Water vapour: feedback or forcing?

Whenever three or more contrarians are gathered together, one will inevitably claim that water vapour is being unjustly neglected by ‘IPCC’ scientists. “Why isn’t water vapour acknowledged as a greenhouse gas?”, “Why does anyone even care about the other greenhouse gases since water vapour is 98% of the effect?”, “Why isn’t water vapour included in climate models?”, “Why isn’t included on the forcings bar charts?” etc. Any mainstream scientist present will trot out the standard response that water vapour is indeed an important greenhouse gas, it is included in all climate models, but it is a feedback and not a forcing. From personal experience, I am aware that these distinctions are not clear to many, and so here is a more in-depth response (see also this other attempt).

First some basics. Long-wave (or thermal) radiation is emitted from the surface of the planet and is largely absorbed in the atmosphere. Water vapour is the principle absorber of this radiation (and acknowledged as such by everybody). But exactly how important is it? In terms of mass, water vapour is much more prevalent (about 0.3% of atmospheric mass, compared to about 0.06% for CO2), and so is ~80% of all greenhouse gases by mass (~90% by volume). However, the radiative importance is less (since all molecules are not created equal). One way to quantify this is to take a radiation model and remove each long-wave absorber (principally the greenhouse gases, but also clouds and aerosols) and see what difference it makes to the amount of long-wave absorbed. This gives the minimum effect from each component. The complementary calculation, using only each particular absorber in turn, gives the maximum effect. Generally these will not be equal because of overlaps in the absorbing spectra (i.e. radiation at a particular frequency can either be absorbed by water vapour or CO2).

Removed absorbers

Fraction LW

Rad. Forcing

absorbed

Tropo. (W/m2)

None

100%

0

H2O

64 (64, RC78)

-56

Clouds

84 (86, RC78)

CO2

91 (88, RC78)

-23

O3

97 (97, RC78)

Other GHG

98

-3

H2O+Clouds

34

H2O+CO2

47

-89

All except H2O+Clouds

85

All except H2O

66 (60-70, IPCC90)

All except CO2

26 (25, IPCC90)

All except O3

7

All except Other GHG

8

All

0%

Instant calculation, global mean, Jan. 1, 1979

RC78=Ramanathan and Coakley (1978)

‘All’ includes aerosols, O3 and other minor gases as additional absorbers.

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