Transcript Projections of Future Climate Change

Lecture 15: Projections of
Future Climate Change
Global Mean Temperature
1
21st Century Climate Change
Dominant influence likely to be increase in
greenhouse gases (anthropogenic)
Projections of temperature change are
made using climate models
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Climate Models – 1 (Not
Necessary)
A climate model is a mathematical
representation of the physical processes
that control climate
Equations are complicated: Computers are
used to solve them
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Climate Models –2 (Not
Necessary)
The most sophisticated climate models
are called General Circulation Models
(GCMs)
These models attempt to simulate all
processes in the atmosphere and ocean
relevant to climate
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Climate Models – 3
Input
Anthropogenic forcing
Climate Model
Output
Climate change
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Emission Scenarios
Emission scenario: Possible future emissions
Reference:
http://www.grida.no/climate/ipcc_tar/wg1/029.
htm
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CO2 Emissions “Scenarios”
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20
15
10
B1
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Calculation of Future CO2
Concentrations -- Method
Model Input
Anthropogenic Emissions
Carbon Cycle Model
Model output
CO2 Concentration increase
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Projected CO2 Concentrations for Various
Emission Scenarios
B1
B1
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Some Scenario Results
Scenario Characterization
A1FI
B1
B2
Rapid economic growth;
strong reliance on fossil
fuels
Moderate economic
growth; much less
reliance on fossil fuels
“Middle-of-the-road”
scenario
Emission CO2
Rate in
Concentration
2100
in 2100 (ppm)
(Pg/yr)
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980
5
550
13
700
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Past and Projected Future CO2
Concentrations on same graph
See the IPCC Synthesis Report, Summary for
Policymakers, Figure SPM-10a, p. 33.
(URL:
http://www.grida.no/climate/ipcc_tar/vol4/engli
sh/fig9-1a.htm)
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Climate Models -- 5
Complication: Models have differing
sensitivities
 models produce different results for
same emission scenarios
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Differing Response of Models
for Same Scenario
Time
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Three Scenarios; three models
Model 1 (Medium sensitivity)
Model 2 (High sensitivity)
Model 3 (Low sensitivity)
Scenario A
Scenario B
Scenario C
Highest sensitivity,
highest emissions
Medium emissions
Low emissions
High emissions
Lowest sensitivity,
lowest emissions
time
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Model Projections of Climate Change
Fig. 9.14 in IPCC Scientific Assessment. Link:
http://www.grida.no/climate/ipcc_tar/wg1/353.htm
From 2001
Report
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Summary: Two Causes for
Large Range in Projections
1. Wide range in emission scenarios
2. Wide range in model sensitivities
#1 due to uncertainty in future human
actions (i.e., it is not a fault of the
models)
#2 is due to our imperfect understanding
of the climate system (i.e., it is a fault
of the models)
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From 2007
Report
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Model Projections
Average temp. for 2090-2100 compared to
average for 1980 – 1990.
All models project warming for all
scenarios
Range: 1.1C – 6.4C
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Meaning
Low end: warming of 1.1C
Achievable IF
1.
2.
Emissions are cut significantly
Lowest-sensitivity model is correct
High end: Warming of 6.4 C
May occur if
1.
2.
Emissions continue to rapidly increase
Highest-sensitivity model is correct
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Constant Concentrations
Suppose concentrations of
greenhouse gases have been
rising, but suddenly stopped
rising at time = tstop
tstop
time
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Temperature
Temperature Response
System is not yet in
equilibrium
Temp. rises as g.h.
gases increase
“Realized” warming
tstop
time
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Realized Warming
This is the warming that has been
observed at time tstop
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Temperature
Temperature Response
Additional Warming
“Realized” warming
tstop
time
Warming continues until
equilibrium is restored
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Warming Commitment
Def: Warming that will occur in the
future due to greenhouse gases added
in the past

Doesn’t include effect of any future
increases in greenhouse gases
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Fig. 1. CC warming commitment (constant concentrations after 2000) for different climate sensitivities
and aerosol forcing levels (L, M, and H on the right of the figure indicate low, mid-, and high magnitudes
for aerosol forcing, respectively)
High
Sensitivity
Medium
Sensitivity
Low Sensitivity
T. M. L. Wigley Science 307, 1766 -1769 (2005)
Published by AAAS
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Meaning of Graphs
With greenhouse-gas composition
constant at 2000 levels, Earth would warm
for centuries (warming commitment)
Commitment depends on sensitivity and
amount of aerosol forcing


Mid-range estimate: Commitment ~ 0.5C
High-end estimate: Commitment ~ 1C
This is warming that is “in the pipeline”
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Reducing Commitment
Only way: Reduce amount of greenhouse
gas in atmosphere
The sooner the reductions occur, the
greater the reduction in the commitment
Problem: Removing greenhouse gases is
very difficult!
More later
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Principal Difficulty
CO2 mixing ratio is 380 ppm

i.e., only 380 out of every million molecules of
air are CO2
CO2 removal would require processing
enormous amounts of air

Lots of energy probably required.
http://news.bbc.co.uk/2/hi/science/nature/6
345557.stm
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What the link reveals
Branson launches $25m climate bid
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Another Problem
Removing CO2 would create waste
Where should it be stored?
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Carbon Capture and Storage
Would reduce future emissions, but
wouldn’t actually remove CO2 from
atmosphere
http://news.bbc.co.uk/2/hi/science/nature/6
345557.stm#carbon
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Carbon Sequestration
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