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Would rain still
fall on the Amazon without the forest?
For 30 years climatologists have been asking what would happen to rainfall over the
Amazon Basin were the forest to disappear, ripped up for cattle pasture, for
soya, for timber or as a result of dramatic changes to the air mass circulation
brought about by global warming and the continuing, business-as-usual emissions
of greenhouse gases. Would rainfall decline significantly? Could it even increase
over the Andes, as the air mass passes unimpeded across the thousands of
kilometres of the Basin?
Most studies of the Amazon Basin, such as those of the UK’s Hadley Centre, indicate that deforestation
would have little effect along the eastern region of the Basin and at worst
would bring about a 15 to 20 per cent reduction in rainfall – one millimetre
less than the 5.8 millimetres daily average – in the central and western part
conclusions are based on climate models parameterized
(tweaked) to agree closely with past data such that when they are ‘played’ back
from present conditions they accord well with general climatic conditions of
the mid 19th century. And, to be more realistic, climatologists such
as Richard Betts at the Hadley Centre have integrated a terrestrial carbon
cycle into their models, with rainfall and temperature as critical factors in
the maintenance of vegetation cover. They took into account the recycling of
rain through evapo-transpiration, which over a long stretch - the 7 million
square kilometre Amazon Basin - is the mechanism whereby the rainforest in the
centre and further extremities inland is deemed to receive adequate watering.
Centre model (HadSM3
coupled to a dynamic global vegetation model –TRIFFID) predicts impacts on surface temperature, on ocean currents and
ultimately on the state of the Amazon rainforests as a result of ‘business-as-usual’
emissions of greenhouse gases, mainly CO2. Hence, the models
predicted that global surface terrestrial temperatures would rise by a good 50
per cent more than indicated in the various IPCC assessment reports, perhaps to
as high as 9 ˚C, and that, consequently, El
Niño-like changes would take place in the Pacific Ocean. The result would be that
the trade winds would falter and the rain-bearing atmospheric Hadley Cell
Circulation (see later) over the tropical Atlantic would diminish sufficiently
to bring about large scale forest dieback over the entire Amazon Basin.
Those who doubt we will reduce greenhouse gas emissions in
time to prevent a 4 ˚C or more increase in average global surface
temperatures (let alone the 2 ˚C rise considered by James Hansen and
others to be the most we can risk) might well say ‘why bother to protect the
Amazon rainforests if they are likely anyway to die back in their entirety.’
Circulation of the
climatologists and meteorologists believe that air currents in the atmosphere
are formed through differences in temperature that bring about heat gradients,
with colder, denser air sinking and hotter, lighter air rising. Hence, the
explanation of the Hadley Cell circulation between Africa and South America is
that cold, dense, dry air sinks over the Sahara region of Africa, forming a
high pressure zone (see Box). That same mass of air is
then drawn over the tropical Atlantic in the form of trade winds from both
hemispheres which converge over the Amazon Basin in what is known as the
Intertropical Convergent Zone (ITCZ) (see Fig. 1).
How the earth’s atmosphere circulates
The circulation of the
earth’s atmosphere modulates surface temperatures over land and sea, and
determines rainfall patterns (see Fig. 1).
Figure 1 The earth’s atmosphere circulates to distribute warmth
The earth’s atmosphere is set in motion because the tropics are heated
up more than the poles. The excess heat over tropical regions is transported towards
the poles by the circulation of the atmosphere and by ocean currents.
the Equator, the hot air with water vapour expands and become less dense, so it
rises, creating low pressure. But as the hot air rises, it cools, the water
vapour condenses and falls as rain. This creates high rainfall in the
Intertropical Convergence Zone in the tropics.
the air mass cools, it increases in density and falls back towards the surface
in the subtropics (30oN and S), creating high pressure. The net circulation is
referred as the Hadley Cell, one on either side of the equator.
the earth did not rotate, there would be a single circulation cell in each
hemisphere. Because of fluid motion on a rotating sphere, the single cell is
broken up into three circulation cells in each hemisphere, named in order from
the Equator: Hadley Cell, Ferrel Cell and Polar Cell.
creates alternating bands of high and low pressures of approximately 30 degree
latitude. Wind arises as air moves horizontally between regions of different
pressures. Very little wind is present at the Equator because air rises
vertically as it heats up. Light, variable winds at the equator are known as
the Doldrums. Similarly, there is little wind at 30 deg. N and S where the air
descends. Air always moves horizontally from an area of high pressure to low
Meanwhile, the Coriolus Force, which is a
consequence of motion on a rotating sphere, deflects the air mass in the mid to
upper troposphere, to the right of the direction of motion in the Northern
Hemisphere and to the left in the Southern Hemisphere. The lower air mass is
more bound by friction to the Earth’s surface than the upper air mass, hence
the opposed direction of motion of the upper troposphere compared to the lower.
The trade winds
pick up masses of water vapour, more than 12 million million (1012) cubic metres worth. When that same air mass, passing over
the Basin, reaches the Andes in the far west of South America, it is forced upwards
because of the topography. It loses its water vapour, all the while releasing
its latent heat of condensation, and this helps to heat and push the air mass
up still further. The next stage is the movement of the air mass in the
direction of North Africa, completing the circulation of the Hadley Cell.
The circulation is thus seen as primarily generated through the thermodynamics
of the system.
challenges accepted view
view of thermodynamics driving the tropical circulation of the Hadley Cell has
now been challenged. Anastassia Makarieva and Victor Gorshkov at the
Theoretical Physics Division of the St Petersburg Nuclear Physics Institute, propose
that the thermodynamic driver of air mass circulation is far secondary to a
much more powerful driver tied to the evaporation and condensation of water
vapour [3-6]. They conclude that the loss of the Amazon rainforests, for
whatever underlying cause, would be disastrous in the extreme. It would
threaten much of South America with unprecedented drought, and lead to
desertification in the central and western part of the Amazon Basin, with repercussions right up into the Andes and beyond.
If they are right, the very existence of the major
river-forming system in the upper moorlands, the páramos, would be threatened,
with horrendous consequences for the generation of fresh water resources in
countries such as Colombia, Peru and Ecuador, let alone in Brazil.
How do they come to that uncompromising view? The answer
lies in their review of hydrological processes and whether they take place over
forested regions of the world, or regions that have lost their forests.
Gorshkov claim that meteorologists and climatologists have ignored an important
atmospheric pumping mechanism that comes into play
when water vapour is first drawn into the lower atmosphere through
evapotranspiration from dense forest, with its relatively high leaf area index,
and then, higher in the lower atmosphere, condenses as a result of declining
temperatures. And they make it very clear that a high leaf area index is vital
to the process and that replacing the forest with pasture or a plantation of
soya, in which evapotranspiration is an order of magnitude lower, simply will
not do. Indeed, without the natural vegetation the Sun’s energy will take the
form of sensible heat, which not only will reduce the potential of rain forming
from evapotranspiration, but will switch the evaporative gradient from the land
mass to the ocean. In fact, in equatorial regions, such as the Amazon Basin,
where the annual solar radiation is more than double that in the higher
latitudes, evapotranspiration from native forest, with its closed canopy and
sub-storey vegetation, will consume as much as 75 per cent of the incoming
radiation – some 560 calories per gram of water – thereby cooling the surface
and powering the process of convection by which towering cumulo-nimbus
clouds may form . Meanwhile, the driver of that
evapotranspiration is the Sun, which in terms of energy received over
the Amazon is equivalent to some 20 Hiroshima-size 15-kiloton bombs going off
every second, day and night, 15 such bombs being
used in evapotranspiration.
The biotic pump
drives the climate and draws water from the oceans
Basically, the biotic
pump, as proposed by Makarieva and Gorshkov, functions as a result of marked
changes in the partial pressure (partial because other gases also contribute)
exerted by water vapour 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 vapour, and so the partial pressure is high. That partial
pressure, plus the higher temperature of air close to the ground, act together
to force the air column upwards against the partial vacuum caused as the water
vapour cools and condenses. That dynamic of evaporation and condensation, forces
the air column upwards against ever reducing pressure and, just as happens in
the expansion chamber of the cooling circuit of a fridge, the upward motion of
the air column causes a loss of heat and a simultaneous drop in temperature, resulting
in a sharp reduction in the saturation pressure of water. Virtually all the water vapour in the vertical plane of the air column therefore condenses
and forms droplets of rain. The fall in temperature
because of the expansion of the air mass is compensated to some extent
by the release of latent heat in condensation.
“In the presence of a large vertical temperature gradient,” Makarieva and Gorshkov said , “the vertical distribution of saturated partial
pressure, pH2O, departs significantly from the static equilibrium;
at any height pH2O is over five times larger than the weight
of the water vapour column above this
height. For this reason practically
all the water vapour ascending in the atmosphere condenses.”
The air at the
base of the air column is then replaced by air moving in horizontally, coming from the ocean. This process of convection,
powered by the partial pressure of water vapour from
evapotranspiration, sucks in the Trade Winds, which have accumulated
significant quantities of water vapour as they pass over the tropical Atlantic
Ocean between Africa and Brazil. That is a very different picture from the
commonly belief that the Trade Winds are driving the air mass circulation
system over the Basin, instead of being sucked in.
As the air above the tropical ocean is also drawing up
water vapour, how can the forest evapotranspiration pull in air from the ocean?
Here, the physicists explain, the multiple layers of surface provided by the
leaves of the natural forest provide considerably more water
vapour per square centimetre than does the ocean and so a differential pressure
will exist between the two, acting along the horizontal plane. Add into the
equation the capillary action which takes place in the xylem (water transport
tubes of the plants) and which draws water into the stomata (pores of the
leaves), from where it evaporates, and also that
chemical compounds and maybe bacteria too act as cloud condensation nuclei
(CCNs) when released from the stomata, and we have an evaporative force that is
finely tuned for generating rain and simultaneously
brings about significant partial pressure differences in both the vertical and
horizontal plane, causing a dynamic disequilibrium and therefore the mass
movement of the air .
It is wonderful to see how the
natural forest keeps the system going during the dry season and even during
drought years, as during a strong El Niño, by increasing leaf coverage and
hence the leaf area index by as much as 25 per cent compared with the wet
season. Indeed, as Myneni and colleagues at Boston University have shown from
satellite images, the forest appears to anticipate the dry season with the
growth in leaf area taking place before the ‘summer’
months have actually taken hold . The increase in leaf area means that the
root system of the forest must draw up more water, and it is now known that the
tap roots, taking water from the water table, also pass water through lateral
roots. This dampens the area around each tree and keeps soil moisture high. The
increase in evapotranspiration and the resulting convection, draw in humid air
brought in by the Trade Winds from the tropical Atlantic in the other hemisphere. That extraordinary process
whereby the rainforest manages its own climate would seem to reinforce the
notion of the biotic pump as described by Makarieva and Gorshkov, and
they point out that the rates of evaporation and precipitation in
tropical rainforests are twice as high as those of evaporation and precipitation
over open oceanic surfaces at similar latitudes.
Nor is such biotic regulation of the water cycle limited to the tropics. The same physics shows that during the late spring and
summer months, undisturbed temperate and boreal forests will generate an evaporative
force, albeit far weaker than in the tropics, which will create a partial
pressure gradient from the ocean to the land. But that biotic pump is switched
off in winter .
“That physical view,” said Makarieva and Gorshkovn 
“is in direct conflict with the traditional paradigm which considers differential heating to be
the major driver of atmospheric circulation. However, this
consideration critically fails in the case of the strongest winds observed on Earth, the hurricanes, that, as is well-known, develop along nearly
“But if differential heating is
not necessary for producing
the strongest winds, perhaps it is not indispensable for producing moderate and weak winds either? The evaporative force
concept that relates wind velocities to spatial differences in the intensity of
condensation rather than heating provides a unifying explanation for both hurricanes and tornadoes as well as for stationary circulation patterns.”
“On a related note,” they
continue, “according to the traditional paradigm the regions of air ascent should be associated with positive
buoyancy. In contrast, observations of atmospheric updrafts indicate a wide range of positive and negative buoyancies. The evaporative force concept resolves
the puzzle. Air pressure depends on two independent variables, temperature and number of air molecules in a unit volume.
Consequently, there are two
independent ways of making local air pressure higher than that in the neighbouring area, so as to
initiate air motion: (1) towarm the air locally (this is what the traditional paradigm of horizontal differential
heating is about) and (2) to
reduce the number of air molecules in the neighbouring area (this
is what condensation is doing in the vertical dimension). Thus, if the condensation is intense, it
can make even dense cold air
rise from the surface by creating a strong weight imbalance in the upper part
of the air column.”
Dire consequences if
The evaporative force
hypothesis of Makarieva and Gorshkov [3-6] predicts that 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 the
forested continent. Whereas the evaporative force over the canopy of a
rainforest is considerably greater than that over the tropical ocean; that is
no longer the case when the forest is gone. On the contrary, the evaporative
force over the ocean is now greater than the biotic pump of the depleted
vegetation, and the ocean will draw the air mass towards it, thus drying out
the continental soils and vegetation in a downward spiral of degradation.
Simultaneously, without the rainforest to recycle rain,
precipitation will decline exponentially as one passes inland from the coast. The western reaches of the
Amazon, as well as the foothills of the Andes, could find themselves receiving
less than one per cent of the rainfall they currently experience; they could
become as dry as the Negev desert of Israel.
Perhaps the extraordinary
drought year of 2005 in the Amazon Basin, which particularly affected the
southwestern region, has given us a foretaste of what would happen if the
forests were to disappear. During that year, the tropical waters off Brazil and up into the Caribbean were a degree or two warmer than normal, with a corresponding
increase in the oceanic evaporative force. That increase may have tipped the
balance, at least for that year - given the degree of deforestation in the
southeastern and southwestern region of the Basin - so as to alter the air mass
movement over the Basin and draw it more towards the ocean rather than
following its normal trajectory over the Amazon.
The conventional explanation for air movements invokes
thermodynamic processes such that hot air rises and the resulting low pressure
draws in cooler, denser air, whether from the ocean or continent. Yet, how can
that be an adequate explanation, when the evidence is exactly the reverse?
Thus, the air flow is from the warmer tropical Atlantic to the cooler Amazon,
made cooler because of the high evapotranspiration and therefore formation of
light-reflecting clouds. And whereas the Sahara is warmer, at least during the
day, than the same latitude Atlantic, how is it that the prevailing winds are not
from the ocean to the land?
“Despite the general meteorological wisdom that warmer air
is lighter and hence rises, so it is an area of low surface pressure (which
presumes wind flow from the ocean to Sahara and from the Amazon and Congo to the ocean), in reality the prevailing winds blow in the opposite direction in all
the three regions.
perfectly explained by the biotic pump,” declared
Makarieva and Gorshkov , “and not by differential heating. In fact it is the
condensation gradient which explains the direction of the winds, whether from
the ocean to the continent or vice versa.”
Further empirical evidence for the biotic pump come from
their unique study of the relationship between the
precipitation pattern over river basins in which they show substantial
differences according to whether or not the region through which the rivers
pass is forested . The Mississippi River Basin is a case in point. Where the
land is forested from the Atlantic coast inland, stretching some 1 750
kilometres, the precipitation stays steady at some 1 000 millimetres over the
course of the year; further inland, where there is no forest, the rainfall
declines exponentially to little more than 200 millimetres. Meanwhile, rainfall
right across the Amazon Basin remains substantially the same at around 2 400
millimetres per year and even increases at the western extremity of the Basin,
for instance in the bio-rich Colombian Amazon, to as much as 4 000 millimetres.
In essence, Makarieva and Gorshkov believe that climate
stability, within a limited temperature range, taking glacial and inter-glacial
periods into account, has been brought about largely through the evolution of
continental forests. In terms of sheer area, the boreal forests of Russia, Canada, and the tropical rainforests of South America and Central Africa, remain critical to
the maintenance of a climate which retains some semblance of stability. Preserving those forests is every bit as important as
concerns over greenhouse gas emissions and consequent global warming.
“Most importantly,” they said , “it was necessary for
natural forests with their high
leaf area index to appear in
the course of biological evolution for evaporation from the forest canopy to
exceed evaporation from the open
water surface. This allowed life to invade the hitherto dry landmasses by sucking moist oceanic
air inland as the forests
marched forward from the coast. Not surprisingly, modern global circulation models devised
without including the physics
of the biotic pump fail radically when attempting to account for the water budget of the strongest biotic
pump on Earth – the Amazon
river basin. The amount of oceanic moisture brought to the Amazon river basin in the models (the modelled atmospheric moisture convergence) proves
to be half the actual amount
empirically estimated from the value of the Amazon runoff. It is obvious that the traditional accounts of
moisture transport in the other
great river basins, including Siberian and North American rivers will similarly
have to be seriously
reconsidered to incorporate the major effects of the forest moisture pumps, the anthropogenic destruction of which is currently threatening to turn
the landmasses back into
of Makarieva and Gorshkov’s thesis are enormous; essentially it means that
South America cannot do without its rainforests, and that instead of quibbling
over how much should be conserved, those countries with substantial areas of
the Amazon Basin should be doing everything in their power to ensure that no
more is destroyed. Forests
are not just carbon sinks or carbon sources if destroyed; they have an
essential and irreplaceable hydrological role in the earth’s climate.
Greg Peachey Comment left 23rd September 2010 17:05:46 I too am supporter of the work of Gorshkov, Makarieva and St Barbe-Baker, and would like to work with you.
I have convened an All Party UK Parliamentary discussion, building on their work on 18 Nov 2010.
I hope you won't mind if I email you all invitations to that event...
Take care, Greg
Stephanie Mott Comment left 30th April 2010 08:08:02 By the way, I have written about the teleconnection between the Amazon and the Mid West of the USA and how US crops in the corn belt depend on the water vapour transported in slow standing Rossby waves out of the Amazon.
Where can I find this??????????????
Ilyan Comment left 16th March 2010 10:10:57 I saw one article that predicted the loss of the Amazon rainforest would lead to severe drought in middle west North America.
Was that omitted because it is too damn scarey?
Krichauff Comment left 16th March 2010 11:11:52 Man of the Trees, Richard St. Barbe Baker predicted this about 100 years ago we have had lots of time to mend our ways but blunder on to doom probably.
Peter Bunyard Comment left 16th March 2010 23:11:00 Thanks for your comments so far. Yes, I am familiar with the wonderful work and studies of Schauberger and St Barbe Baker (whom I did meet on several occasions). Meanwhile Makarieva and Gorshkov have thrown down the gauntlet to those - the great majority of climatologists and meteorologists - who have totally overlooked the role of life and in this instance the great forests of the planet in actually managing the mass movements of air which bring rainfall to the deep interior of continents. Hence forests are the generators of evolution in providing the means by which adequate rainfall can maintain the richness of the Amazon to the western reaches of the Basin, some thousands of kilometres from the Atlantic coast of Brazil.
We do indeed threaten our very survival by eliminating the vast stretches of rainforest and in my opinion, doing everything in our power to prevent further forest destruction is every bit as important as our concerns with CO2 emissions.
By the way, I have written about the teleconnection between the Amazon and the Mid West of the USA and how US crops in the corn belt depend on the water vapour transported in slow standing Rossby waves out of the Amazon. Roni Avissar, then at Duke University and now in Florida, is one of the principal researchers in the field.
Rory Short Comment left 16th March 2010 23:11:29 The work of Anastassia Makarieva and Victor Gorshkov makes absolute intuitive sense to me because I take joy in the fact that we live in, and are components of, a bio-sphere not a mechano-sphere. Our thinking n the recent past, and unfortunately still in the present, has been driven by a belief, and perhaps even a desire, that somehow we live in a mechano-sphere.
Peter Goldsbury Comment left 31st October 2010 07:07:34 We love this article and are honoured to reference Victor and Anatassia's lessons along with others in a active reasearch project at the school and community in the Whirinaki Rainforest, Te Urewera, New Zealand. That is all about sharing resources that allow people to better understand the real value of the critical ecosystem services that rainforests provide to support ongoing and well life on our planet. Details at
Todd Millions Comment left 11th April 2010 06:06:38 If Mr Bunyard or anyone else wishes too see this biotic pump in action,a wintre spent on the west side of Vancouver isle,north of long beach,would provide an exellent example before your eyes,even with it rising from the clear cut stumps.
It impressed me when i lived their-hard too measure the rains again reaching for the sky-but the minds eye trying to stretch it to amazonoan scale is indeed awe inspring.
Further-imagine this in western europe,Before the great beech and oak forests were lost for-sheep!
Alick Bartholomew Comment left 16th March 2010 23:11:19 This important research corroborates the insights 80 years ago of Viktor Schauberger who demonstrated the close connection between trees, the Earth’s water cycle and the production of rain, and who saw the tropical rainforest as the powerhouse for balancing climatic extremes on the planet. He called the forests ‘the cradle of water’ in the sense that they produce the most balanced and best quality fresh water. The forest canopy receives rain imbued with the Sun’s energy and draws up water from deep in the soil, transpiring abundant and high quality water which balances the two energy sources.
Schauberger proposed a complex water cycle below ground level, where the water molecules from precipitation are broken down to provide energy for new growth, then to be recycled into new water. He believed that that new virgin water is created by the coming together of molecular hydrogen and free oxygen in the depths of the soil, stimulated by the trees’ root systems. Schauberger was an intuitive, and it is good to see some of his insights supported scientifically.
Thank you, Peter Bunyard, for this excellent article!
Mary Paul Comment left 16th March 2010 04:04:59 Wonderfaul article. I would thoroughly agree with the Russian researchers. Having read some of the work of VIktor Schauberger where he describes the hydrological cycle (trees are essential to keep the ground temperature cooler than the rain so as to allow the rain to penetrate) as against the half-hydrological cycle. the latter is seen when the lack of vegetation allows the earth to become so warm that rain does not penetrate, but flows over the top and is rapidly evaporated, and so ground waters are not able to be replenished. Schauberger was an Austrian and did absolutely brilliant work.
Mike G Comment left 4th December 2012 08:08:35 stumbled across this article while researching a Jules Verne novel about the Amazon.
Sure would be good if Rush Limbaugh and his cohorts would just pay attention and give up on their boneheaded view that human contribution to global worming is a hoax. The longer they wait, the more foolish they look.