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Water vapour: feedback or forcing?

Filed under: — gavin @ 6 April 2005 - (Deutsch)

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.

The table shows the instantaneous change in long-wave aborption when each component or combination of components is removed using the radiation code from the GISS GCM. (The source code is available for those who have the patience to get it to work). This isn’t a perfect calculation but it’s quick and easy and is close enough to the right answer for our purposes. (N.B. This is very similar to what was done by Ramanathan and Coakley (1978) using a single column model – their numbers are in the table for reference). [Update Oct 2010:The numbers in this post have been somewhat updated and published in Schmidt et al (2010). ]. Because of the overlaps, the combined changes are larger than the changes due to each individual component. Another calculation is the instantaneous radiative forcing at the tropopause, but that is complicated for clouds, O3 and Aerosols which have impacts on solar radiation as well as the long wave, so I only give that value for the ‘pure’ greenhouse gases.

The overlaps complicate things, but it’s clear that water vapour is the single most important absorber (between 36% and 66% of the greenhouse effect), and together with clouds makes up between 66% and 85%. CO2 alone makes up between 9 and 26%, while the O3 and the other minor GHG absorbers consist of up to 7 and 8% of the effect, respectively. The remainders and uncertainties are associated with the overlaps which could be attributed in various ways that I’m not going to bother with here. Making some allowance (+/-5%) for the crudeness of my calculation, the maximum supportable number for the importance of water vapour alone is about 60-70% and for water plus clouds 80-90% of the present day greenhouse effect. (Of course, using the same approach, the maximum supportable number for CO2 is 20-30%, and since that adds up to more than 100%, there is a slight problem with such estimates!).

Since we are looking at the whole of the present-day greenhouse effect (around 33 C), it is not surprising that the radiative forcings are very large compared to those calculated for the changes in the forcing. The factor of ~2 greater importance for water vapour compared to CO2 is consistent with the first calculation.

So where does the oft quoted “98%” number come from? This proves to be a little difficult to track down. Richard Lindzen quoted it from the IPCC (1990) report in a 1991 QJRMS review* as being the effect of water vapour and stratiform clouds alone, with CO2 being less than 2%. However, after some fruitless searching I cannot find anything in the report to justify that (anyone?). The calculations here (and from other investigators) do not support such a large number and I find it particularly odd that Lindzen’s estimate does not appear to allow for any overlap.

While water vapour is indeed the most important greenhouse gas, the issue that makes it a feedback (rather than a forcing) is the relatively short residence time for water in the atmosphere (around 10 days). To demonstrate how quickly water reacts, I did a GCM experiment where I removed all the water in the atmosphere and waited to see how quickly it would fill up again (through evaporation from the ocean) . The result is shown in the figure. It’s not a very exciting graph because the atmosphere fills up very quickly. At Day 0 there is zero water, but after only 14 days, the water is back to 90% of its normal value, and after 50 days it’s back to within 1%. That’s less than 3 months. Compared to the residence time for perturbations to CO2 (decades to centuries) or CH4 (a decade), this is a really short time.

Only the stratosphere is dry enough and with a long enough residence time (a few years) for the small anthropogenic inputs to be important. In this case (and in this case only) those additions can be considered a forcing. Oxidation of anthropogenic methane (which is a major source of stratospheric water) and, conceviably, direct deposition of water from increases in aircraft in the lower stratosphere, can increase stratospheric water and since that gives a radiative forcing effect, they do appear on the forcings bar chart (under “H2O from CH4“). Some scientists have argued that changes to irrigation and other land use changes (which effect evaporation) are also direct forcings to water vapour amounts, but I think it’s cleaner to think of that as an indirect water vapour response to the change.

When surface temperatures change (whether from CO2 or solar forcing or volcanos etc.), you can therefore expect water vapour to adjust quickly to reflect that. To first approximation, the water vapour adjusts to maintain constant relative humidity. It’s important to point out that this is a result of the models, not a built-in assumption. Since approximately constant relative humidity implies an increase in specific humidity for an increase in air temperatures, the total amount of water vapour will increase adding to the greenhouse trapping of long-wave radiation. This is the famed ‘water vapour feedback’. A closer look reveals that for a warming (in the GISS model at least) relative humidity increases slightly in the tropics, and decreases at mid latitudes.

How do we know that the magnitude of this feedback is correctly simulated? A good test case is the response to the Pinatubo eruption. This caused cooling for up to 3 years after the eruption – plenty of time for water vapour to equilibriate to the cooler sea surface temperatures. Thus if models can simulate the observed decrease of water vapour at this time, it would be a good sign that they are basically correct. A good paper that demonstrated this was Soden et al (2002) (and the accompanying comment by Tony DelGenio). They found that using the observed volcanic aerosols as forcing the model produced very similar cooling to that observed. Moreover, the water vapour in the total column and in the upper troposphere decreased in line with satellite observations, and helped to increase the cooling by about 60% – in line with projections for increasing greenhouse gases.

To be sure there are still some lingering uncertainties. Some recent data indicates that tropical upper tropopsheric water vapour does not quite keep up with constant relative humidity (Minschwaner and Dessler, 2004) (though they still found that the feedback was positive). Moist convection schemes in models are constantly being refined, and it’s possible that newer schemes will change things . However, given the Pinatubo results, the models are probably getting the broader picture reasonably correct.

*R.S. Lindzen, 1991. Quart. J. Roy. Met. Soc., 117, pp. 651-652

111 Responses to “Water vapour: feedback or forcing?”

  1. 101

    [...] in discussions of climate as a positive feedback on other longer-duration warming phenomena. Here is a sensible discussion if you are interested. If you are not, note that water vapour considered [...]

  2. 102

    [...] for anything of this sort is A discussion of water vapor is given here. RealClimate These guys are climate scientists, so if you’re looking for a quick explanation, this isn’t it, [...]

  3. 103

    [...] by the Clausius-Clapeyron relationship (further discussion on water vapor feedback here, here, and at realclimate. This means two important things: First, the influence of water vapor on the 33 K greenhouse effect [...]

  4. 104

    [...] vapor is indeed a very important greenhouse gas, see here for more. It is included in the models, of [...]

  5. 105

    [...] Originally Posted by prashamk I found the following while surfing around: Can the experts put some light? The 95% figure is very wrong. True figure is about 66% or 36% depending on whether you count the overlap. (IE if you remove the water vapour and leave everything else, you reduce the GE by 36% or so, if you remove everything else and leave the water vapour you leave about 66% of the GE.) That is without considering clouds, which, as you point out, ameliorate the effect. (See: Water vapour: feedback or forcing?) [...]

  6. 106

    [...] like "not making up the figures"). Water with clouding is only 81% of the total GHG’s. RealClimate Thanks BW for letting me revise it. Which one relates to our climate change the most?? [...]

  7. 107

    [...] c, Water. Source: (estimates 67% to 85% of greenhouse affect), and Freidenreich and Ramaswamy, Solar Radiation [...]

  8. 108

    [...] effect than CO2(humiidity and cloud cover). But water vapour is more of a feedback component. Technical stuff [...]

  9. 109

    [...] for 2.5W/m^2. For total GHG’s and their effects upon the planet, then this is good reading. RealClimate __________________ Sustainability – is the environmental/economic and social interests combined. [...]

  10. 110

    [...] RealClimate Clouding is one factor they haven’t fully evaluated as yet, but with water vapour and clouding they are really only 66 to 85% You’ll have to read the article to explain. But water is a feedback only because we can not artificially increase or decrease water in the air. By warming the ocean though, we are removing a forcing that creates clouding and rain and can lead to a further rise in temperatures. By deforestation we are removing a fast source of evaporation (by transpiration) that helps cool the lower land levels and creates rainfall (especially monsoonal rain). __________________ Sustainability – is the environmental/economic and social interests combined.…/frontpage.pdf [...]

  11. 111

    [...] -          Water vapor is the most important greenhouse gas (dwarfing the effect of CO2.) Again, even though the first statement is in principle correct, the second is dead wrong. Water vapor doesn’t change due to increased emissions, but it changes in response to the increase in temperature: warm air can hold more water vapor. Therefore water vapor works as a positive feedback: If the climate warms, it causes even more warming, and vice versa. Water vapor is in a fast equilibrium with the biosphere, and it is not a climate forcing (cause), but rather a feedback (reinforcing effect). If we wouldn’t have changed the climate, water vapor would not have changed either. The effect of water vapor is definitely included in climate models: A large part of the warming from CO2 occurs indirectly via water vapor as a positive feedback. See also here and here. [...]

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