 yr B.P. Estimated reduction in Greenland Ice Sheet Area and Thickness Drought is increasing most places The most important spatial pattern (top) of the monthly Palmer Drought Severity Index (PDSI) for 1900 to 2002. The time series (below) accounts for most of the trend in PDSI. Example from South West Australia Tasmania Water Catchment Models AWBM Assumption: no change in land use Assumption: no change in land use Example: Hydro Tasmania Inflow Prediction Most important lake 1.7 1.6 1.5 1.4 1.3 1.2 1.1 1.0 0.9 Factor of 1.0 represents no change in inflows Arthur's Lake Great Lake Trevallyn Dam Factors <1.0 represents drying 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 Factors >1.0 represents wetter Jan 1.7 1.6 1.5 1.4 1.3 1.2 1.1 1.0 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 Feb Mar Apr May Jun Jul Aug Sep Oct Nov Lake Burbury Lake Mackintosh Feb Mar Apr Great Lake factors well below 1.0 and thus drying predicted Others have drier Summers/Autumns and wetter Winters Lake Rowallan Jan Dec May Jun Jul Aug Sep Oct Nov Dec