Lecture 13Lecture 13
Radiative ForcingRadiative Forcing
Radiative ForcingRadiative Forcing
Definition: A change in the net radiation at Definition: A change in the net radiation at the top of the atmosphere due to some the top of the atmosphere due to some external factor.external factor.
Net RadiationNet Radiation
Net radiation = Incoming - Outgoing Net radiation = Incoming - Outgoing
Positive net radiationPositive net radiation Incoming > OutgoingIncoming > Outgoing
Negative net radiationNegative net radiation Outgoing > IncomingOutgoing > Incoming
Positive & Negative ForcingPositive & Negative Forcing
Positive forcing Positive forcing warming warming
Negative forcing Negative forcing cooling cooling
Forcing and FeedbacksForcing and Feedbacks
Radiative forcing (external)
Climate system
Internal response
(including feedbacks)
Forcing and FeedbacksForcing and Feedbacks
““Forcing” is produced by an Forcing” is produced by an externalexternal process, e.g.process, e.g. Changes in solar fluxChanges in solar flux Volcanic eruptionsVolcanic eruptions Human actionsHuman actions
A feedback is a A feedback is a responseresponse to temperature to temperature changeschanges Example: Increased water vapor due to Example: Increased water vapor due to
warmingwarming
More More
Anthropogenic increases in greenhouse Anthropogenic increases in greenhouse gases are considered forcingsgases are considered forcings
Increases in greenhouse gases that are Increases in greenhouse gases that are caused bycaused by temperature changes are temperature changes are feedbacks feedbacks
The same gas can be involved in forcings The same gas can be involved in forcings and feedbacks, e.g., COand feedbacks, e.g., CO22
Forcing: Forcing: COCO22 increase from burning of fossil fuels increase from burning of fossil fuels
FeedbackFeedback temp temp decay decay COCO22
Comparing Causes of Temperature Comparing Causes of Temperature ChangeChange
Assumption: Larger radiative forcing Assumption: Larger radiative forcing larger effect on temperaturelarger effect on temperature
Comparisons followComparisons followSource: Intergovernmental Panel on Climate Change (IPCC)
Positive Radiative ForcingsPositive Radiative Forcings
Largest – by Largest – by farfar: increased greenhouse : increased greenhouse gases gases Increase is almost entirely Increase is almost entirely anthropogenicanthropogenic
Long-Lived Greenhouse GasesLong-Lived Greenhouse Gases
0.340.34HalocarbonsHalocarbons
2.642.64TotalTotal
0.160.16NN22O (nitrous oxide)O (nitrous oxide)
0.480.48CHCH44(methane)(methane)
1.661.66COCO22
Forcing (WmForcing (Wm-2-2))GasGas
More About Greenhouse GasesMore About Greenhouse Gases
Radiative transfer model
Adding greenhouse gas reduces outgoing Adding greenhouse gas reduces outgoing longwave radiation (OLR) at top of longwave radiation (OLR) at top of atmosphereatmosphere
Initial EquilibriumInitial Equilibrium
Absorbed Shortwave
OLR
Top of atmosphere
Now, add greenhouse gas
Keep temperatures fixed
Reduced Upward FluxReduced Upward Flux
Absorbed Shortwave
OLR
Top of atmosphere
Net Downward FluxNet Downward Flux
Net Flux
Top of atmosphere
Result: A positive radiative forcing
Negative Radiative ForcingsNegative Radiative Forcings
Largest: Increase in sulfate aerosolsLargest: Increase in sulfate aerosols Mostly anthropogenic Mostly anthropogenic
Anthropogenic Sulfate AerosolsAnthropogenic Sulfate Aerosols
Coal and diesel fuel contain sulfurCoal and diesel fuel contain sulfur
Burning of these fuels produces sulfur Burning of these fuels produces sulfur dioxide (a gas)dioxide (a gas)
In the atmosphere, this gas is converted In the atmosphere, this gas is converted into particles into particles
Effect of Anthropogenic Sulfate Effect of Anthropogenic Sulfate Aerosols on TemperatureAerosols on Temperature
Direct effectDirect effect The aerosols themselves reflect sunlightThe aerosols themselves reflect sunlight This is similar to the effect of volcanic aerosolsThis is similar to the effect of volcanic aerosols
Indirect effectIndirect effect Sulfate aerosols act as Sulfate aerosols act as condensation nuclei This increases the droplet concentration in cloudsThis increases the droplet concentration in clouds Result: Increased cloud albedoResult: Increased cloud albedo
Both effects tend to increase the Earth’s Both effects tend to increase the Earth’s albedoalbedo
Evidence for Indirect EffectEvidence for Indirect Effect
Bright streaks are areas of enhanced albedo
Cause: Emissions from ships
Streaks called “ship tracks”
Cause of Ship TracksCause of Ship Tracks
• Ship exhaust contains aerosols
• The aerosols cause more droplets to form
• Cloud albedo is increased
Total Total AnthropogenicAnthropogenic Effect on Effect on ClimateClimate
Total Anthropogenic Climate Forcing =Total Anthropogenic Climate Forcing =
sum of all anthropogenic forcingssum of all anthropogenic forcings
Mainly, greenhouse gases (+) Mainly, greenhouse gases (+)
++
sulfate aerosols (-)sulfate aerosols (-)
Net Anthropogenic Radiative Net Anthropogenic Radiative Forcing (1750 – 2005)Forcing (1750 – 2005)
Best Estimate:1.6 W/mBest Estimate:1.6 W/m22
PositivePositive..
Solar IrradianceSolar Irradiance
Some evidence suggests solar irradiance Some evidence suggests solar irradiance may have increased latelymay have increased lately
Current estimate of forcing: very smallCurrent estimate of forcing: very small
Note: Evidence is very weak!Note: Evidence is very weak!
Engineering Solution to Global Engineering Solution to Global Warming?Warming?
Sulfate aerosols cool the EarthSulfate aerosols cool the Earth
Why not deliberately increase sulfur Why not deliberately increase sulfur emissions?emissions?
Answer: Sulfur emissions contribute to Answer: Sulfur emissions contribute to acid rainacid rain
Damage to forests
Erosion of statues
Words on gravestone obliterated
Effect on Aquatic LifeEffect on Aquatic Life
http://www.elmhurst.edu/~chm/vchembook/195lakeeffects.html
Canadian lakes have acidified by U. S. Canadian lakes have acidified by U. S. pollutionpollution
Reducing acid rainReducing acid rain
Sulfur emissions have been Sulfur emissions have been reducedreduced
Result: Less acid rainResult: Less acid rain
Acid Rain vs. Global WarmingAcid Rain vs. Global Warming
Reducing sulfur emissions reduces acidityReducing sulfur emissions reduces acidity
Reducing sulfur emissions reduces cooling Reducing sulfur emissions reduces cooling effect of sulfate aerosolseffect of sulfate aerosols
Result: Accelerated warmingResult: Accelerated warming
http://www.sciencedaily.com/releases/1999/07/990708075951.htm