Updates on the IPCC Fourth Assessment Report (AR4)

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Updates on the IPCC Fourth Assessment Report
(AR4) Gerald A. Meehl NCAR Boulder, CO
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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
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AR4 WG1 timetable
All runs needed for WGI
Documentation needed (papers or reports)
All papers/documentation in press or appeared
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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)
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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
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Global mean surface temperatures have increased
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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)
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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
Little Ice Age
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Global Warming is real
Land Ocean
Ocean
8 of top 10 warmest years have occurred in the
last decade
Land
NOTE warming greatest over land
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Instrumental Observed Temperature Trends - ANNUAL

IPCC TAR (2001)
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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

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most of the warming observed over the last 50
years is attributable to human activities---IPC
C Third Assessment Report, 2001
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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, 17691772)
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Surface temperature change in the 21st century
from 21 models early century
mid century late century
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Changes in hydrologic cycle by the end of the
21st century temperature
precipitation soil moisture
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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
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Effects in Agricultural and Biological Systems
related to Frost Daysan 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

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Changes in frost days in the late 20th century
show biggest decreases over the western and
southwestern U.S. in observations and the model
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Future changes in frost days from the climate
model show greatest decreases in the western and
southwestern U.S., similar to late 20th century
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Heat Waves Have effects on human mortality,
economic impacts, ecosystem and wildlife impacts
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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
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SummaryGlobal warming is real, and most of the
warming since the late 20th century has been due
to human activityCimate models can be used to
reduce uncertainty by1. 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 climate3. 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
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Large-scale changes in atmospheric circulation
affect regional pattern of changes in future
frost days
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
H
cold
L
warm
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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)
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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
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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
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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)