Water Vapor Feedback and Global Wetting

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Even the most ardent climate change denier acknowledges that CO2 concentrations in the atmosphere have increased.  Most will even admit that CO2 does indeed absorb heat energy, making it a greenhouse gas.  However, many of the denier arguments start from the assumption that the heat absorbed by the extra CO2 is not enough to drive changes in the Earth's climate.  In once sense, they're right.  The expected increase in CO2 in 2050 would only lead to 0.5°C of warming by 2050.

Of course, that last sentence leaves off the key phrase: "if nothing else changed."  The 0.5°C number, which I poached from Hansen's book, assumes that nothing else changes.  In reality, if you change one part of the climate system, in this case, by adding a little bit of extra heat, that change will cause other changes in the atmosphere and ocean.  If these other changes lead to more heating, then we have a positive feedback loop that will take a little bit of warming by CO2 and turn it into a  larger heating.  Climate scientists have identified many feedbacks in the climate system (atmosphere, ocean, and ice).  Some of these feedbacks are positive (they amplify warming), while some are negative (they counteract warming).  The earth's "climate sensitivity" is then the sum of all of these feedbacks.  Many lines of evidence suggest that the climate sensitivity is strongly positive, that is that a small increase in CO2 leads to a larger amount of warming.    

My favorite climate feedback is the water vapor feedback.  Like CO2, water vapor (H2O) is a greenhouse gas.  If the amount of water vapor in the atmosphere increases, then the atmosphere will trap more heat and the planet will warm.  The basic idea behind the water vapor feedback is

1. The atmosphere contains a mixture of gases.  The ability of these gases to absorb heat is represented by the gray haze in the image:
wv_001.jpg
2. CO2 in the atmosphere increases.  Now, the atmosphere absorbs more heat (more gray haze),  leading to a small amount of warming in both the atmosphere and the ocean:
wv_003.jpg
3. The warming increases the rate of evaporation over the ocean (blue arrows), leading to more H20 in the atmosphere:
wv_005.jpg
4.  The water vapor absorbs more heat (more haze), and the Earth gets warmer:
wv_007.jpg
Of course, the warming from water vapor isn't any different than that from CO2, so the new warming will lead to more water vapor and more warming, etc, but I'm sick of making pictures.

While the warming from the water vapor feedback is important, warmer temperatures are only one consequence.  Water vapor in the atmosphere tends to move from warm areas like the tropics, where there is a lot of evaporation, to cooler areas, like our own temperate latitudes.  At these latitudes, the additional water vapor leads to more rain and snow, meaning that global warming will lead to wetting over large parts of the globe.  

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This page contains a single entry by Andy Pershing published on April 26, 2011 5:20 PM.

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