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Radiative Forcing
Definition: A change in the net radiation at
the top of the atmosphere due to some
external factor.
Net Radiation
Net radiation = Incoming - Outgoing
Positive net radiation

Incoming > Outgoing
Negative net radiation

Outgoing > Incoming
Positive & Negative Forcing
Positive forcing  warming
Negative forcing  cooling
Forcing and Feedbacks
“Forcing” is produced by an external process, e.g.
Changes in solar flux
 Volcanic eruptions
 Human actions
A feedback is a response to temperature changes
Example: Increased water vapor due to warming
Anthropogenic increases in greenhouse gases are
considered forcings
Increases in greenhouse gases that are caused by
temperature changes are feedbacks

The same gas can be involved in forcings
and feedbacks, e.g., CO2
Forcing:



CO2 increase from burning of fossil fuels
Largest – by far: increased greenhouse gases
Increase is almost entirely anthropogenic
Feedback

temp  decay  CO2
Initial Equilibrium
Top of
atmosphere
Absorbed
Shortwave
OLR
Now, add greenhouse gas
Keep temperatures fixed
Reduced Upward Flux
Top of
atmosphere
Absorbed
Shortwave
OLR
Net Downward Flux
Top of
atmosphere
Net Flux
Result: A positive radiative forcing
Negative Radiative Forcings
Largest: Increase in sulfate aerosols

Mostly anthropogenic
Effect of Anthropogenic Sulfate
Aerosols on Temperature
Direct effect


The aerosols themselves reflect sunlight
This is similar to the effect of volcanic aerosols
Indirect effect



Sulfate aerosols act as condensation nuclei
This increases the droplet concentration in
clouds
Result: Increased cloud albedo
Both effects tend to increase the Earth’s
albedo
Solar Irradiance
Some evidence suggests solar irradiance
may have increased lately
Current estimate of forcing: very small
Note: Evidence is very weak!
Climate Sensitivity
The change in equilibrium temperature
per unit of radiative forcing
Temperatur
e
Change
in
equilibri
um temp
Start in
equilibriumApply
New Equilibrium
Temp
Temp. rises
Time
Example
Suppose Sensitivity = 2C per unit of
forcing (1 Wm-2)
Radiative forcing = 3 Wm-2
Then, eventual warming = 2 x 3 = 6C
Differing Sensitivities
System 2 is
twice as
sensitive
2
C
1
C
Same
Comparing Models
Double CO2 content of model atmosphere

Radiative forcing ~ 4 W/m2
IPCC has compared many climate models
Results used to estimate actual climate
sensitivity of Earth
Sensitivity Estimates
Model sensitivities have a range of 2C to
4.5C for a doubling of CO2
(A technical point – don’t memorize.)
The Role of Feedbacks
Model sensitivity is determined by the
strength of the feedbacks in the model
Positive feedbacks increase sensitivity
Negative feedbacks decrease
sensitivity
Differences in Model Sensitivity
Main Cause of Variation: Cloud
Feedbacks
In most models, cloud feedback is
positive

However, magnitude varies a lot from one
model to another
Thermal Inertia
Determines rate of temperature
change
Rate of Warming
Thermal inertia: resistance of system to
temp. change

Measured by heat capacity
Higher heat capacity  slower warming
Temperature
Change (C)
System 1: 70% of
warming has
System 2:at70%
occurred
t = of
warming has
1.2
occurred at t = 2.4
Time
Earth-Atmosphere System
Most of the heat capacity is in oceans
Presence of oceans slows down warming
Comparison
Look at two systems with same radiative
forcing and sensitivity, but different heat
capacities
Summary
Positive (negative) radiative forcing causes
warming (cooling)
System warms (cools) until equilibrium is
restored
Amount of eventual warming (cooling)
depends on radiative forcing and sensitivity

Eventual warming (cooling) = sensitivity x rad.
forcing
Rate of warming is inversely proportional to
heat capacity
More Realistic Situation
Previous examples assumed radiative
forcing applied instantaneously

i.e., all GHG & aerosols added
instantaneously
Real life: GHG & aerosols added
gradually
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 – 3
Input
Anthropogenic forcing
Climate Model
Output
Climate change
28
Calculation of Future CO2
Concentrations -- Method
Model Input
Anthropogenic Emissions
Carbon Cycle Model
Model output
CO2 Concentration increase
29
Climate Models -- 5
Complication: Models have differing
sensitivities
 models produce different results for
same emission scenarios
30
Differing Response of Models
for Same Scenario
Time
31
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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Impacts
Arctic: large reduction in summer sea
ice

Arctic could be ice-free in summer by end
of century
Permafrost = soil that remains frozen
throughout the year

Warming  softening of permafrost
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Impacts
Glaciers and Ice Sheets
Mountain glaciers will continue to shrink
Greenland ice sheet will very probably lose
mass
Antarctica (?)
34
Impacts: Sea Level
Melting glacial ice and thermal
expansion will cause sea level to rise
Estimated rise


Low-emission scenario: 18 – 38 cm
High-emission scenario: 26 – 59 cm
Estimates are probably too low

Contribution from ice sheets was not taken
into account!
35
Impact of Rising Sea Level
Greatest in countries with heavily
populated coastal regions, e.g.
Bangladesh and in small-island nations
36
Fresh Water Supplies
Warming  shrinking glaciers, reduced
snowfall in mountains
Problem: 1/6 of world population depends
on glacial & snow melt for drinking water
37
Precipitation
Models project increases in precipitation in
some regions, decreases in others
Regions of decrease include:


Southwestern U. S., Mexico, Central America,
Caribbean
Mediterranean
Regions of increase include:

Canada, most of Asia
38
Soil Moisture, Runoff
precip.  soil moisture and runoff
But, can have soil moisture even with
precip.
39
Effect on California
Warming  less snowfall in mountains
 less summer runoff
 less water in summer for
irrigation
hydroelectric power
drinking water
Loss of salmon habitat
40
More about precipitation
Models project increased variability
 increased flooding and increased
droughts!
Another problem: increased demand for
water.
41
Agriculture
Reductions in soil moisture  reduced
crop yields
However, areas with increased soil
moisture could benefit

(If warming isn’t too large.)
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Other Potential Agricultural Benefits of
Warming
Increased growing season in higher latitudes

Could benefit Canada, Russia
Beneficial effects of increased CO2 could
offset damaging effects of reduced soil
moisture


Called “CO2 fertilization”
Only works if warming is relatively small
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Ecosystems
In past, ecosystems have been able to
adapt, but …
“ resilience of many ecosystems is likely to
be exceeded by 2100”
Effects of climate change aggravated by


increased human demands
fragmentation of habitats
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Ecosystems, continued
Up to 30% of species at “increasingly high
risk of extinction” if average global temp
increase above 2 -3C
Oceans becoming more acidic

Will hurt organisms that make shells
45
Carbon Cycle
Now, biosphere is a net “sink” of carbon

i.e., carbon uptake > carbon released
By mid-century, biosphere likely to
become a net source of carbon


i.e., carbon release > carbon uptake
(mainly due to increased rate of decay)
Ocean carbon uptake will diminish
Result: Faster rise of CO2
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Impacts on U. S. Forests
Each tree species requires a specific
environment for optimum growth
Climate change will cause a shift in tree
habitats
Projections of habitat changes

http://www.fs.fed.us/ne/delaware/atlas/web
_atlas.html#
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Tropical Cyclones – basic info
Called hurricanes in Atlantic, eastern
Pacific
Called typhoons in western Pacific (north of
equator)
Energy source: heat stored in oceans
Theory: warmer oceans  stronger storms

(There is evidence this already happening)
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Forest Fires
In western U. S., warming  more forest
fires
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Human Health
More deaths from heatwaves

Like 1995 Chicago heat wave
Increases in some tropical diseases
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Adaptation

Strategies for coping with climate change
Mitigation

Strategies for reducing the rate of climate
change
Why Adaptation?
Some warming is inevitable because of
thermal inertia
Adaptation Examples
Sea-level rise


Build coastal defenses
(e.g., dikes, as in the Netherlands)
Expensive!

Relocation of affected populations
(Millions of refugees possible.)
More Examples
Agriculture


Increased use of irrigation
Development of heat- and droughtresistance crops
(Genetic engineering?)
Human health and comfort

Increased use of air conditioning
(Would require increased electricity)
Mitigation
Reduction of greenhouse-gas emissions

Probably the most effective way
Carbon capture and storage

Send emissions into ground or deep ocean
Geoengineering (more later)


Increase albedo (e.g., sulfur in stratosphere)
Remove CO2 (e.g., artificial trees)
Reducing Global Emissions
This is a massive problem
Very large reductions are required to make
much difference
International cooperation is required
Some Methods to Reduce
Emissions
Using alternative energy sources, e.g.,
solar, wind, nuclear

(France: 80% of power generation is nuclear)
Increased efficiency


Improved gas mileage in cars
Compact fluorescents instead of incandescent
light bulbs
(Problem: CFs contain mercury)
Geoengineering Schemes
Disposal of CO2 in the Deep Ocean
Carbon Capture and Storage
Aerosol Injection into the Stratosphere
Injection of other small reflectors into
Stratosphere
Shielding the Earth from Afar
Placing reflective plastic over deserts
Modify ocean reflectivity
Damming the Bering Strait