An introduction to climate change - Climate Change: Biological and Human Aspects - page 9

(a)

55% Carbon dioxide
15% Methane
6% Nitrous oxide
7% Other CFCs
17% CFCs 11 and 12

(b)

63% Carbon dioxide
18% Methane
6% Nitrous oxide
4% Other CFCs & HCFCs
9% CFCs 11 and 12

Fig. 1.2 Thecontributionfromeachoftheprincipalanthropogenicgreenhousegasesduetothechangeinwarming

(radiativeforcing)(a)from1980to1990(excludingozone,whichmayormaynotbesigniicantandisdiicult
toquantify).DatafromIPCC(1990).(b)Thedatafor2005.DatafromIPCC(2007).

the infrared radiation at its absorptive frequencies. Indeed, if we were to look at the
Earth from space, solely in water-vapour frequencies, our planet would appear as mist-
veiled as Venus. This is true even over the dry Sahara Desert. But the concentration
of water vapour is not constant throughout the atmospheric column. Tropospheric
water vapour, in the atmospheric layer closest to the ground, varies considerably over
the surface. In the first 1-2 km of the atmosphere (lower part of the troposphere),
the amount of water vapour potentially in a unit volume increases with temperature.
In the troposphere above this point, the water-vapour greenhouse effect is most
important but is harder to quantify. Nonetheless, current computer models of the
global climate account for water-vapour feedback, whereby a warmer world sees
more evaporation, hence more water vapour, and this tends to double the warming
that one would expect from just a model of fixed water vapour. The ability of current