Global Warming - 1 An Assessment The balance of the evidence... PowerPoint 97 PowerPoint 97 To download: Shift LeftClick Please respect copyright on this material

Reaction Topic F Greenhouse gases warm the earth, increasing evaporation. F Do you think changes in cloudiness will cool or warm the earth? What is your reasoning?

Key Points F Scientific assessment describes present state of knowledge F Global temperature has increased during 20 century, continuing through the 1990s – beyond reasonable doubt ( virtually certain) F Some of this increase is due to human activities – the balance of the evidence (likely) F Impacts are complex, vary with region, very difficult to quantify reliably F Any action to limit future increases will take decades to become effective – certain (virtually certain)

Scientific Assessments F There are accepted standards of what constitutes scientific evidence F Review and discussion by knowledgeable peers establishes consensus – What is thoroughly established – What is plausible, but definitive evidence lacking – What is poorly understood or legitimately controversial F Scientists are paid to be skeptical. Some will always disagree

The Intergovernmental Panel on Climate Change F IPCC95 Cover

Prediction IPPC 1990 Report F Latest predictions show less warming F Uncertainties were underestimated F IPCC90 Fig 8

Notes on Science Assessment F Current consensus on attribution – Balance of the evidence F Regional predictions much less secure than global average F Precipitation predictions much less secure than temperature F Impacts on ecosystems, agriculture, etc only in general terms

Judgements of Confidence IPCC Third Assessment Report-2001 Summary for policymakers - Footnote 7 Summary for policymakers - Footnote 7 Summary for policymakers - Footnote 7 F Statement –Virtually Certain –Very Likely –Likely –Medium Likelihood –Unlikely –Very unlikely –Exceptionally unlikely F Chance True – > 99% – 90-99% – 60-90% – 33-66% – 10-33% – 1-10% – < 1%

Key Points F Scientific assessment describes present state of knowledge F Global temperature has increased during 20 century, continuing through the 1990s – beyond reasonable doubt ( virtually certain) F Some of this increase is due to human activities – the balance of the evidence (likely) F Impacts are complex, vary with region, very difficult to quantify reliably F Any action to limit future increases will take decades to become effective – certain (virtually certain)

Global Average Temperature F Is this a natural fluctuation or attributable to human activities? F Sept Every month for past 19 has been a record high!

Past NH Summer Temperature F tree rings, ice cores,documents F IPCC95 Fig 10

IPPC 3rd Assessment Report-1 F Global temperature increased over the 20th Century by about 0.6 o C : virtually certain F This increase is largest for any century within past 100 years: likely F Snow cover and ice extent decreased –Snow by 10% since 1960s very likely –Lakes & rivers by 2 wks since 1900 very likely –Arctic sea-ice by 40% since 1950s likely F Global average sea level rose cm (4-8 inches) during 20th century F Cloud cover increased 2% over mid-high latitude land during 20th century likely

Key Points F Scientific assessment describes present state of knowledge F Global temperature has increased during 20 century, continuing through the 1990s – beyond reasonable doubt ( virtually certain) F Some of this increase is due to human activities – the balance of the evidence (likely) F Impacts are complex, vary with region, very difficult to quantify reliably F Any action to limit future increases will take decades to become effective – certain (virtually certain)

What Do the Models Say? Partially Tuned Simulations F What happened ? F IPCC95 Fig 15

Model Tuning F Models are simplified representations of reality F They represent what are believed to be the most significant processes F Representations sometimes involve coefficients which must be empirically determined F Tuning = adjusting coefficients within plausible ranges to achieve desired model output

Is the Aerosol Effect Real? We think we understand it but could be surprised F Radiative forcing per unit sulfate aerosol uncertain a priori F Good simulations of Mt. Pinatubo – aerosol in stratosphere, not troposphere F Warming in sulfate areas only at night – observed and modeled F Better agreement for global rise for past 100 years – some tuning

After Eruption of Mt. Pinatubo F IPCC95 Fig 14

Sensitivity to Aerosols -1 for 3 values of cloud feedback F greenhouse gases only F IPCC95 Fig 16a

Sensitivity to Aerosols -2 for 3 values of cloud feedback F greenhouse gases and aerosols F IPCC95 Fig 16b

Estimated Current Forcing F IPCC 1995

Key Points F Scientific assessment describes present state of knowledge F Global temperature has increased during 20 century, continuing through the 1990s – beyond reasonable doubt ( virtually certain) F Some of this increase is due to human activities – the balance of the evidence (likely) F Impacts are complex, vary with region, very difficult to quantify reliably F Any action to limit future increases will take decades to become effective – certain (virtually certain)

Impacts of Climate Change F Regional analysis is critical F Current climate models disagree on regional changes F Changes in precipitation at least as important as changes in temperature F Impacts on ecosystems, water resources, agriculture, extreme events (floods, droughts, hurricanes,…). Some beneficial,vary with region F Other stresses use to land use, acid rain F Less wealthy countries more vulnerable F Need to build resilience to multiple stresses

Key Points F Scientific assessment describes present state of knowledge F Global temperature has increased during 20 century, continuing through the 1990s – beyond reasonable doubt ( virtually certain) F Some of this increase is due to human activities – the balance of the evidence (likely) F Impacts are complex, vary with region, very difficult to quantify reliably F Any action to limit future increases will take decades to become effective – certain (virtually certain)

IPPC 3rd Assessment Report-2 F Atmospheric CO2 –increased 31% since 1750 virtually certain –present concentrations largest in 420,000 yr and likely in 20,000,000 yr F 3/4 anthropogenic emissions CO2 due to fossil fuel burning, remainder mainly land use change F Human influences will continue to change atmospheric composition throughout the 21st Century F Global average temperatures are projected to rise under all IPPC scenarios.

Changes in Radiative Forcing Business as Usual Scenario

Projected Future Warming Intermediate cloud feedback F For high or low feedback multiply by 1.8 or 0.6 F Must examine credibility F IPCC95 Fig 18

Emissions Scenarios CO 2 Gt C /y

Concentration Scenarios CO 2 ppmv

Key Points F Scientific assessment describes present state of knowledge F Global temperature has increased during 20 century, continuing through the 1990s – beyond reasonable doubt ( virtually certain) F Some of this increase is due to human activities – the balance of the evidence (likely) F Impacts are complex, vary with region, very difficult to quantify reliably F Any action to limit future increases will take decades to become effective – certain (virtually certain)

Sources of Information F Horel and Geisler Chapter 8 F Climate Change Houghton et al Cambridge University Press F IPCC (Working Group 1) “Third Assessment Report.” Summary for Policy Makers Summary for Policy MakersSummary for Policy Makers F IPPC “Regional Impacts of Climate Change” –Introduction Introduction