PPT11 - Fluid Earth Circulations

PPT11 - Fluid Earth Circulations - Greenhouse gases (CO2,...

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Greenhouse gases (CO2, H2O, CH4) Trace constituents of the atmosphere that absorb infra-red radiations emitted by the Earth’s surface Greenhouse gases act as a “blanket” which keeps heat close to the Earth’s surface… How can greenhouse gases change and affect climate? Greenhouse warming depends on the concentration of greenhouse gases in the atmosphere (mainly H2O, CO2 and CH4) Greenhouse gases are found in small concentrations in the atmosphere and absorb infra- red radiations emitted by the Earth’s surface as it gets warmed by the sun Greenhouse gases act as a “blanket” which keeps heat close to the Earth’s surface. How can we change greenhouse gases concentration and affect climate? Infrared radiations emitted by the earth’s surface are partially absorbed by greenhouse gases (mainly H2O, CO2 and CH4) We have already explained the underlying reasons for greenhouse warming (see radiation balance lectures) The Earth’s atmosphere is quite transparent to incoming solar radiations (maximum emission in the visible range), but it is largely opaque to the infrared radiations emitted by the Earth’s surface. This absorption is due to the presence of small quantities of greenhouse gases, mainly H2O, CO2 and CH4 Increasing the concentration of any of these greenhouse gases would increase the % of infra red emission absorbed by the atmosphere and would warm the planet (i.e. would decrease the size of the windows of low absorption between 7 and 15 microns). Decreasing their concentration would allow more IR to escape to outer space and cool the planet H2O: -Absorbs IR at different wavelengths -Most abundant GHG (its atmospheric concentration depends on temperature) On the top figure above, we identify the two greenhouse gases (H2O and CO2) responsible for absorption at different wavelengths You will note that H2O vapor, in particular, absorbs IR radiations at many different wavelengths including IR emitted by the Earth’s surface (to the right of the green dividing line) and is therefore a very effective greenhouse gas (the graph above shows that water vapor absorbs strongly (up to 100%) of the IR radiations emitted by the earth surface with wavelength of about 1.1, 1.5, 1.9, 2.5, 5.5 to 7.5, >20 um; the earth’s surface emits IR at wavelength > 5 micrometers) H2O is also the most abundant greenhouse gas in the atmosphere and its mean concentration in the atmosphere depends mainly on temperature. The maximum amount of water vapor (in g per m3) that air can hold before condensation occurs increases sharply with air temperature (increasing water content beyond this maximum or decreasing the temperature of saturated air result in condensation) Since our planet is a “water planet” with 70% of its surface covered by ocean, most of the lower atmosphere is near saturation (except at the interior of large continents)
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We can see that air must be very dry at the poles, where temperature is low, and very humid in the tropics. . H2O
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PPT11 - Fluid Earth Circulations - Greenhouse gases (CO2,...

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