May 28, 2008
Climate Engineering Proposals Seen As Cutting Global Rainfall

Researchers at Lawrence Livermore National Laboratory, led by atmospheric scientist Govindasamy Bala, find that climate engineering to cancel the warming effects of CO2 would reduce net global rainfall.

In the new climate modeling study, which appears in the May 27-30 early online edition of the Proceedings of the National Academy of Sciences, Bala and his colleagues Karl Taylor and Philip Duffy demonstrate that the sunshade geoengineering scheme could slow down the global water cycle.

The sunshade schemes include placing reflectors in space, injecting sulfate or other reflective particles into the stratosphere, or enhancing the reflectivity of clouds by injecting cloud condensation nuclei in the troposphere. When CO2 is doubled as predicted in the future, a 2 percent reduction in sunlight is sufficient to counter the surface warming.

This new research investigated the sensitivity of the global mean precipitation to greenhouse and solar forcings separately to help understand the global water cycle in a geoengineered world.

While the surface temperature response is the same for CO2 and solar forcings, the rainfall response can be very different.

“We found that while climate sensitivity can be the same for different forcing mechanisms, the hydrological sensitivity is very different,” Bala said.

The global mean rainfall increased approximately 4 percent for a doubling of CO2 and decreases by 6 percent for a reduction in sunlight in his modeling study.

“Because the global water cycle is more sensitive to changes in solar radiation than to increases in CO2, geoengineering could lead to a decline in the intensity of the global water cycle” Bala said.

Sunlight is probably more important than global atmospheric since the sunlight hits the surface of the oceans and cause more localized heating right on the surface of the oceans where the heat does the most to cause water to evaporate.

I can imagine at least one method to counteract the reduction in water evaporation: Use large floating windmills on the ocean to pump up and spray water into the atmosphere. But I have no idea what scale of windmills would be needed to do this. I suspect it would not be cost effective.

Here's another idea: Use satellites to block sunlight from hitting Earth. But only block the sunlight over land. That way the oceans would still get the full force of solar radiation. Still, satellites are a far more expensive way to reduce solar radiation as compared to spraying aerosols into the atmosphere. So this approach has problems as well.

Anyone have a good idea on how to climate engineer to lower temperatures without lowering water evaporation from the oceans?

Share |      Randall Parker, 2008 May 28 10:45 PM  Climate Engineering


Comments
TangoMan said at May 29, 2008 12:03 AM:

Randall,

I can imagine at least one method to counteract the reduction in water evaporation: Use large floating windmills on the ocean to pump up and spray water into the atmosphere. But I have no idea what scale of windmills would be needed to do this. I suspect it would not be cost effective.

The solution you offer to solve the follow-on problem of the reduced water cycle caused by a global warming solution is a solution in its own right. Check out the work of John Latham and view his solution in this documentary (minute 14:00 to minute 25:00)

Allan said at May 29, 2008 5:02 AM:

First of all, I don't believe in AGW ... but for the sake of discussion, I read somewhere that as the earth warms and there is less snow on the ground then even more heat is retain ... due to less heat being reflected back into space.

Sooooo, if that is true, then we should all put shiny tin roofs on our houses like my grandmother had.

Actually this thought occurred to me the other day as I was thinking about how to shade the a/c on top of a friends house. I was thinking I could rig up a something shiny to shade the a/c and maybe even angle it to reflect some more light onto her solar panels.

Brett Bellmore said at May 29, 2008 11:00 AM:

I can see serious complaints from farmers if we used orbital shades to keep sunlight off their fields. If you want to cool the Earth efficiently, I would think that the best place to deny sunlight to would be the extremely dark, and biologically unproductive ocean areas. This wouldn't require as much shading to intercept a given degree of heating, and would have much less impact on the amount of energy entering the food chain.

It could probably result in about the same amount of rain *over land*, if targeted properly.

Ben said at September 24, 2008 10:24 AM:

The satellite option does not have to expensive. In fact, it can be much cheaper than aerosols (not to mention reversible). You can send up a machine that assembles the shade in space, rather than sending up the shade itself.
I recently saw a story about a sheet that is just one atom thick: http://www.sciencedaily.com/releases/2008/09/080922122519.htm

If anyone has any feedback on this idea please email me at itstheemes@gmail.com

Thank you

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