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Main Findings

The text below was largely composed in November 2006. The Russian version of "Main Findings" is based on a seminar given in the Theoretical Physics Division of Petersburg Nuclear Physics Institute in January 2008. The English and Russian versions are somewhat different in structure. Besides, Russian version contains an additional section on hurricanes.

Biotic pump of atmospheric moisture
Fig. 4(1). Due to their high leaf area index, natural forests maintain high transpiration fluxes (thick dark blue arrow), which exceed the evaporation fluxes over the ocean (thin dark blue arrow). The evaporated moisture undergoes condesation and disappears from the gas phase. Air in the atmospheric column above the forest rarifies. In the result, there appears ascending air motion over the forest canopy, which, in its turn, "sucks in" moist air from the ocean (light blue arrow). It then returns to the ocean in the upper atmosphere (dotted arrow) after precipitation of moisture over the continent.
 
Desert is locked for moisture
Fig. 4(2). The negligible evaporation from the ground surface in deserts (thin black arrow) locks them for the flux of the moisture-rich surface oceanic air.
 
Monsoon circulation
Fig. 4(3). (a) Winter monsoon: evaporation from the warmer oceanic surface is larger than evaporation from the colder land surface; surface air moves from land to the ocean. (b) Summer monsoon: evaporation from the warmer land surface is larger than evaporation from the colder oceanic surface; surface air moves from ocean to land.

It is here that the decisive role of natural vegetation in keeping land moistened becomes evident. Plant leaves are powerful sources of water vapor. Water is supplied by the roots to the leaves and evaporates into the atmosphere through stomata (leaf openings), which can be closed or opened at the plant's discretion. Solar radiation is the ultimate source of energy for this biological process called transpiration. Natural forests feature high leaf area index, i.e. there are several (up to a dozen) leaf blades along the vertical. Therefore, evaporation from the forest canopy (i.e., from several leaf planes) can greatly exceed evaporation from the one plane of the open water surface of the ocean. The evaporative force, which causes air to ascend, is larger over the natural forest canopy than over the ocean. In the result, the moisture-rich oceanic air flows to the forest-covered continent, Fig. 4(1). As it ascends and its moisture precipitates, the air dries out and returns to the ocean in the upper atmosphere.

What happens when the natural vegetation cover is completely destroyed? In deserts evaporation from the ground surface is practically absent, while oceanic evaporation is always substantial. The upward-directed evaporative force is always greater over the ocean than in the desert. It makes oceanic air rise and effectively "sucks in" the desert air to the ocean, Fig. 4(2), where it replaces the rising oceanic air masses at the oceanic surface. Thus, due to the absence of evaporation from the ground surface, deserts appear to be locked for oceanic moisture year round, Fig. 4(2).

In the intermediate case when the natural forests with their high leaf area index are destroyed, but there remains some vegetation cover on land, horizontal air fluxes can be directed either from the ocean to land or vice versa depending on the environmental conditions, in particular, surface temperature (e.g., the monsoon climate), Fig. 4(3). The annual amount of precipitation received by land will be then intermediate between the cases of natural forest and desert.

Blossoming flower

We believe that the results obtained are novel and original in two main aspects. First, it is shown for the first time that the vegetation cover fully determines the magnitude of the incoming ocean-to-land flux of moist air, Figs. 4(1-3). So far in all evaluations of the biotic impact on water cycle this magnitude has been considered as an abiotic geophysical constraint imposed on the ecosystem. The role of ecosystems in maintaining the terrestrial water cycle has been thought to be reduced to minor impacts like changing the ratio of runoff R and evaporation E. Indeed, if one puts P = E + R, i.e. all water precipitated at rate P either evaporates at a rate E or runs off to the ocean at a rate R, then, at constant P, an increase in evaporation E associated with a richer vegetation would mean less runoff. This is a common statement in modern ecohydrology. Indeed, one can often hear that if one cuts forests, one gets more streamwater. The flaw of such reasoning is revealed in our approach. When the forests are cut, not only E, but also P itself approaches zero, because the incoming fluxes of moist oceanic air weaken and ultimately cease to exist, cf. Figs. 4(1) and 4(2).

Hadley circulation
Fig. 4(4). Hadley circulation (trade winds): evaporation is more intensive on the equator, where the solar flux is larger than in the higher latitudes; surface air flows towards the equator year round; seasonal displacements of the convergence zone follow the displacement of the area with maximum insolation.

Second, until now the proposed physical principle predicting that air must flow from the areas with weaker evaporation to areas with intensive evaporation and the introduced evaporative force as atmospheric circulation driver have never been considered in the atmospheric science. These findings are likely to invoke important revisions of our current understanding of global atmospheric circulation. For example, they provide a novel explanation for Hadley circulation, Fig. 4(4).

As far as in the stationary case solar radiation is the source of energy for evaporation, the increase of solar flux towards the equator should be accompanied by a corresponding increase of the flux of evaporation and the evaporative force. The intensive ascent of moist air on the equator has to be compensated by horizontal air fluxes originating at higher latitudes and moving towards the equator, where they ascend and travel back in the upper atmosphere, Fig. 4(4). Subsidence of dry air masses at the non-equatorial border of Hadley cells diminishes water vapor concentration in these areas, producing an additional, unrelated to the geography of solar radiation, decrease of the evaporative force. This creates favorable conditions for Ferrel circulation, i.e. movement of air masses from subtropics to higher latitudes.

Blossoming flower

Post-deforestation precipitation drop
The derived estimate for the deforestation-caused decrease of annual precipitation in a local area at distance x from the ocean initially situated in an extensive forest-covered river basin. Pf(x) is the annual local precipitation in the forested river basin, Pd(x) is the post-deforestation annual precipitation in the same area, l ~ 600 km. For example, for the inner continental areas located at over 1000 km from the ocean coast, deforestation causes at least a fivefold reduction of annual precipitation.

To summarise, the results obtained show that the precipitation regime on land is fully determined by the degree of disturbance of the natural vegetation cover. We calculate that elimination of the forest cover in world's largest river basins would have the following consequences: at least one order of magnitude's decline of the river runoff, appearance of droughts, floods and fires, significant desertification of the coastal zone and complete desertification of the inner parts of the continents. Conversely, gradual restoration of natural tree cover will lead to restoration of human friendly water conditions on most part of the Earth's landmasses, including modern deserts and other arid zones. It is necessary to immediately stop any attempts of destroying the extant natural forest remnants and, in particular, those bordering with the ocean or inner seas. Further on, it is necessary to initiate a world-wide company on facilitating natural gradual recovery of aboriginal forest ecosystems on territories adjacent to the remaining natural forests. Only extensive contiguous natural forests will be able to run a stable water cycle and subsequently intensify it, gradually extending the river basins at the expense of newly recovering territories.