Updates on the IPCC Fourth Assessment Report (AR4) Gerald A. Meehl NCAR Boulder, CO

The IPCC Fourth Assessment Report (AR4) The Intergovernmental Panel on Climate Change (IPCC) consists of about 190 governments that commission assessments performed by the international climate science community on the state of human knowledge of climate and climate change Working Group 1: Climate science Working Group 2: Climate impacts and adaptation Working Group 3: Mitigation

AR4 WG1 timetable All runs needed for WGI IPCC WG1 approvalJan rd draft, review of SPMSept th LA meetingJun nd draft, Govt/expert reviewMar rd LA meetingDec st draft, expert reviewAug nd LA meetingMay 2005 Zero order draft, internal review…… 1st LA meetingSep 2004 Climate sensitivity workshop (July, 2004, Paris) ……. IPCC approval of outlineNov nd Scoping meetingSep st Scoping meetingApr 2003 All papers/documentation in press or appeared Documentation needed (papers or reports)

Working Group I Contribution to the IPCC Fourth Assessment Report Climate Change 2007: The Physical Science Basis Chapter 1: Historical Overview of Climate Change Science Chapter 2: Changes in Atmospheric Constituents and in Radiative Forcing Chapter 3: Observations: Surface and Atmospheric Climate Change Chapter 4: Observations: Changes in Snow, Ice and Frozen Ground Chapter 5: Observations: Oceanic Climate Change and Sea Level Chapter 6: Paleoclimate Chapter 7: Couplings Between Changes in the Climate System and Biogeochemistry Chapter 8: Climate Models and their Evaluation Chapter 9: Understanding and Attributing Climate Change Chapter 10: Global Climate Projections Chapter 11: Regional Climate Projections 22 CLAs, (7 Americans) 142 LAs, (27 Americans)

Climate models are a lot like weather forecast models, but include interactive ocean, land surface, and sea ice components, and also account for changes in atmospheric constituents like greenhouse gases

Global mean surface temperatures have increased

Latest view of last 2000 years of Northern Hemisphere Temperature Change Mann, M.E., Ammann, C.M., Bradley, R.S., Briffa, K.R., Crowley, T.J., Jones, P.D., Oppenheimer, M., Osborn, T.J., Overpeck, J.T., Rutherford, S., Trenberth, K.E., Wigley, T.M.L. (EOS, 2003)

“Medieval Warm Period” Latest view of last 2000 years of Northern Hemisphere Temperature Change Mann, M.E., Ammann, C.M., Bradley, R.S., Briffa, K.R., Crowley, T.J., Jones, P.D., Oppenheimer, M., Osborn, T.J., Overpeck, J.T., Rutherford, S., Trenberth, K.E., Wigley, T.M.L. (EOS, 2003) “Little Ice Age”

8 of top 10 warmest years have occurred in the last decade “Global Warming” is real … Land Ocean Land & Ocean NOTE: warming greatest over land

Instrumental Observed Temperature Trends - ANNUAL IPCC TAR (2001)

Natural forcings do not fully explain observed late 20 th century warming Climate models with only “natural” forcings (volcanic and solar) do not reproduce observed late 20 th century warming When increases in anthropogenic greenhouse gases and sulfate aerosols are included, models are able reproduce observed late 20 th century warming

“…most of the warming observed over the last 50 years is attributable to human activities” ---IPCC Third Assessment Report, 2001

Climate change commitment: at any point in time, we are committed to additional warming and sea level rise from the radiative forcing already in the system (Meehl et al., 2005: How much more warming and sea level rise? Science, 307, 1769—1772)

Surface temperature change in the 21 st century from 21 models early century mid century late century

Changes in hydrologic cycle by the end of the 21 st century temperature precipitation soil moisture

Precipitation intensity is projected to increase particularly in the northern tier of states (warmer air can hold more moisture, so that for a given event more precipitation falls) Dry days in between precipitation events increase mostly in the southern tier of states, but in the Pacific Northwest both precipitation intensity and dry days in between events increase

Effects in Agricultural and Biological Systems related to Frost Days—an example from a climate model Changes in frost days affect:  Range shifts (latitudinal or altitudinal)  Change in growing season length  Earlier flowering; emergence of insects; earlier mating; loss of habitat, shorter hibernation

Changes in frost days in the late 20 th century show biggest decreases over the western and southwestern U.S. in observations and the model

Future changes in frost days from the climate model show greatest decreases in the western and southwestern U.S., similar to late 20 th century

Heat Waves Have effects on human mortality, economic impacts, ecosystem and wildlife impacts

Climate models can be used to provide information on changes in extreme events such as heat waves Heat wave severity defined as the mean annual 3-day warmest nighttime minima event Model compares favorably with present- day heat wave severity In a future warmer climate, heat waves become more severe in southern and western North America, and in the western European and Mediterranean region Meehl, G.A., and C. Tebaldi, 2004: More intense, more frequent and longer lasting heat waves in the 21st century. Science, 305, Observed Model Future

Summary Global warming is real, and most of the warming since the late 20 th century has been due to human activity Cimate models can be used to reduce uncertainty by: 1. Quantifying the effects of forcing factors that influenced climate during the 20 th century (and over the past 1000 years) 2. Running multi-member ensembles to quantify the range of model responses for future climate 3. Analysis of extreme events such as future changes in frost days, heat waves and precipitation intensity shows how the models can provide information on what changes could occur, and why they could occur

Large-scale changes in atmospheric circulation affect regional pattern of changes in future frost days H L Anomalous ridge of high pressure brings warmer air to northwestern U.S.causing relatively less frost days compared to the northeastern U.S. where an anomalous trough brings colder air from north cold warm

Climate model shows an increase in the average number of heat waves per year in the future (top) and an increase in heat wave duration (bottom) (model grid points near Chicago and Paris)

The Chicago (1995) and Paris (2003) heat waves show large positive 500 hPa height anomalies For present-day heat waves near Chicago and Paris, the climate model also simulates large positive 500 hpa height anomalies

Atmospheric circulation in heat waves becomes more intense for future climate ( ) compared to present-day ( ) Future change in base state (mean) atmospheric circulation due to increased CO2 is conducive to more intense heat waves

Kerr, R.A., Science 307 (11February 2005), adapted from K.R. Briffa and T.J. Osborn, Science 295 (22 MARCH 2002), AND A. Moberg et al., Nature 322 (10 FEBRUARY 2005)