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)