Greenhouse Gases and Climate Modeling GLOBAL ENERGY BUDGET - 4 Greenhouse Gases and Climate Modeling

WHY DO SOME GASES CONTRIBUTE TO THE GREENHOUSE EFFECT & OTHERS DO NOT? Gas molecules absorb/emit radiation in two ways Changing the rate at which the molecule rotates Changing the amplitude with which a molecule vibrates

CHANGE IN ROTATION Molecules rotate at discreet frequencies If the frequency of the incoming wave is just right, the molecule absorbs the photon The molecule emits the photon when the rotation slows down Depends on structure of molecule

H2O ROTATION BAND Strong absorption feature of Earth’s atmosphere H2O molecule absorbs IR radiation of 12μm or longer Virtually 100% of infrared radiation > 12μm absorbed

H2O ROTATION BAND

CHANGE IN AMPLITUDE OF VIBRATION If the frequency at which the molecule vibrates matches frequency of incoming wave, molecule absorbs photon and vibrates more Bending mode of CO2 allows molecule to absorb IR radiation about 15 μm λ

CHANGE IN AMPLITUDE OF VIBRATION

15 μm CO2 BAND Strong absorption feature of Earth’s atmosphere Important to climate because it occurs near peak of Earth’s outgoing radiation very little of Earth’s outgoing radiation can escape because it is absorbed by CO2

OTHER GREENHOUSE GASES CH4, N2O, O3 and freons More effect on outgoing radiation than low concentrations would suggest Absorb at different wavelengths than H2O & CO2

O2 & N2 Poor absorbers of IR radiation Perfectly symmetrical molecules Electromagnetic fields unable to interact with symmetrical molecules

EFFFECT OF CLOUDS ON RADIATION BUDGET Quantification of effect difficult Many types of clouds Cumulus – water Cumulonimbus – water Stratus – water Cirrus – ice

CLOUD TYPES

CLOUD EFFECTS Day – cool Earth by reflecting sunlight back to space Without clouds albedo would be ~0.1 At 0.1 Te would increase 17C Night – warm Earth – re-emit outgoing IR radiation

CLOUD EFFECTS Stratus – low, thick Cirrus – high, thin Increase albedo Reflect incoming solar radiation Radiate at higher temperature, and according to Stefan-Boltzmann law radiate more energy to space Cirrus – high, thin Increase greenhouse effect Ice crystals more transparent to incoming solar radiation Radiate at lower temperatures and according to Stefan-Boltzmann law radiate less energy to space

CON TRAILS FROM JETS?

EARTH’S GLOBAL ENERGY BUDGET

PRINCIPLE OF PLANETARY ENERGY BALANCE At the top of the atmosphere, the net downward solar radiation flux (incoming minus reflected), must equal the outgoing infrared flux

CLIMATE MODELING Climate system complex Computer models based on data used to simulate climate systems GCM – General Circulation Model (aka Global Climate Model) - includes 3-d representation of atmosphere (winds, moisture, energy) Weather (clouds, precipitation) Require huge amounts of computer power

One Dimensional Climate Model Radiative-Convective Model (RCM) Climate system approximated by averaging incoming solar and outgoing IR over Earth’s entire surface Vertical dimension divided into layers Temperature of each layer calculated Energy received or emitted Convection Latent heat release

RCMs Allow estimation of greenhouse effect magnitude uses concentrations of greenhouse gases in atmosphere Models accurately predict ∆Tg (33C) Allow prediction of temperature increase due to GHG Doubling CO2 from 300ppm to 600ppm would produce a 1.2C increase The temperature change ∆T0 in the absence of any climate system feed back loops

CLIMATE FEEDBACKS Amplify or moderate radiative effect due to GHG concentrations Water Vapor Feedback Snow and Ice Albedo Feedback The IR Flux/Temperature Feedback The Cloud Feedback (Uncertain)

THE WATER VAPOR FEEDBACK If Earth’s surface temperature , then water vapor  (precipitation) If water vapor , then greenhouse effect , and surface temp  If Earth’s surface temperature , then water vapor  (evaporation) If water vapor , then greenhouse effect , and surface temp 

THE WATER VAPOR FEEDBACK

THE WATER VAPOR FEEDBACK Incorporated into RCM by assuming fixed relative humidity in troposphere RCM predicts doubling CO2 doubles the equilibrium change in surface temperature compared to the effect without water vapor

Mathematically Speaking . . . Comparing equilibrium temperature with and without water vapor feedback (from Ch 2) ∆Teq = ∆T0 + ∆Tf ∆Teq = 1.2C+ 1.2C  2.4C

The Feedback Factor The ratio of the equilibrium response to forcing (the response with feedback) to the response without feedback  = temperature change with feedback = 2.4 C  2 temperature change w/out feedback 1.2 C Negative feedback loop if 0 <  < 1 Positive feedback loop if 1 <  STRONGLY POSITIVE

SNOW & ICE ALBEDO FEEDBACK Snow & ice have higher albedo than land & water Increases in snow and ice coverage should decrease surface temperature Positive feedback loop Snow & ice restricted to middle & high latitudes, 2- or 3-d models are required

SNOW & ICE ALBEDO FEEDBACK

THE IR FLUX/TEMPERATURE FEEDBACK Strong negative feedback loop Stabilizes Earth’s climate on short time scales If Earth’ surface temperature , outgoing IR flux , if outgoing flux , surface temperature would  More energy is lost from the system System can fail if the atmosphere contains too much water vapor Venus – runaway Greenhouse Effect

THE IR FLUX/TEMPERATURE FEEDBACK

THE CLOUD FEEDBACK (UNCERTAIN) Adds significant uncertainty to climate models Clouds can warm or cool, depending on height Form at some locations and not others Most current GCMs Net positive feedback for doubled CO2 Increase in cirrus clouds (warming) outweighs any increase in stratus clouds (cooling)