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