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How forests attract rain: A new hypothesis

Last modified June 03, 2009 08:25

The role of vegetation in the earth hydrological cycle remains controversial. Local people in many partially forested regions believe that forests “attract” rain, but climatology had until recently no scientific explanation for that believe. [I have heard these believes expressed myself regularly in Africa and Asia when having discussions with local farmers. I could only answer that there was no theory to explain this, particularly not as a local scale effect. KS.]

Dear Mohan Reddy Vishwavaram,
thanks for your intervention.
What you are arguing is in line with the recent

Sheil D, Murdiyarso D (2009) How forests attract rain: an examination of a new hypothesis. Bioscience 59:341-347.

My summary (KS):

"The role of vegetation in the earth hydrological cycle remains controversial.
Local people in many partially forested regions believe that forests “attract” rain, but climatology had until recently no scientific explanation for that believe. [I have heard these believes expressed myself regularly in Africa and Asia when having discussions with local farmers. I could only answer that there was no theory to explain this, particularly not as a local scale effect. KS.]

A new hypothesis suggests that these local people may be correct and that forest cover plays a much greater role in determining rainfall than previously recognized. Deforestation has already reduced vapor flows derived form forests by almost 5% per year with little sign of slowing. The need for understanding how vegetation cover influences climate has never been more urgent. Makarieva et al. (2006) and Makarieva and Gorshkov (2007) have developed a hypothesis to explain how forests attract moist air and how continental regions such as the Amazon basin remain wet. The implications are substantial.

The above and what follows is based on or taken from Sheil and Murdiyarso (2009), who have attempted to explain the basic ideas of Makarieva and Gorshkov and their significance for a wider audience. See also Hance (2009) and Pearce (2009) about this paper. They state that deforestation has been implicated as contributing to declining rainfall in various regions (including the Sahel, West Africa, Cameroon, Central Amazonia, and India), as well as weakening monsoons, but the links remained uncertain. Observations suggest that extensive deforestation often reduces cloud formation and rainfall, and accentuates seasonality. Forest clearings can cause a distinct, convection driven “vegetation breeze” in which moist air is drawn out of the forest. Atmospheric turbulence resulting from canopy roughness and temperature driven convection are thought to explain the localized increase in rainfall sometimes associated with fragmented forest cover. Much of the latter statements were taken by Sheil and Murdiyarso (2009) from Bonan (2008). Researchers have previously puzzled over a missing mechanism to account for observed precipitation patterns and Makarieva and Gorshkov’s hypothesis offers a solution they call a “pump”.

Pressure gradients driven by temperature and convection are considered to be principal drivers of air flows in conventional meteorological science. Makarieva and Gorshkov argue that the importance of evaporation and condensation have been overlooked. At the global average lapse rate water vapour rises and condenses. The reduction in atmospheric volume that takes place during this gas to liquid phase change causes a reduction in air pressure. This drop in pressure has routinely been overlooked. So atmospheric volume reduces at a higher rate over areas with more intensive evaporation. The resulting low pressure draws in additional moist air from areas with weaker evaporation. This leads to a net transfer of atmospheric moisture to the areas with the highest evaporation. Sheil and Murdiyarso (2009) discuss various local consequences. Forest loss and diminished evaporation can for example reduce the penetration of monsoon rains and reduce the duration of the wet season. Clearing enough forest within a larger forest zone may switch net moisture transport “from ocean to land” into “from land to ocean”, leaving forest remnants to be dessicated. Clearing a band of forest near the coast may suffice to dry out a wet continental interior.

Makarieva and Gorshkov’s hypothesis suggests that forest loss will be associated with a loss of stabilizing feedbacks and increased climatic instability. In Brazil’s Atlantic forests just such a correlation has been detected between reduced tree cover and increased local interannual variation in rainfall. Have forests evolved to generate rain? This idea touches on the much-debated possibilities of emergent self-stabilizing behavior (or “Gaia”). Trees and forests have evolved numerous times in the history of the earth, suggesting a repeated trend to generate rich self-watering terrestrial habitats. There is scope for self-stabilizing interactions to arise. We need to unravel the feedback processes and thresholds that operate spatially at different scales, and the influences that act upon them.

Acceptance of the biotic pump would add to the values that society places on forest cover. By raising regional concerns about water, acceptance of the biotic pump demands attention from diverse local actors, including many who may otherwise care little for maintaining forest cover".

With best wishes, Kees Stigter, founding president of INSAM

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