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With the development of atmospheric General Circulation Models (GCMs) we can ask such questions. [Shukla and Mintz. 1982. Science 215:1498-1501.] Shukla and Mintz contrasted a world where water on land was not limiting, a world where as much evapotranspiration as possible takes place, with a world in which the green evaporators were gone. In the world without stomates it was 15 to 25 C warmer for land areas north of 20 S. There was 125 Watts per meter squared less evaporative cooling. No bio-air conditioning! Since air got less water vapor, the calculated cloudiness was less and more sunlight got to the ground, 172 to 258 Watts per meter squared, and thus even greater sensible heating of the air by the ground (148 Watts per meter squared more than in our stomate rich world). Sort of like a shopping center parking lot.

Without transpiration, thermal low pressure cells developed over the continents (average continental surface pressures fell by 5 to 15 mb -- that is a lot!] The excess upward motions over the continents in these thermal lows are compensated for by sinking air over the oceans building the strength of the subtropical anticyclones, especially the North Pacific where pressures 10 mb higher than we experience (Lots of fair days in Hawaii!).

With such mean pressure field changes, the circulation of the atmosphere would be different including greater equatorial pacific trade winds (speedy transit for Christopher Columbus to the new world), greater maritime flow into Alaska a bonanza for Bonanza Creek) and very strong equatorward flow along the western littorals of the Americas (more smog holding capacity for you guys in LA!). There would also be a significant northward displacement of the jet stream northward.

Rainfall: Europe and Asia had almost no rainfall in the transpiration free world.
It rained like it does in our world only in places like India and Southeast Asia (near tropical waters). Over North America only 1 mm per day of model rain fell to the earth in the transpiration free world (that is about 14 inches per year and who needs trees anyway). That North America gets even that much is because of our proximity to the sea on three sides. Poor dry central Asia! In our world central Asia relies on recycled water from transpiration from the vegetation elsewhere in central Asia. The Himalayas to the south are a dam to the moist tropical air from the Indian Ocean.

Well, all these changes are of little worry in that the real world, non-GCM model experiment can't be done. But this computer assisted thought experiment puts into view the role of life on earth. The physical world of weather and climate is not so immune to the will of the biosphere. Now for the Shukla and Mintz concluding paragraph: "Finally, on the question of whether the earth's vegetation cover and its modification by man have a significant influence on climate, and whether deforestation and afforestation, soil destruction and soil reclamation, or crop irrigation appreciably affect rainfall; the answer given by this study is that they do, if they are of large magnitude and large horizontal extent." Shukla and Mintz's stomate-free world is like ours would be if life had never left the sea! It is clear that at the planetary evapotranspiration controls atmospheric circulation, weather and climate to a very substantial degree. In your future schematics, point your climate-vegetation arrow both ways with large magnitude. (Also see the blog “Bullish on Clouds”)

Photo: Luca Galuzzi,, licensed under the Creative Commons Attribution-Share Alike 2.5 Generic license.


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