The Greenhouse Effect
The Greenhouse EffectLan MaGlobal Warming: Problems & Solutions17 September, 2007
What to cover today: How do we calculate the Earth’s surfacetemperature? What makes a gas a greenhouse gas and howdoes the increasing greenhouse gases in theatmosphere cause global warming?
Radiationfrom EarthSolar input Source: NASA – Visible Earth
Solar input depends on the distance between theSun and the planets and the size of the planetsEarth!1 AU 1.5x108 kmrE 6378kmS0 1367W/m2
Is that all?It turns out that not allincoming solar radiation isabsorbed at Earth’ssurface. Part of it isdirectly reflected fromclouds, the surface itself,and air molecules. Thisreflectance of solar input iscalled albedo; the presentvalue for Earth is about0.3.http://www.geo.lsa.umich.edu/ html
Solar inputS0 1367W/m2Albedo(0.3)*S0Radiationfrom EarthSource: NASA – Visible Earth
The amount of radiation emitted is proportional tothe surface temperature of that body radiation emitted σTE4σ Stephan-Boltzmannconstant 5.67x10-8 Wm-2K-4
Solar inputS0 1367W/m2Albedo(0.3)S0Radiation from Earth(5.67x10-8 W/m2K4)TE4Input OutputS0πrE2(1-α) 4πrE2σTE4TE (S0(1-α)/4σ)1/4 255K -18 C !!!Yikes! That’s freezing cold!!!Source: NASA – Visible Earth
Let’s apply the same calculation toVenus and Mars and see Venus252KEarth255KMars216KTheoretical temp735K288K218KActual temp
What parameters do we need to calculatethe Earth’s average surface temperature? Solar input (depends on distance fromSun and size of planet) Albedo (depends on landform and cloudcover) Blackbody assumption (perfectabsorption and perfect emission) Greenhouse effect
Wait a second, there is another twist!About 25% of the incoming solar radiation is absorbed byvarious gases like H2O, CO2, O2, and O3 before it hitsthe surface. So the calculation would be:Input OutputS0(1 - α - atmos absorption) 4σTE4 615 W/m2TE (S0(1 - α - atmos absortion)/4σ)1/4 228K -45 C !!!Opps that’s going the wrong direction!
Thankfully, greenhouse gases also save the day!We know the average surface temperature of Earth isabout 15 C, by back calculation we see:4σ(TE)4 4σ(288K)4 1560 W/m2GH Effect 1560 W/m2 – 615 W/m2 945 W/m2Let us compare:solar input1367 W/m2greenhouse effect 945 W/m2
Thankfully, greenhouse gases also save the day!The greenhouse effect is HUGE ! From the “noatmosphere” model of -18 C to an observed averagesurface temp of 15 C, it is a 33 C effect !Furthermore, if this were linear then we would get 1 Cincrease for every 7 W/m2. Thankfully this is not thecase, but adding more greenhouse gases into theatmosphere still poses an appreciable amount ofwarming to Earth!
Let’s come back to the planets Venus has a really thickatmosphere (90 times Earth’ssurface pressure.) Moreimportantly, this atmosphereconsists of 96% CO2735KVenus298KEarth218KMars
Let’s come back to the planets Mars has a very thinatmosphere such that thegreenhouse effect is nearlynegligible735KVenus298KEarth218KMars
Let’s come back to the planets Earth has an atmospherecomposed of 78% N2, 21% O2,and only 1% of traces gasesincluding greenhouse gases735KVenus298KEarth218KMarsEarth is so far the only life-sustainingplanet not just because of the rightdistance from the Sun but also due to amoderate greenhouse effect.
That was a lot of math Now what ?!
Past atmosphere
Present atmosphere
Future atmosphere?Source: IPCC AR4
What are greenhouse gases?Source: Global Warming Art
What are greenhouse gases?Greenhouse gases can effectively absorbradiation emitted from the Earth’s surface.Such absorption excites molecules to vibrateand rotate and sometimes change theirdipole moment.Symmetric and linear molecules like O2 and N2do not have a dipole moment (and cannotproduce one).
Dipole Moment Amount of charge separated by distance There needs to be a change in dipolemoment- a change of charge for amolecule to interact with the energyradiating from Earth’s surface(electromagnetic radiation)
What are greenhouse gases?ν1symmetricν2bending15 μmν2asymmetric4.3 μmOCOOCOOCO
Sources of greenhouse gases:CO2Natural: ocean, volcano, decompositionAnthropogenic: fossil fuel burning, exhaustCH4Natural: aerobic decomposition (wetland, cows)Anthropogenic: fossil fuel burning, agricultureN2ONatural: soil and oceanAnthropogenic: fertilizers (nitrification of ammonium)CFCs /HCFCsO3Anthropogenic: refrigerant, aerosol propellantNatural: photolysisAnthropogenic: NOx VOCs
Concern?Source: IPCC AR4
Acknowledgement Professor Kristie Boering (Chemistry) Professor John Chiang (Geography) Professor Inez Fung (Earth & PlanetaryScience) Zan Stine (EPS Ph.D. candidate)
Discussion
If we cannot predict the weather for more than 10days, why should we believe in models thatpredict our climate 50 years from now?
Trends!
Is global warming also caused by the ozone hole,which is allowing more harmful and intense UVradiation to penetrate through?13 July, 200713 September, 2007Source: NASA – Ozone Hole Watch
Overlap between O3 hole and GW Some of the most prominent ozone destructivechemical species, Chlorofluorocarbons (CFCs,entirely man-made), are also greenhouse gases(change in dipole moment!) Hydro-chlorofluorocarbons (HCFCs, also manmade), the replacement for CFCs, are lessdestructive to ozone but are still potentgreenhouse gases
Stratospheric Ozone CFCs are now banned, but due to long life-timethey will be around for another half a decade orso. The ozone hole still occurs during Antarcticspring but is hopefully on its way to shrink. We don’t need to regenerate ozone. Ozone inthe stratosphere is naturally produced andwhen we are not “poking” it as much it willgradually recover.
Two types of Ozone Ozone in the stratosphere (upper atmosphere)absorb harmful UV and is thus good, but thatis also where most ozone loss occurs. Ozone in the troposphere (lower atmosphere) isa form of smog / pollution generated by NOxand VOCs and thus is bad. (BTW ozone istoxic- it causes breathing difficulties, lungtissue damage, and damages rubber and someplastics)
Thankfully, greenhouse gases also save the day! The greenhouse effect is HUGE ! From the “no atmosphere” model of -18 C to an observed average surface temp of 15 C, it is a 33 C effect ! Furthermore, if this were linear then we would get 1 C increase for every 7 W/m2. Thankfully this is not the case, but adding more greenhouse gases .
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