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Global Warming: Earth evolution or man made? The case for being man made. Nathan Bindoff ACECRC, IASOS, CSIRO MAR University of Tasmania TPAC.

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Presentation on theme: "Global Warming: Earth evolution or man made? The case for being man made. Nathan Bindoff ACECRC, IASOS, CSIRO MAR University of Tasmania TPAC."— Presentation transcript:

1 Global Warming: Earth evolution or man made? The case for being man made. Nathan Bindoff ACECRC, IASOS, CSIRO MAR University of Tasmania TPAC

2 Projected impacts of climate change 1°C2°C5°C4°C3°C Sea level rise threatens major cities Falling crop yields in many areas, particularly developing regions Food Water Ecosystems Risk of Abrupt and Major Irreversible Changes Global temperature change (relative to pre-industrial) 0°C Falling yields in many developed regions Rising number of species face extinction Increasing risk of dangerous feedbacks and abrupt, large-scale shifts in the climate system Significant decreases in water availability in many areas, including Mediterranean and Southern Africa Small mountain glaciers disappear – water supplies threatened in several areas Extensive Damage to Coral Reefs Extreme Weather Events Rising intensity of storms, forest fires, droughts, flooding and heat waves Possible rising yields in some high latitude regions Stern report (2006) Why the concern about climate change?

3 Changing Atmosphere

4 Radiative change 1750-2005 Role of aerosols

5 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

6 Global mean temperatures compared with past (NH) Very likely that last 50 years was warmer than any period in last 500 years

7 Sea level is rising in 20 th century Rates of sea level rise: 1.8 + 0.5 mm yr -1, 1961-2003 1.7 + 0.5 mm yr-1, 20 th Century 3.1 + 0.7 mm yr -1, 1993-2003 SPM-3b

8 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….”

9 Climate models, essential to hypothesis testing Observations 1980-2000 Mean Model 1980-2000

10 What is attribution? Anthropogenic greenhouse gas increases very likely caused most of the observed warming since mid-20 th century extremely unlikely due to natural variation Observations Solar + volcanic TS-23 Attribution to man All forcing = GHG + Aerosols + solar + volcanic

11 Continental warming likely shows a significant anthropogenic contribution over the past 50 years Observations All forcingnatural forcing SPM-4

12 Affirmative case has shown –many strands of evidence for climate change over a broad range of variables, that are consistent with conceptual/qualitative models of climate change –that known man induced changes in atmosphere are very likely to explain global and continental surface temperature records for the last century –it is extremely unlikely that solar variations can explain global mean observations –The above results are quantitative, formally rigorous, and robust, combining both observations and physical models –There are many more formal attribution and detection studies for other climate variables (precipitation, ocean warming) that support the above case. Summary for the affirmative case

13 125,000 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 A paleoclimate perspective

14 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

15 “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) I am not the only one, IPCC WGI “Headlines”

16 Emission Scenarios High – fossil fuel intensive future Low – greater alternative energy sources Mix of economics, technology change. Do not include mitigation Aerosols HighLowMedium

17 Role of volcanoes on radiative forcing

18 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 High Emissions Low Emissions

19 Precipitation increases are very likely in high latitudes in 2090-2099 Decreases are likely in most subtropical land regions in 2090-2099 Figure SPM-6, TS-30, 10.9 Projections of Future Changes in Climate

20 Scenarios for sea-ice North. Hem. Summer South. Hem. Winter

21 Scenarios for sea-ice Antarctic Winter Arctic Summer 1980-2000 2080-2100

22 Other observed changes in the climate system

23 Future Climate: Greenland Ice Sheets 19003660217026103030 =1.4m Ice Sheets: a key risk for future climate

24 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) Projections of Future Climate: Ice Sheets

25 Scenarios for Extremes- frost, heat waves, growth

26 Past successes –Montreal Protocol (1987,1989) –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 Causes for optimism

27 Global atmospheric CO 2 budget Pep Canadell, Corinne LeQuere, Mike Raupach, Gregg Marland, Skee Houghton, Tom Conway, Philippe Ciais Atmospheric accumulation = F Foss + F LUC + F LandAir + F OceanAir

28 Projections of Future Changes in Climate

29 The IPCC is a “remarkable example” of mobilizing expert analysis to inform policymakers Jeffrey Sachs (Nature, 12 August 2004) The IPCC assessments are “dull as dishwater” Tim Flannery, The Weather Makers

30 The structure of the IPCC for the Fourth Assessment Report Co-chairs: Susan Solomon, USA Dahe Qin, China

31 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 21 st century. Ch. 10, Fig. 10.15

32 Glacier contribution to sea-level since 1961 Increased glacier retreat since the early nineties Mass loss from glaciers and ice caps: 0.5 ± 0.18 mm yr -1, 1961- 2003 0.77 ± 0.22 mm yr -1, 1991- 2003

33 Ice sheet contributions to sea level rise Antarctic ice sheet loses mass mostly through increased glacier flow Greenland mass loss is increasing Loss: glacier discharge, melting Mass loss of Greenland: 0.05 ± 0.12 mm yr -1 SLE, 1961-2003 0.21 ± 0.07 mm yr -1 SLE, 1991-2003 Mass loss of Antarctica: 0.14 ± 0.41 mm yr -1 SLE, 1961-2003 0.21 ± 0.35 mm yr -1 SLE, 1991-2003

34 Attribution are observed changes consistent with expected responses to forcings  inconsistent with alternative explanations Observations All forcing = GHG + Aerosols + solar + volcanic Solar+volcanic TS-23

35 Climate models, and climate model credibility Observations 1980-2000 Mean Model 1980-2000

36 Heat content change: time evolution Key points for 1961- 2003: consistency of products oceans absorbed 0.21 ± 0.04 W m –2 (0-3000m) over the earth’s surface. 70% of this energy is absorbed in top 700 m 0.1°C warming (0-700m) 1993-2003 has higher rates of warming (0.50 ± 0.18 W m –2 ) decadal variability, cooling since 2003

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