26 April 2013 [BR for Everyone]
Biotic pump and temperature in Scientific American

Article in Scientific American explores the link between the biotic pump and the surface temperature changes caused by deforestation.

In brief, the effect is as follows. Consider hot land bordering with a cold ocean. Let us imagine that, for some reason, moist saturated air starts traveling from the ocean inland. Then, as the air enters the warm continental areas, the air temperature grows and water vapor becomes unsaturated. If the land is humid, water vapor will be added to the incoming air via evaporation. When the water vapor is added, the air pressure grows (as it falls when vapor condenses and disappears from the gas phase).

Thus, if the land-ocean temperature difference is sufficiently large, this evaporation into the incoming air and the associated pressure rise can overcome the pressure fall associated with condensation in the ascending and cooling air. In such a case, no net pressure gradient may form. As a result, the hot and moist land will be locked for atmospheric moisture.

This creates a peculiar situation. If the air is dry, the thermally driven convection (i.e. the warm air rising above the desert) could make the cold air from elsewhere converge to the warm and dry desert. But that will not make any positive effect on the hydrological cycle -- the dry air does not bring rain. If, on the other hand, the air is moist, there will be an increase of water vapor partial pressure as the air moves towards higher temperatures. This effect will diminish the condensation-induced pressure gradient and can, under certain circumstances, erode it altogether.

This means that where there is deforestation and the local temperatures grow because of decreased transpiration (a well-known effect), this temperature increase creates unfavorable conditions for the penetration of moist air to the continent. Dry air might be welcome, but not moist.

Quantitatively, this effect was described in two papers: in the International Journal of Water (2010) and in a new paper in Russian here. (See p. 12 and Fig. 8 here for how it applies to the Amazon basin.) It explains why the monsoon winds cannot penetrate deep into a deforested continent, e.g. into Sahara or Australian desert. Indeed, as soon as the first rains moisten the poorly vegetated land (often causing floods), there is an intense evaporation from the hot moist surface into the incoming air. This destroys the needed pressure differences that would otherwise drive the moist air further into the continental interior.

Therefore, it is very important that forests maintain low surface temperature such that the temperature gradient between land and ocean is minimized. The biotic pump works most efficiently when the horizontal temperature difference is small, like between the Amazon forest and the Atlantic ocean. Deforestation and loss of the biotic temperature control on land can be expected to be impairing the regional water regime.

In other words, not only the high transpiration of forests (something that we have been always mentioning) but also their capability to ensure local (not global) cooling of land surface is important for the biotic pump. So to speak, transpiration acts constructively to make the biotic pump working, while the biotically mediated surface cooling prevents the unfavorable conditions that could interfere with its work.

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