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.
The main problem with water on land is that it cannot be accumulated there once and for all. As far as the landmasses are elevated above the sea level, all water accumulated on land in soil, lakes, rivers, glaciers etc. ultimately leaves to the ocean under the force of gravity. To keep land moistened and, hence, terrestrial life thriving, it is necessary to continuously compensate the gravitational runoff of water from the continents by a reverse, ocean-to-land, atmospheric moisture flow. If there is no influx of atmospheric moisture to land from the ocean, all water accumulated on land in soil, lakes and rivers will be lost via runoff in but a few years!
Fig. 1. Regions of the world where the dependence of precipitation on distance from the ocean was studied.
Moisture evaporated from the oceanic surface is delivered to land by winds. As the moisture-laden oceanic air flows inland from the coast, some of its moisture precipitates and is lost to runoff. Therefore, from the geophysical point of view, the deeper inland, the less the moisture content of the flow, the less the rainfall.
This pattern is indeed observed in deforested regions over the globe. Using the rainfall station data we found that annual precipitation P (mm year−1) on territories deprived of natural forest cover decreases exponentially with distance x from the ocean with an e-folding length of no more than several hundred kilometers, Fig. 2a. This means that the passive geophysical atmospheric flows can only moisten a narrow band of land near the coast. The much more extensive inner parts of the continents invariably remain arid.
Fig. 2. Annual precipitation P versus distance x from the source of moisture on deforested territories (a) and in natural forests (b).
In sharp contrast with this pattern, precipitation over territories covered by the remaining natural forests of the Earth (Amazonia, Equatorial Africa, Siberia) does not decrease with distance from the ocean and may even grow over several thousand kilometers, Fig. 2b.
From the purely geophysical point of view, this remarkable phenomenon is quite unexpected and unexplainable. For example, the innermost part of the deforested North-East China (region 2 in Figs. 1 and 2a) at about 2000 km from the Pacific ocean coast receives as little as 100 mm annual precipitation. For comparison, the innermost part of the Yenisey river basin covered by extensive natural forests (region 8 in Figs. 1 and 2b) is many thousand kilometers away from any of the oceans, representing one of the innermost continental areas on the planet, see Fig. 1. In spite of this unfavorable geophysical position, the upper reaches of Yenisey receive as much as 800 mm annual precipitation, and the forests flourish.
High precipitation creates high soil moisture content, which, in its turn, maximizes biological productivity. With natural selection coming into play, higher productivity is associated with higher competitive capacity. Therefore, evolution of terrestrial life forms can be expected to culminate in a state when entire continents are covered by ecosystems functioning at a maximum possible power limited by solar radiation only. In such a state soil moisture content and, hence, local water losses to runoff, should be equally high at any distance from the ocean. This means that the most productive, most competitive ecological communities (i.e. forests) must be able to transport moisture inland from the ocean in quantities sufficient for compensation of the high runoff losses associated with the high soil moisture content. The existence of such forest moisture pumps resolves the geophysical enigma of high precipitation and intense water cycles in the inner parts of the forest-covered continents.
Fig. 3. Water vapor partial pressure and the evaporative force in the terrestrial atmosphere as dependent on atmospheric height z. (a) Saturated partial pressure of water vapor pH2O(z) and weight of water vapor Wa(z) in the atmospheric column above height z at the observed mean tropospheric lapse rate of 6.5 oC km−1. (b) The upward-directed evaporative force f equal to the difference of the upward directed pressure gradient force f↑, which is associated with the non-equilibrium vertical distribution of the atmospheric water vapor, and the downward directed weight f↓ of a unit volume of the saturated water vapor.
Having established that the forest pumps of atmospheric moisture must exist, it was necessary to investigate the physical principles of their functioning. These proved to be non-trivial. Atmospheric air is in approximate hydrostatic equilibrium, which means that at any height z air pressure is balanced by the weight of atmospheric column above z. In contrast, atmospheric water vapor is out of hydrostatic equilibrium. At any height z the partial pressure of water vapor is about four times larger than the cumulative weight of water vapor above z, Fig. 3a. This effect, well-known from observations but so far unappreciated in its importance, has to do with the observed vertical gradient of atmospheric temperature, which decreases with height by approximately 6.5 oC km−1. Water vapor is a condensable gas. The maximum amount of water vapor that can be held in the air exponentially declines with decreasing air temperature. Hence, when the air temperature drops sufficiently rapidly with height, the upper atmosphere appears to be so cold that it cannot hold enough water vapor for its weight to balance the high partial pressure of water vapor in the lower, warmer atmosphere.
As far the cumulative pressure of the moist air exceeds the weight of atmospheric column, there appears an upward-directed force, Fig. 3b, which causes air and water vapor to ascend. At the surface, the ascending air volumes must be replaced by the air flowing horizontally from the neighboring areas into the region of ascent. Provided they are appropriately directed, these horizontal air fluxes can be used by the biota to transport of moisture from the ocean.
When the water vapor ascends, it enters the cooler upper atmospheric layers, condenses and precipitates. The upward-directed force driving the ascending and, hence, horizontal, air motions is caused by the vertical distribution of water vapor, so when the amount of water vapor in the atmosphere diminishes, the force diminishes as well and the atmospheric motions slow down. The stationary value of this force, which can be termed the evaporative force, is determined by the rate at which water vapor is added into the atmosphere to compensate for its condensation, i.e. by the rate of evaporation. Based on these physical considerations, the fundamental physical principle can be formulated that horizontal fluxes of air and water vapor are directed from areas with weaker evaporation to areas with stronger evaporation.