How Forests Attract Rain: An Examination of a New Hypothesis, Sheil and Murdiyarso 2009

Compendium Volume 2 Number 1 July 2018

Highlighting the significance of Makarieva and Gorshkov’s “biotic pump” hypothesis (above), Sheil and Murdiyarso explain it in layman’s terms in this article for the benefit of a broader public, and examine its validity. They point out that the biotic pump hypothesis offers an explanation for a question not otherwise resolved in conventional climate theory.

Conventional theory offers no clear explanation for how flat lowlands in continental interiors maintain wet climates. Makarieva and Gorshkov show that if only “conventional mechanisms” (including [rain] recycling) apply, then precipitation should decrease exponentially with distance from the oceans. Researchers have previously puzzled over a missing mechanism to account for observed precipitation patterns (Eltahir 1998) [Sheil & Murdiyarso 2009: 342].

They explain the biotic pump hypothesis and how it resolves the puzzle:

Air currents near Earth’s surface flow to where pressure is lowest. According to Makarieva and Gorshkov, these are the areas that possess the highest evaporation rates. In equatorial climates, forests maintain higher evaporation rates than other cover types, including open water. Thus, forests draw in moist air from elsewhere; the larger the forest area, the greater the volumes of moist air drawn in. This additional moisture rises and condenses in turn, generating a positive feedback in which a large proportion of the water condensing as clouds over wet areas is drawn in from elsewhere. The drivers (solar radiation) and basic thermodynamic concepts and relationships are the same as in conventional models, thus most behaviors are identical— the difference lies in how condensation is incorporated.

Makarieva and Gorshkov’s estimates, incorporating volume changes from condensation, imply that when forest cover is sufficient, enough moist air is drawn in to maintain high rainfall inside continents. The numbers now add up: thus, condensation offers a mechanism to explain why continental precipitation does not invariably decline with distance from the ocean [Sheil & Murdiyarso 2009: 342].

Commenting on the relevance of the hypothesis, the authors conclude:

Acceptance of the biotic pump would add to the values that society places on forest cover. By raising regional concerns about water, acceptance of Makarieva and Gorshkov’s biotic pump demands attention from diverse local actors, including many who may otherwise care little for maintaining forest cover [Sheil & Murdiyarso 2009: 346].

Sheil, Douglas & Daniel Murdiyarso, 2009, How forests attract rain: an examination of a new hypothesis, BioScience 59:4,

For the full PDF version of the compendium issue where this article appears, visit Compendium Volume 2 Number 1 July 2018