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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
IPCC WG1 approvalJan 2007
3rd draft, review of SPMSept 2006
4th LA meetingJun 2006
2nd draft, Govt/expert reviewMar 2006
3rd LA meetingDec 2005
1st draft, expert reviewAug 2005
2nd LA meetingMay 2005
Zero order draft, internal review……
1st LA meetingSep 2004
Climate sensitivity workshop(July, 2004, Paris)
…….
IPCC approval of outlineNov 2003
2nd Scoping meetingSep 2003
1st Scoping meetingApr 2003
All runs needed for WGI
Documentation needed (papers or reports)
All papers/documentation in press or appeared
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 ChangeMann, 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)
Latest view of last 2000 years of Northern Hemisphere Temperature ChangeMann, 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”
“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 20th century warming
• Climate models with only “natural” forcings(volcanic and solar) do not reproduce observed late 20th century warming
• When increases in anthropogenic greenhouse gases and sulfate aerosols are included, models are able reproduce observed late 20th
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 21st century from 21 models
early century mid century late century
Changes in hydrologic cycle by the end of the 21st 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 lengthEarlier flowering; emergence of insects; earlier mating; loss of habitat, shorter hibernation
Changes in frost days in the late 20th 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 20th century
Heat WavesHave 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, 994--997.
Observed
Model
Future
Summary
Global warming is real, and most of the warming since the late 20th 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 20th 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
HL
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 (2080-2099) compared to present-day (1961-1990)
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)