"The experimental results therefore provide evidence that condensation and not buoyancy is the major mechanism driving airflow, thus lending strong support to one of the main tenets of the [Biotic Pump Theory]."
The "Biotic Pump Theory"
"maintains that the primary motive force of atmospheric circulation derives from the intense condensation and sharp pressure reduction that is associated with regions where a high rate of evapotranspiration from natural closed-canopy forests provides the "fuel" for cloud formation. The net result of the "biotic pump" theory is that moist air flows from ocean to land, drawn in by the pressure changes associated with a high rate of condensation. "
The authors conclude,
"The general implications are that the great forests of the world play a fundamental role in air mass circulation through providing water vapour via evapotranspiration, and are therefore the “fuel” for a high rate of cloud condensation. Airflow circulation is the net result, bringing the rain to the deep interior of continents. The Biotic Pump theory suggests that the hydrological role of rainforests is by far their most important climatic contribution, and that large-scale deforestation may well be as detrimental in its consequences for the well-being of the planet as are greenhouse gas emissions. Indeed, it may be that in macro-climatological terms whether forests are net absorbers or emitters of greenhouse gases is relatively insignificant compared to their hydrological role."
Experimental evidence of condensation-driven airflow
Atmospheric convection, which leads to air mass circulation, is generally considered to result from the lower atmosphere acting as a heat engine, with the kinetic energy for convection deriving from differences in temperature, according to the general principle that hot air rises and cold air sinks. However, as Makarieva et al. (2013) point out, when hot air rises in the lower atmosphere it cools because of expansion and when the same, but now cooler, air sinks it heats up, such that the overall gain or loss in kinetic energy is zero. The same cooling and heating happens when air expands and forces air elsewhere to compress; there is no net energy gain to do work. In other words, a strict application of the first law of thermodynamics to the atmosphere would yield a rate of kinetic energy generation equal to zero. Instead, the same authors (2013) present the notion that the potential energy, derived from an outside source (the Sun), is stored in the evapotranspiration of water which, on condensing, converts into kinetic energy, and so drives the process of air mass convection. During daylight hours closed-canopy forests pump more than double the quantity of water vapour per square metre of surface compared to the ocean in the same latitude, the net result being that condensation in cloud-forming over the forest causes surface air to flow upwards, thereby generating low pressure at the surface which, in turn, establishes an ocean-to-land pressure gradient (Makarieva et al., 2013, 2014). By means of evapotranspiration, rainforests, whether in the equatorial tropics or in boreal regions during summer months, feed the lower atmosphere with water vapour, up to some 3 % of atmospheric pressure, and thereby provide the source material for cloud condensation. The partial pressure change, with the corresponding kinetic energy release, drives convection, according to the biotic pump theory (BPT). From that point of view, it is the hydrological cycle, including water evaporation and condensation, which drives convection and therefore the circulation of the air masses. That is in sharp contrast to the orthodox view of convection and air mass circulation, which explains the movement of the air mass through latitudinal differences, helped on by the release of latent heat.
The proposition that a high rate of evapotranspiration from forested regions is a prime mover of major air mass convection has remained contentious. Meesters et al. (2009) rejected the BPT on the grounds that the ascending air motions induced by the evaporative/condensation force would rapidly restore hydrostatic equilibrium and thereby become extinguished. In reply Makarieva et al. (2009) pointed out that condensation removed water vapour molecules from the gas phase and reduced the weight of the air column. That removal must disturb hydrostatic equilibrium and make air circulate under the action of the evaporation/condensation force (Makarieva, 2009). The mass of an air column is equal to the number of air molecules in the column multiplied by their molecular masses. When the number of air molecules in the column is preserved, its weight remains unchanged and independent of density. Hence, heating of the air column does not change its weight. In contrast, condensation changes the number of gas molecules in the air column and instantaneously reduces the weight of the air column irrespective of the effects it might have on air density (Makarieva, 2009). In effect, the BPT states that the major physical cause of moisture fluxes is not the non-uniformity of atmospheric and surface heating, but that water vapour is invariably upward-directed as a result of the rarefaction of air from condensation (Makarieva, 2013). The BPT, therefore, maintains that the air pressure sustains its disequilibrium because of the reduction in total weight of the air column as condensation occurs, that being a continuous process as the ascending moist air cools. In fact, when the initial bulk air pressure in the lower atmospheric levels no longer equals the bulk weight of the air column, the initial hydrostatic equilibrium of air as a whole is disturbed and an accelerating upward motion is initiated in the air column. This upward motion of expanding and cooling moist air sustains the continuous process of condensation and does not allow the hydrostatic equilibrium of air as a whole to set in. The motion continues as long as there is water vapour in the rising air to sustain condensation (Makarieva, 2009). Within the concept of the biotic pump it is the physical mechanism of condensation which drives the upward airflow in the lower atmosphere by removing molecules from the air column, and thus generates the surface horizontal winds, such as the Trade Winds.
This paper describes a series of experiments, in a specially-designed structure, to test the physics of condensation and its potential to cause airflows. The results have provided data for a careful analysis of the physics involved, showing that condensation and the subsequent release of kinetic energy from the partial pressure change do indeed account for the observed airflow. The experimental results therefore provide evidence that condensation and not buoyancy is the major mechanism driving airflow, thus lending strong support to one of the main tenets of the BPT. The results are significant and unambiguous. At least at the laboratory scale, using only conventional physics, such as is employed in climatological studies, the primary force driving convection appears to be the kinetic energy released in the implosive events which take place during condensation, with a sudden reduction – > 1200-fold – in the air volume of one gram-molecule of water vapour (18 g) as it transforms to liquid and ice. Air density changes are shown to be orders of magnitude less important in convection processes compared to partial pressure changes on condensation. The macro-physics of the experiment is not fundamentally different from that in the lower atmosphere at large. The same laws apply and are widely used by hydrologists, meteorologists and climatologists. Those opposed to the biotic pump theory should therefore re-consider their position and take into account that the physics underlying the theory may not only be correct, but that it operates in the atmosphere at large.
The general implications are that the great forests of the world play a fundamental role in air mass circulation through providing water vapour via evapotranspiration, and are therefore the “fuel” for a high rate of cloud condensation. Airflow circulation is the net result, bringing the rain to the deep interior of continents. The Biotic Pump theory suggests that the hydrological role of rainforests is by far their most important climatic contribution, and that large-scale deforestation may well be as detrimental in its consequences for the well-being of the planet as are greenhouse gas emissions. Indeed, it may be that in macro-climatological terms whether forests are net absorbers or emitters of greenhouse gases is relatively insignificant compared to their hydrological role.