Physicists challenge tropical air-flow theory


A controversial new theory about tropical weather patterns holds that the Amazon rainforest, which plays a huge role in cooling the planet, is far more vulnerable to desertification than current climate models would suggest.

The theory has been developed by two Russian physicists in a series of articles, the most recent of which appeared in the July print edition of Proceedings of the Royal Society. It identifies evapotranspiration primarily from tropical forests as the key driver of air circulation and, thus, rainfall, in watersheds such as the Amazon and Congo river basins. Evapotranspiration, they argue, acts as a gigantic “biotic pump.”

Authored by Anastassia Makarieva and Victor Gorshkov of the Petersburg Nuclear Physics Institute’s theoretical-physics division, the articles have met with strong skepticism among some climate scientists. These critics, voicing a long-standing view among climatologists, attribute Amazon air circulation not to evapotranspiration, but to temperature differentials. Solar radiation warms air to a greater degree over land than over the Atlantic Ocean, they say, prompting the air over the Amazon Basin to rise and thereby create areas of low pressure that draw moist air into the region from the Atlantic. Evapotranspiration, they argue, is largely irrelevant to this process.

The climatologists assert Makarieva and Goshkov provide no convincing evidence to counter this explanation. And they express concern that environmentalists might prematurely embrace the alternative theory simply because it would further boost the case for forest conservation. That, they add, could wind up doing the forest-protection cause more harm than good if the theory is shown to be off-base, as these critics believe it to be.

Michael Coe, a climate scientist with the Woods Hole Research Center in Falmouth, Massachusetts, says Makarieva and Gorshkov’s “methods and assumptions are flawed from beginning to end.” Marcos Costa, a climate scientist from the Federal University of Viçosa, Brazil, is equally critical, saying the Russians’ ideas are “very convenient for hard-core conservationists” out to alarm the world about the severity of the gathering climate crisis. Says Costa: “This [theory] is a matter of convenience, not science.”

But Makarieva and Gorshkov stand by their theory, and they claim the prevailing one it challenges is long overdue for reappraisal. In a recent e-mail to EcoAméricas, they wrote: “A peculiar feature of the modern meteorological community is that the majority believes that science behind atmospheric circulation is clear and settled (‘warm air rises’) … That ‘warm air rises’ has become a dogma that seems to block the normal scientific scrutiny. The fact that a balloon rises because it is warm does not mean that everything that moves does so because it is warm.”

What the Russians have offered, in essence, is a new explanation for the vast, natural recycling system that scientists have long acknowledged as essential for the growth and survival of tropical rainforests. Evapotranspiration—the pumping of water into the lower atmosphere mainly by plants, though also from other sources such as lakes and rivers—effectively converts some rainwater into vapor, which forms cumulonimbus clouds. The clouds then release the vapor as rain, such that in the Amazon Basin the same drop of water can get recycled as many as six times as the air stream passes over the region on its way to the Andes.

This process is crucial to the cooling of the planet. As much as 75% of the incoming solar radiation hitting the Amazon Basin is absorbed, for instance, with the region receiving energy equivalent to some 20 Hiroshima-sized atomic bombs going off every second, day and night. The fundamental importance of this extraordinary system to world climate is widely accepted. What is disputed by Makarieva and Gorshkov is not the process itself but what drives it.

In their series of articles, the Russian scientists suggest that the above-described hydrological cycle is largely powered not by temperature differentials, as is widely believed, but by the forest itself. That is a radically different explanation than the one offered by meteorologists, who for many years have held that thermodynamics—that is, differences in temperature—accounts for the major patterns of air-mass circulation.

A crucial such pattern is the tropical Hadley Cell circulation, in which air mass is carried from Africa to South America and then lifted higher into the atmosphere and sent back to Africa in a giant circular movement. Makarieva and Gorshkov challenge the idea that these air-mass movements are mainly the result of air warming up and rising because of topography, creating an area of low pressure that draws air from the east. Instead, they contend that the evapotranspiration-fueled “biotic pump” powers this air-circulation system by generating a partial pressure gradient.

As proposed by Makarieva and Gorshkov, the pump runs on changes in the pressure exerted by phased transitions of water vapor at different altitudes in the air column above the rainforest. Just above the canopy, warm temperatures permit the air to hold large quantities of water vapor, so the total air pressure is high. That pressure pushes the air immediately above the canopy upwards into the partial vacuum caused by the cooling and compression of the water vapor that occurs with condensation in the colder upper part of the air column. Virtually all the water vapor in the vertical plane of the air column condenses and forms droplets of rain. In this way, the forest sucks in ocean air, creating the humid trade winds that blow ashore with their bounty of water vapor. In effect, evapotranspiration is the primer for the pump, which sustains the rainforests across the entire Amazon Basin.

In challenging the prevailing explanation for air movements, Makarieva and Gorshkov question how thermodynamic processes can explain wind patterns in the Amazon. The air flow there, they say, is from the warmer tropical Atlantic to the Amazon, which is made cooler by high evapotranspiration and the resulting formation of light-reflecting clouds.

And, they ask, as the Sahara is warmer than the same latitudes of the Atlantic, at least during the day, how it is that the prevailing winds are not from the ocean to the land? “This is perfectly explained by the biotic pump, but not by differential heating,” the two physicists said in a recent e-mail explanation, reflecting a point they have made in their articles.

Makarieva and Gorshkov acknowledge that solar radiation also produces water vapor over the tropical ocean, but they say the multiple layers of surface provided by the leaves of the natural forest provide considerably more water vapor per square centimeter than does the ocean. The result, they say, is a differential pressure between the two acting along the horizontal plane.

They also point to the capillary action which takes place in the xylem (the water transport tubes of the plants) and which draws water into the stomata, or pores of the leaves, from where it evaporates. This, they assert, creates an evaporative force that is finely tuned for generating rain and simultaneously bringing about significant differences of moist air pressure in both the vertical and horizontal planes, causing a dynamic disequilibrium and therefore contributing to the movement of the air masses.

Makarieva and Goshkov say a high leaf area index, such as that associated with the closed canopy of a tropical rainforest, is vital to the process. So replacing forest with pasture or soy crops, in which sustainable evapotranspiration is an order of magnitude lower, is no substitute, they argue. Indeed, without the natural forest vegetation, they say, the sun’s energy will take the form of sensible heat—that is, heat that changes the temperature of a material without changing its state (such as turning water into vapor)—which will greatly reduce the potential of rain forming from evapotranspiration.

This is no esoteric scientific debate: if the Russian physicists are right, the consequences for the region would be huge. According to their theory, a continental region devoid of coastal and inland forests and located next to a warm tropical ocean will display surface air mass movements in reverse of those found in a forested continent. While the evaporative force over the canopy of a rainforest is considerably greater than that over the tropical ocean, they say, the evaporative force over the ocean will become greater than the vegetation on deforested land. The ocean, they add, will draw the air mass towards it, drying out the continental soils and vegetation in a downward spiral of degradation.

So by effectively arguing that water vapor rising from plants and trees drives the hydrological cycle, they imply that deforestation in such river basins as the Amazon and Congo could disrupt rainfall patterns far more extensively than currently assumed.

Makarieva and Gorshkov posit that if the Amazon forest were destroyed, the Amazon’s western reaches and the foothills of the Andes could receive less than 1% of their current rainfall. And within a short period, perhaps within a few decades, their rainfall could diminish to 22 millimeters a year. This is roughly the same precipitation rate as that of the Negev Desert.

The consequences of a breakdown in the region’s air flows and the moisture they bring would be manifold, involving everything from the soybean production that underpins Brazil’s foreign-exchange earnings to the hydropower stations that generate the vast majority of the country’s electricity.

If the rains now carried by the low-level jet stream from the Amazon down to the southeast were to fail, Brazil’s main grain-growing regions would lose at least a third of their rainfall and the River Plate Basin—site of Argentina’s soy boom—would suffer. Agricultural output in the other countries that share the Amazon basin, including Colombia, Ecuador, Peru and Bolivia, would also tumble.

The U.S. Corn Belt presumably would be affected as well. Climate studies by climatologist Roni Avissar, formerly of Duke University and now with the University of Miami, and colleagues have suggested that agriculture in the U.S. Midwest depends largely on rains brought from the Amazon Basin in the spring and early summer, just when they are most needed to generate a good harvest.

Such teleconnections, or climate anomalies that are related but occur far apart, are produced by streams of water-laden air that move transversely out of the Amazon Basin to the higher latitudes of the Americas, gaining their impetus through the energy that is released as water condenses into rain.

Climatologists Coe and Costa say it would be a mistake to base such concerns on Makarieva and Gorshkov’s theory, which they consider groundless. Costa asserts Makarieva and Gorshkov are assigning evapotranspiration a role it cannot play.

“Evapotranspiration cools the surface but the same water heats the atmosphere when it condenses,” Costa says. In other words, he argues, there is no net loss of heat. Adds Costa: “[T]he notion that the hydrological cycle is driven by the forest is absurd”.

Coe of the Woods Hole Institute agrees. “This is a ridiculously narrow and specific assumption that ignores the huge differences in geography, atmospheric circulation and increasing solar radiation that exist between the regions they are comparing,” he says. “The Congo and the Amazon straddle the equator and therefore are entirely different from the other basins. The rainfall does not decrease with distance from the ocean in these cases because the sun causes very strong heating of the continent, which creates strong low pressure over the continental interior. The stronger the low pressure, the greater the upward motion and opportunity for rainfall.”

Meanwhile, Peter Cox, a climatologist at Britain’s Exeter University, says the underlying processes described by the Russian scientists must be at least partially represented in existing climate models.

Makarieva and Gorshkov reject the criticism of their theory. Addressing Cox’s comment, they say: “The vapor-driven process of air pressure gradient formation [the concept behind their theory] has never been discussed in the climate literature.”

And on Costa and Coe’s points they respond: “It is true that upon condensation, the energy that has been harbored by evaporation is released. But it is of great significance where this energy is released.”

The physicists argue that the resulting temperatures vary according to the height above the earth’s surface at which the energy is released and to the type of region above which it is released—the effect being different in the tropics compared, for example, with the mid-latitudes. Evapotranspiration can—and does—have a cooling effect, they insist.

- Sue Branford and Peter Bunyard

Michael Coe
Woods Hole Institute
Falmouth, MA, United States
Tel: (508) 548-9900, ext. 136
Marcos Costa
Professor of Climatology
The Federal University of Viçosa
Viçosa, Minas Gerais state, Brazil
Tel: +(55 31) 3899-1899
Peter Cox
Exeter University
Exeter, U.K.
Tel: +(44 1392) 725220, ext. 5220
Victor Gorshkov
Petersburg Nuclear Physics Institute
Gatchina, Russia
Tel: +7(813-71) 46025
Anastassia Makarieva
Petersburg Nuclear Physics Institute
Gatchina, Russia
Tel: +7(813-71) 46025