A new study Identification of human-induced changes in atmospheric moisture content (Santer 2007) was published last week, inspiring me to revisit the water vapour argument. A popular skeptic argument (well, a ranking of #20 is no mean effort) is that water vapour is the most important greenhouse gas, rendering CO2 warming relatively ineffective. Water vapour is indeed the most dominant greenhouse gas. The radiative forcing for water is around 75 W/m2 while carbon dioxide contributes 32 W/m2 (Kiehl 1997). Water vapour is also the dominant positive feedback in our climate system and a major reason why temperature is so sensitive to changes in CO2.
Unlike external forcings such as CO2 which can be added to the atmosphere, the level of water vapour in the atmosphere is a function of temperature. Water vapour is brought into the atmosphere via evaporation - the rate depends on the ocean and air temperature and is governed by the Clausius-Clapeyron relation.
If extra water is added to the atmosphere, it condenses and falls as rain or snow within a week or two. Similarly, if somehow moisture was sucked out of the atmosphere, evaporation would restore water vapour levels to 'normal levels' in short time.
As water vapour is directly related to temperature, it's also a positive feedback - in fact, the largest positive feedback in the climate system (Soden 2005). As temperature rises, evaporation increases and more water vapour accumulates in the atmosphere. As a greenhouse gas, the water absorbs more heat, further warming the air and causing more evaporation.
How does water vapour fit in with CO2 emissions? When CO2 is added to the atmosphere, as a greenhouse gas it has a warming effect. This causes more water to evaporate and warm the air more to a higher (more or less) stabilized level. So CO2 warming has an amplified effect, beyond a purely CO2 effect.
How much does water vapour amplify CO2 warming? Without any feedbacks, a doubling of CO2 would warm the globe around 1°C. Taken on its own, water vapour feedback roughly doubles the amount of CO2 warming. When other feedbacks are included (eg - loss of albedo due to melting ice), the total warming from a doubling of CO2 is around 3°C (Held 2000).
The amplifying effect of water vapor has been observed in empirical studies such as Soden 2001 which observed the global cooling after the eruption of Mount Pinatubo. The cooling led to atmospheric drying which amplified the temperature drop. A climate sensitivity of around 3°C is also confirmed by numerous empirical studies examining how climate has responded to various forcings in the past.
Satellites have observed an increase in atmospheric water vapour by about 0.41 kg/m² per decade since 1988. A detection and attribution study (Santer 2007), otherwise known as "fingerprinting", was employed to identify the cause of the rising water vapour levels. Fingerprinting involves rigorous statistical tests of the different possible explanations for a change in some property of the climate system.
Results from 22 different climate models (virtually all of the world's major climate models) were pooled and found the recent increase in moisture content over the bulk of the world's oceans is not due to solar forcing or gradual recovery from the 1991 eruption of Mount Pinatubo. The primary driver of 'atmospheric moistening' was found to be the increase in CO2 caused by the burning of fossil fuels.
Basic theory, observations and climate models all show the increase in water vapor is around 6 to 7.5% per degree Celsius warming of the lower atmosphere. The observed changes in temperature, moisture, and atmospheric circulation fit together in an internally and physically consistent way. When skeptics cite water vapour as the most dominant greenhouse gas, they are actually invoking the positive feedback that makes our climate so sensitive to CO2 as well as another line of evidence for anthropogenic global warming.
Posted by John Cook on Sunday, 30 September, 2007
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