Simple Sam learns about water vapour feedback

Posted on August 31, 2010


Sam came back to me with a question about climate feedback cycles and water vapour.

“In that diagram you showed before, I noticed that water vapour was put in a different feedback loop from the other greenhouse gases,” says Sam.  (See here.)  “Why is that?  Is it because no one can blame people for water vapour like they can CO2?”

So suspicious,  Sam.   No, water vapour is different from those other greenhouse gases.

“Yes, John Tyndall said it was the most powerful greenhouse gas of all.  Much more powerful than carbon dioxide.”  (See John Tyndall article.)

Sam, I’m proud of you.  You’ve been doing your homework.  Yes, water vapour is the most powerful of the natural greenhouse gases, but it’s not classified with the others because it acts as a climate feedback, not a driver.

“What’s the difference?”

A climate driver, or a climate forcing, is something that pushes the climate along in a certain direction on a sustained basis.  Water vapour doesn’t do that.  Water vapour responds.  It amplifies the effect of carbon dioxide, methane, and other climate drivers, by responding to heat.  Something like CO2 can be both driver and feedback, but water vapour’s role is as a feedback.

It’s easy to understand why when you think of it.


Add heat to the climate system, Sam, and water evaporates.  As a vapour, it out-greenhouses the other main greenhouse gases.  For a while.  But then it rains.  Or turns into dew.  Or dumps snow on Washington, D.C.  Suddenly, it’s not water vapour anymore, just water.  Or snow.

The reason water vapour does not act as a climate driver is because it loops through the system too quickly to have a sustained effect on its own momentum.  It’s not like CO2, for instance, which lingers for hundreds or thousands of years.

However, as an amplifier of the effect of other greenhouse gases and other climate drivers, water vapour’s role is very large.  Understanding water vapour’s specific role as an amplifier of the effect of other greenhouse gases is a vital part of understanding the mechanism of global climate change.  It comes down to two questions, really.  One, is there a strong and positive water vapour feedback?  And, two, do the climate models accurately reflect the way water vapour acts in Earth’s climate?

“What do you mean by that? –“positive feedback””

Sound a little touchy-feely to you, Sam?  It has nothing to do with that.  When talking climate, “positive feedback” means it reinforces the effect of a climate forcing; “negative feedback” means it slows or mitigates it.

Now the second question, I’m going to put off dealing with for now since we haven’t begun to talk about climate models yet. …

“Sure, put it off.”

… But the first question, is there a strong and positive water vapour feedback, can be answered:  Yes, and the evidence is overwhelming.

For instance, scientists studied the effect of the eruption of Mount Pinatubo in 1991.  It was the second largest volcanic eruption in the 20th century, and the first that could be tracked using satellites and modern technology.  (The largest happened in Alaska in 1912.)

Volcanoes have a net cooling effect on the Earth’s climate, and one of the ways they cool the climate is through the release of massive amounts of aerosols into the atmosphere.  Aerosols reflect light and thus have the opposite effect on the climate as greenhouse gases.  That is, they cool rather than warm.  And less heat means less evaporation, right?

And that is what happened.  Worldwide, wherever the clouds from Mount Pinatubo cooled the landscape, the relative humidity went down.

“So what?  It was less muggy.”

Well, if you’re lowering the amount of water vapour in the air, then you are dialing down the effect of water vapour as a greenhouse gas.  You should expect the climate to cool even more, to cool more, that is, than the effects of Mount Pinatubo’s aerosols could explain alone.

This also happened.

The study of Mount Pinatubo confirmed the water vapour feedback effect in both aspects:  temperature changes affect relative humidity, and the change in relative humidity reinforces and amplifies the temperature changes.

Studying Mount Pinatubo showed that the effect works when the climate forcing cools the temperature.  Other research concerning El Niño , for instance, shows that the affect works when the climate forcing warms the temperature.  (See Dessler & Wong, 2009, below.)

You can read more about the Mount Pinatubo research here.


Here is the study referred to:

Soden, B. J., R. T. Wetherald, G. L. Stenchikov, and A. Robock (2002), Global cooling after the eruption of Mount Pinatubo: A test of climate feedback by water vapor, Science, 296, 727-730.

And here are some other studies which address water vapour feedback:

Chen, J. Y., Del Genio, A. D., Carlson, B. E., and Bosilovich, M. G. (2008) The spatiotemporal structure of twentieth-century climate variations in observations and reanalyses. Part I: Long-term trend, Journal of Climate 21, 2611–2633.

Dessler, A. E., and S. C. Sherwood (2009), A matter of humidity, Science, 323, doi: 10.1126/Science.1171264, 1020-1021.

Dessler, A. E., and S. Wong (2009), Estimates of the water vapour climate feedback during the El Niño Southern Oscillation, J. Climate, 22, doi: 10.1175/2009JCLI3052.1, 6404-6412.

Dessler, A. E., P. Yang, and Z. Zhang (2008), The water-vapour climate feedback inferred from climate fluctuations, 2003-2008, Geophys. Res. Lett., 35, L20704, doi: 10.1029/2008GL035333.

Forster, P. M. D., and M. Collins (2004), Quantifying the water vapour feedback associated with post-Pinatubo global cooling, Climate Dynamics, 23, 207-214.

McCarthy, M. P., Thorne, P. W., and Titchner, H. A. (2009) An analysis of tropospheric humidity trends from Radiosondes, Journal of Climate 22, 5820–5838.

Sherwood, S. C., Roca, R., Weckwerth, T. M., and Andronova, N. G. (2010) Tropospheric water-vapor, convection, and climate, Reviews of Geophysics 48, RG2001.

Soden, B. J., D. L. Jackson, V. Ramaswamy, M. D. Schwarzkopf, and X. Huang (2005), The radiative signature of upper tropospheric moistening, Science, 310, 841-844.

Willett, K. M., Gillett, N. P., Jones, P. D., and Thorne, P. W. (2007) Attribution of observed surface humidity changes to human influence, Nature 449, 710–716.

The following study presents a different view to the ones given above.

Paltridge, G., A. Arking, and M. Pook (2009), Trends in middle- and upper-level tropospheric humidity from NCEP reanalysis data, Theor. Appl. Climatol., doi: 10.1007/s00704-009-0117-x, 351-359.

Andrew Dessler, who headed up some the above cited studies, comments about the problematical Paltridge study:

There is a recent paper by Paltridge et al. [2009] that shows that water vapor in the tropical upper troposphere in the NCEP/NCAR reanalysis decreased over the past few decades.  I have repeated this calculation with more modern and sophisticated reanalysis data sets (ECMWF interim reanalysis and MERRA reanalysis) and this result does not hold in those data sets.  Given all of the other evidence that the water vapor feedback is positive, all of the ways that long-term trends in reanalyses can be wrong, and lack of verification in more reliable reanalysis data sets, I conclude that the Paltridge et al. result is almost certainly wrong. (See here.)

Posted in: Climate School