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Global Warming - 1 An Assessment The balance of the evidence ... To download: Shift LeftClick PowerPoint 97 Please respect copyright on this material Reaction Topic Greenhouse gases warm the earth, increasing evaporation. Do you think changes in cloudiness will cool or warm the earth? What is your reasoning? Key Points Scientific assessment describes present state of knowledge Global temperature has increased during 20 century, continuing through the 1990s – beyond reasonable doubt ( virtually certain) Some of this increase is due to human activities – the balance of the evidence (likely) Impacts are complex, vary with region, very difficult to quantify reliably Any action to limit future increases will take decades to become effective – certain (virtually certain) Scientific Assessments There are accepted standards of what constitutes scientific evidence 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 Scientists are paid to be skeptical. Some will always disagree The Intergovernmental Panel on Climate Change IPCC95 Cover Prediction IPPC 1990 Report Latest predictions show less warming Uncertainties were underestimated IPCC90 Fig 8 Notes on Science Assessment Current – consensus on attribution Balance of the evidence Regional predictions much less secure than global average Precipitation predictions much less secure than temperature Impacts on ecosystems, agriculture, etc only in general terms Judgements of Confidence IPCC Third Assessment Report-2001 Summary for policymakers - Footnote 7 Statement – – – – – – – Virtually Certain Very Likely Likely Medium Likelihood Unlikely Very unlikely Exceptionally unlikely Chance – – – – – – – > 99% 90-99% 60-90% 33-66% 10-33% 1-10% < 1% True Key Points Scientific assessment describes present state of knowledge Global temperature has increased during 20 century, continuing through the 1990s – beyond reasonable doubt ( virtually certain) Some of this increase is due to human activities – the balance of the evidence (likely) Impacts are complex, vary with region, very difficult to quantify reliably Any action to limit future increases will take decades to become effective – certain (virtually certain) Global Average Temperature Is this a natural fluctuation or attributable to human activities? Sept 1998. Every month for past 19 has been a record high! Past NH Summer Temperature tree rings, ice cores,documents IPCC95 Fig 10 IPPC 3rd Assessment Report-1 Global temperature increased over the 20th Century by about 0.6o C : virtually certain This increase is largest for any century within past 100 years: likely 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 Global average sea level rose 10-20 cm (4-8 inches) during 20th century Cloud cover increased 2% over mid-high latitude land during 20th century likely Key Points Scientific assessment describes present state of knowledge Global temperature has increased during 20 century, continuing through the 1990s – beyond reasonable doubt ( virtually certain) Some of this increase is due to human activities – the balance of the evidence (likely) Impacts are complex, vary with region, very difficult to quantify reliably Any action to limit future increases will take decades to become effective – certain (virtually certain) What Do the Models Say? Partially Tuned Simulations What happened 1940-1970? IPCC95 Fig 15 Model Tuning Models are simplified representations of reality They represent what are believed to be the most significant processes Representations sometimes involve coefficients which must be empirically determined 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 Radiative forcing per unit sulfate aerosol uncertain a priori Good simulations of Mt. Pinatubo – aerosol in stratosphere, not troposphere Warming – in sulfate areas only at night observed and modeled Better agreement for global rise for past 100 years – some tuning After Eruption of Mt. Pinatubo IPCC95 Fig 14 Sensitivity to Aerosols -1 for 3 values of cloud feedback greenhouse gases only IPCC95 Fig 16a Sensitivity to Aerosols -2 for 3 values of cloud feedback greenhouse gases and aerosols IPCC95 Fig 16b Estimated Current Forcing IPCC 1995 Key Points Scientific assessment describes present state of knowledge Global temperature has increased during 20 century, continuing through the 1990s – beyond reasonable doubt ( virtually certain) Some of this increase is due to human activities – the balance of the evidence (likely) Impacts are complex, vary with region, very difficult to quantify reliably Any action to limit future increases will take decades to become effective – certain (virtually certain) Impacts of Climate Change Regional analysis is critical Current climate models disagree on regional changes Changes in precipitation at least as important as changes in temperature Impacts on ecosystems, water resources, agriculture, extreme events (floods, droughts, hurricanes,…). Some beneficial,vary with region Other stresses use to land use, acid rain Less wealthy countries more vulnerable Need to build resilience to multiple stresses Key Points Scientific assessment describes present state of knowledge Global temperature has increased during 20 century, continuing through the 1990s – beyond reasonable doubt ( virtually certain) Some of this increase is due to human activities – the balance of the evidence (likely) Impacts are complex, vary with region, very difficult to quantify reliably Any action to limit future increases will take decades to become effective – certain (virtually certain) IPPC 3rd Assessment Report-2 Atmospheric CO2 – increased 31% since 1750 virtually certain – present concentrations largest in 420,000 yr and likely in 20,000,000 yr 3/4 anthropogenic emissions CO2 due to fossil fuel burning, remainder mainly land use change Human influences will continue to change atmospheric composition throughout the 21st Century 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 For high or low feedback multiply by 1.8 or 0.6 Must examine credibility IPCC95 Fig 18 Emissions Scenarios CO2 Gt C /y Concentration Scenarios CO2 ppmv Key Points Scientific assessment describes present state of knowledge Global temperature has increased during 20 century, continuing through the 1990s – beyond reasonable doubt ( virtually certain) Some of this increase is due to human activities – the balance of the evidence (likely) Impacts are complex, vary with region, very difficult to quantify reliably Any action to limit future increases will take decades to become effective – certain (virtually certain) Sources of Information Horel and Geisler Chapter 8 Climate Change 1995 - Houghton et al Cambridge University Press IPCC (Working Group 1) “Third Assessment Report.” Summary for Policy Makers IPPC “Regional Impacts of Climate Change” http://www.grida.no/climate/ipcc/regional/ – Introduction