Greenhouse Effect

Thermal radiation Objects emit electromagnetic radiation –The hotter they are, the faster the energy output (  T 4 ) –The hotter they are, the shorter the wavelength Sun: mostly emits visible (short-wave) Earth: mostly emits infrared (IR) N 2, O 2 don’t readily absorb IR

Surface Temp – No Atmosphere Solar output Insolation Albedo Surface Temperature Longwave radiation

Planet with No Atmosphere Solar energy input IR output to space Temperature rises if input > output Output rises as temperature rises Temperature steady when input = output Surface energy changes with absorption and radiation

Add CO 2 Solar output Insolation Albedo Surface Temperature Longwave radiation CO 2 Atmospheric IR radiation Net effect: surface temp increase

Why CO 2 ? Gases with more than 3 atoms strongly absorb and emit long-wave IR –Water (H 2 O) thermally controlled –Carbon Dioxide (CO 2 ) residence time ~ 100 yr –Methane (CH 4 ) reacts with oxygen

Planetary Greenhouses Venus: ° C Earth: + 35 ° C Mars: + 1 ° C

When It Matters Most When/where surface cools radiatively –At night –In the winter –Near the poles

Geologic Factors Land distribution (polar land holds snow) Silicate weathering (consumes CO 2 ) Uplift (exposes weatherable rocks) Tectonic Volcanism: CO 2 emissions

Orbital Factors Lower axial tilt  warmer winters, cooler summers  more winter snow, less summer melt Less eccentricity  less annual insolation

Past Climate Swings Little Ice Age: sunspot minimum? Pliocene/Pleistocene ice ages: orbital cycles Archaean “Snowball Earth” –initiation: CH 4 consumed by biogenic O 2 –ending: volcanic CO 2 accumulation Warm Carboniferous: CO 2 depleted by coal accumulation, triggering Permian glaciations Warm Mesozoic: high seafloor spreading Cool late Cenozoic: Tethys collisions

Feedback Activity Predict effects of changes Add to feedback diagram Predict effect on surface temperature Solar output Insolation Albedo Surface Temperature Longwave radiation CO 2

Select Evidence there’s lots

Mauna Loa CO 2 Record Pre-industrial level Source: U.N. Environmental Programme

Ice Core CO 2 and Temperature Kurt M. Cuffey and Françoise Vimeux, Nature 412, (2 August 2001)

Past CO 2 levels Curve: Keeling, C.D. and T.P. Whorf In Trends: A Compendium of Data on Global Change. Oak Ridge, TN: Carbon Dioxide Information Analysis Center.

Hockey Stick Curve Science, 307, 828 (11 Feb 2005).

Lines of Evidence Radiation physics Climate modeling Paleoclimate records Geographic warming pattern Sea surface temperature, sea level Ice retreat

Sources of Greenhouse Gases Volcanoes Fires Methane-producing organisms Fossil fuel burning Release of CO 2 from warming oceans Methane from permafrost thawing

Human Impact Trends Energy, food demand increasing Coastal populations increasing disproportionately CO 2 emission rise exceeded projections

Future Climate Predictions Decreased snow pack  freshwater Longer growing season decreased sea ice Sea-level rise Warmer ocean Tropical and mid-latitude cyclones

Human Impact Predictions Coastal flooding Freshwater supplies Heat waves and droughts Tropical diseases Insect impacts

Mitigation Carbon-neutral energy –CO 2 sequestration –biomass –solar, wind, geothermal –nuclear Conservation and efficiency Cost accounting –carbon tax –cap/trade