Download 1 Lecture Aims Equilibrium Climate Sensitivity (ECS) Feedbacks

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Scottish Alliance for Geoscience, Environment & Society
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• Show some applications of climate models
• Key concept is feedbacks.
• Attribution of recent change to human
activities
• Predictions of climate change
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Equilibrium Climate
Sensitivity (ECS)
Why do we use Equilibrium Climate Sensitivity?
Definition:
•
established measure for climate models
•
comparison of climate models across hierarchy
•
traceability of climate model generations
•
focus on long-term changes
Equilibrium change in the annual mean global
•
independence of scenarios
surface temperature following a doubling of the
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ECS is not a projection
atmospheric (equivalent) CO2 concentration.
What determines ECS?
Feedbacks.
[Glossary AR4]
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• Act to amplify (or decrease) warming from changes in
CO2 and other greenhouse gases.
– Blackbody – warmer planet emits more radiation. (Negative
feedback)
– Water vapour – warmer atmosphere can store more water
vapour. Water vapour absorbs IR so is a GHG.
• Most important in the upper troposphere
• Warmer world will have more moisture in the atmosphere and so
will trap more heat. +ve feedback.
– Clouds
• +ve feedback – “trap” IR radiation
• -ve feedback – reflect back solar radiation.
– Ice/Albedo feedback.
• Ice is white and reflects lots of solar energy back to space.
• Melt ice and more solar radiation absorbed which in turn warms the
climate..
Changes in water
vapour will change
atmospheric
absorption.
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ECS with only blackbody feedback: about 1.15°C
Forcing
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• Why is ECS uncertain?
– Different parameterisations (see Lecture 1)
lead to different cloud, and other responses,
to climate forcings.
Response
Climate System
various Feedbacks
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observations: past transient climate change
1. Water Vapour Feedback:
positive
2. Cloud Feedback:
positive
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climatology and its variations
3. Albedo Feedback:
positive
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processes in climate models
4. Lapse Rate Feedback:
negative
Very recent research has produced various constraints
on ECS
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New in AR4:
• various constraints yield PDFs
• likelihoods can be determined
• a best estimate can be given
• lower bound is well constrained
• upper bound is difficult to constrain
due to short term response and
uncertainties in past climate change
Climate sensitivity (°C)
most likely value of ECS:
about 3°C
From Friedlingstein et al, 2006. Plots shows additional CO2 from
feedbacks between climate change and carbon cycle. Values vary
between 25 and 225 ppm at 2100 mostly due to land-carbon cycle
feedbacks.
Ch 10, Fig. Box 10.2, Fig. 1a,c
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Global mean temperatures are rising faster with time
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Gobal mean
temperature
Global average
sea level
Period
Northern hemisphere
Snow cover
Rate
50 0.128±
±0.026
100 0.074±
±0.018
Years °/decade
From Kevin E. Trenberth, NCAR
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Heat waves are increasing: an example
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• How can we decide what observed
changes are due to?
• Using climate models simulate the last
century+ with our best estimate of
forcings.
• Do some statistical comparison with
observations & draw conclusions
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Sunspot Number
200
Volcanic Aerosol depth
0.2
0
1700
0
1850
2000
Large tropical volcanoes inject SO2 into the Stratosphere
where it stays for 2-3 years. Effect is to make an aerosol
that scatters light increasing the earth’s reflectivity.
2000
Solar activity (sunspots etc) & irradiance (total radiation) changes with 11year solar cycle. There are long term changes in solar activity – the
Maunder Minimum being one example. Converting this to changes in
solar irradiance can be done though very uncertain. “Sun-like” starts
which show activity variations have been used to estimate irradiance
changes. Recent work (astronomical) and modelling (Lean et al) suggests
there may be no significant long term variation in solar irradiance.
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CO2 MMR*106
380
Mauna Loa
Observatory
360
1800
340
1400
320
1200
300
1000
280
260Ice cores
1700
1800
1900
Greenhouse gas
concentrations have
changed over the last
century. Their effect is
to decrease IR
transmission by the
atmosphere
CH4 MMR*109
Flasks
1600
2000
Ice cores
800
600
1700
1800
Year
1900
2000
Year
Thanks to Met Office
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• are observed
changes
consistent with
expected
responses to
forcings
inconsistent
with alternative
explanations
SPM-4
likely shows a
significant
anthropogenic
contribution
over the past
50 years
All forcing
Observations
Solar+volcanic
Observations
All forcing
natural forcing
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Annual
Trend 1979 to 2005
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Surface
Troposphere
Ch 3
Global ocean heat
content from different
analysis
1955
1980
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• extremely unlikely without
external forcing
• very unlikely due to known
natural causes alone
2005
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Scenario is consistent “storyline” of how future development might
happen.
From the IPCC SRES Summary for Policy Makers “Future
greenhouse gas (GHG) emissions are the product of very complex
dynamic systems, determined by driving forces such as
demographic development, socio-economic development, and
technological change. Their future evolution is highly uncertain.
Scenarios are alternative images of how the future might unfold …
The possibility that any single emissions path will occur as
described in scenarios is highly uncertain. “
Four different narrative storylines were developed …. Each storyline
represents different demographic, social, economic, technological,
and environmental developments
The scenarios cover a wide range of the main demographic,
economic, and technological driving forces of Greenhouse gases
and sulphur emissions…. Each scenario represents a specific
quantitative interpretation of one of four storylines.
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• For 4th Assessment report each modelling group
asked to asked to carry out simulations with:
• B1 (“Low”). Global Population that peaks in midcentury, move towards service and information
economy with emphasis on global solutions to
economic, social & environmental sustainability
• A1B (“Medium”) Very rapid economic growth,
population peaking in mid-century and regional
convergence
• A2 (“High”) Heterogeneous world. Self-reliance
and preservation of local identities. Population
increases throughout the century.
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Best estimate for
low scenario (B1)
is 1.8°C (likely
range is 1.1°C to
2.9°C), and for
high scenario
(A1FI) is 4.0°C
(likely range is
2.4°C to 6.4°C).
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Projected warming
in 21st century
expected to be
greatest over land
and at most high
northern latitudes
and least over the
Southern Ocean
and parts of the
North Atlantic
Ocean
Near term
projections
insensitive to
choice of
scenario
Longer term
projections
depend on
scenario and
climate model
sensitivities
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• Equilibrium Climate Sensitivity.
– Uncertain due to uncertain feedbacks mainly, in models, from
clouds.
• Observations show climate change over the 20th century
– Could be due to various human and natural drivers.
– Strong evidence using models and observations that human
factors (greenhouse gases) are the main driver.
• Future Projections
Precipitation increases very likely in high latitudes
– Uncertain with modelling uncertainty as important as scenario.
– Warming greatest at high latitudes and least in Southern Ocean
and North Atlantic.
– Precipitation is likely to increase at high latitudes and decrease
in land sub-tropics.
Decreases likely in most subtropical land regions
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• Write a short critical essay on attribution of
recent change.
• You might like to consider possible errors
in models and forcings.
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See the latest
IPCC report.
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