Download Forcing

Lecture 13
Radiative Forcing
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
Radiative forcing
(external)
Internal response
(including feedbacks)
Climate
system
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
More
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
Feedback

temp  decay  CO2
Comparing Causes of Temperature
Change
Assumption: Larger radiative forcing 
larger effect on temperature
Comparisons follow
Source: Intergovernmental Panel on Climate
Change (IPCC)
Positive Radiative Forcings
Largest – by far: increased greenhouse
gases

Increase is almost entirely anthropogenic
Long-Lived Greenhouse Gases
Gas
Forcing (Wm-2)
CO2
1.66
CH4(methane)
0.48
N2O (nitrous oxide) 0.16
Halocarbons
0.34
Total
2.64
More About Greenhouse Gases
Radiative transfer model
Adding greenhouse gas reduces outgoing
longwave radiation (OLR) at top of
atmosphere
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
Anthropogenic Sulfate Aerosols
Coal and diesel fuel contain sulfur
Burning of these fuels produces sulfur
dioxide (a gas)
In the atmosphere, this gas is converted
into particles
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
Evidence for Indirect Effect
Bright streaks are
areas of enhanced
albedo
Cause: Emissions
from ships
Streaks called
“ship tracks”
Cause of Ship Tracks
• Ship
exhaust
contains
aerosols
• The
aerosols
cause more
droplets to
form
• Cloud
albedo is
increased
Total Anthropogenic Effect on
Climate
Total Anthropogenic Climate Forcing =
sum of all anthropogenic forcings
Mainly, greenhouse gases (+)
+
sulfate aerosols (-)
Net Anthropogenic Radiative
Forcing (1750 – 2005)
Best Estimate:1.6
Positive.
2
W/m
Solar Irradiance
Some evidence suggests solar irradiance
may have increased lately
Current estimate of forcing: very small
Note: Evidence is very weak!
Engineering Solution to Global
Warming?
Sulfate aerosols cool the Earth
Why not deliberately increase sulfur
emissions?
Answer: Sulfur emissions contribute to
acid rain
Damage to forests
Erosion of statues
Words on gravestone obliterated
Effect on Aquatic Life
http://www.elmhurst.edu/~chm/vchembook
/195lakeeffects.html
Canadian lakes have acidified by U. S.
pollution
Reducing acid rain
Sulfur emissions have been reduced
Result: Less acid rain
Acid Rain vs. Global Warming
Reducing sulfur emissions reduces acidity
Reducing sulfur emissions reduces cooling
effect of sulfate aerosols
Result: Accelerated warming
http://www.sciencedaily.com/releases/199
9/07/990708075951.htm