Transcript ATM306-Lecture

3. Climate Change
3.1 Observations
3.2 Theory of Climate Change
3.3 Climate Change Prediction
3.4 The IPCC Process
3.1 Observations
Need to consider:
• Instrumental climate record of the last century
or so
• Recent changes in greenhouse gases and
other quantities
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Acknowledgement: Steve Arnold (University of Leeds, UK)
Important Questions Concerning the Climate Record
• How much is the world warming?
• Is the recent warming unusual?
• How rapidly is climate changing compared to
earlier changes?
• Have precipitation and atmospheric moisture
changed?
• Are atmospheric/oceanic circulations
changing?
• Has climate variability (e.g., extremes)
changed?
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Observed Climate Variables
• Temperature (global mean, regional, diurnal
range, upper air...)
• Precipitation, humidity, cloud cover
• Snow cover
• Sea-ice thickness and extent
• Natural modes (El Nino, North Atlantic
Oscillation, Atlantic Multidecadal Oscillation)
• Climate extremes
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Use of ‘Anomalies’
• Anomalies are changes relative to some
particular reference period used to
emphasise positive and negative excursions
around a long-term mean
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Global Mean Land-Surface Temperatures
• Temperature anomalies
(difference from 19611990 mean)
• Surface temperature trend
~0.05OC / decade since
1850.
• Urban “heat island” effect
is negligible (~0.006OC /
decade)
• Differences between
compilations arise from
weighting of observation
density
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Global Mean Land-surface and Sea Surface Temperatures
• Recent land warming
dominated by NH continents
• Ocean warming slower
• 20th-century T Change
~0.7±0.2 OC
– Compare 0.45 OC
estimated by IPCC in 1995.
Land
SST
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Trends
http://youtu.be/e0vj-0imOLw
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Spatial Pattern of Surface Temperature Changes
1901-2005
1975-2005
Grey areas: not enough data
White crosses: significant trend
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Summary of Surface Temperature Changes
• In the 20th century there has been a consistent largescale warming of the land and ocean surface.
• Rate of warming appears to be accelerating
• Some regional details can be explained by
atmospheric circulation changes
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Upper Air Temperatures
• Weather balloons (radiosondes)
– T at discrete levels
– Difficulties with changes in instruments
• Satellite (Microwave Sounding Unit – MSU and stratospheric
Sounding Unit - SSU) since 1979
– T over broad regions
– Problems with orbit drift, calibration, etc. -> spurious T trends
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Upper Air Temperatures
• Stratosphere cooling at > 0.5 OC/decade
• Volcanic eruptions cool troposphere and heat stratosphere
• Troposphere up to ~10km has warmed at slightly faster rate than
surface since 1950s.
• Stratosphere cooled markedly since 1979.
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Changes in the Cryosphere
• Snow cover
• Sea-ice extent
• Mountain glaciers
- Average Arctic temperatures have increased
at almost twice the global average rate in
past 100 years.
WMO International Polar Year 2007-2008
www.ipy.org
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Observed Northern Hemisphere Snow Cover
TOP
• Annual snow-cover
extent decreased by
10% since 1966
• Mostly accounted for
by spring/summer
since 1980s
BOTTOM
• Annual snow cover
anomalies correlate
with T (dashed line)
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Observed Northern Hemisphere Sea-Ice Extent
• http://svs.gsfc.nasa.gov/vis/a000000/a003400
/a003456/AMSR_E_SeaIce_to_09_14_2007_
512x288.m1v
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Observed Northern Hemisphere Sea-Ice Extent
Summer minimum Arctic sea ice extent
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Observed Northern Hemisphere Sea-Ice Extent
NASA Goddard Space Flight Center (orange line shows average minimum
ice extent (for 1979-2010 period). 2012 has lowest sea ice extent in more
than 30 years.
Observed Mountain Glacier Length
• 169 glacier length records
• Glacier retreat is worldwide
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Summary of Observed Cryospheric Changes
• Consistency of cryospheric changes and
temperature changes
• NH snow cover correlates well with spring
temperature changes
• Reduced sea-ice extent consistent with
increases in spring temperatures
• Small changes in Arctic winter ice, despite
large changes in winter T
• Major retreat of glaciers consistent with 20thcentury T changes
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Precipitation
• Wetter: East North and
South America, Eurasia
• Drier: Sahel, S Africa,
Mediterranean, S Asia
• More rain, less snow in
northern regions
• Increased frequency of
heavy precipitation
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Precipitation
• Wetter: East North and
South America, Eurasia
• Drier: Sahel, S Africa,
Mediterranean, S Asia
• More rain, less snow in
northern regions
• Increased frequency of
heavy precipitation
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•Beware of the AMO!
Precipitation
• Increases in amounts of very heavy precipitation (1958-2007)
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The map shows percent increases in the amount falling in very heavy
precipitation events (defined as the heaviest 1 percent of all daily events)
from 1958 to 2007 for each region. There are clear trends toward more very
heavy precipitation for the nation as a whole, and particularly
in the Northeast and Midwest (updated from Groissman et al,2004)
Atmospheric/Oceanic Circulations (El Nino)
• Occurrence of
strong El Ninos
may be higher
since 1980s
• But significance
is low due to
strong contribtion
of natural
variability
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Atmospheric/Oceanic Circulations (NAO)
• North Atlantic
Oscillation (NAO) is a
measure of circulation
patterns in the NH
• Positive index implies
warmer Europe
• Greater occurrence of
positive indices since
1980s
• NAO trending towards
negative values
recently.
• Difficult to separate
natural variability and
long-term change
http://www.cgd.ucar.edu/cas/jhurrell/indices.html
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Climate Extremes (extreme temperatures)
10th percentile
Trends in days/decade 19512003 (relative to 1961-1990).
90th percentile
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Climate Extremes (heatwaves)
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Climate Extremes (precipitation)
Contribution from ‘very wet’ days
(95th precentile) to total decadal
precipitation.
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Sea-level Rise
Annual average global mean sea level
Coastal tide gauges
Reconstructed
Satellite altimetry
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Sea-level Rise
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Changes in Greenhouse Gases
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