Tropical forests – Planet’s heart as well as lungs

Pearce (2009) describes a new theory, dubbed the ‘biotic pump,’ which claims that forests create winds that pump moisture across continents. Developed by two Russian meteorologists, this theory awards the lush tropical forests, known as the planet’s lungs, a new title – the ‘planet’s heart.’ According to this theory, even localised deforestation near coasts can adversely affect the biotic pump, eventually converting wet continents into deserts. The good news is that planting coastal forests may convert deserts into forests.

(The following write-up by Asitha Jayawardena is entirely based on Pearce (2009). For enhanced readability, the reference (i.e. Pearce, 2009) is usually mentioned at the beginning of a section only.)

The ‘biotic pump’, a new theory

The lush tropical forests, especially in the Amazon and tropical Africa, have always prided themselves as the planet’s lungs. Now, as Pearce (2009) reveals, they can claim another title of planetary proportions – the planet’s heart – thanks to a new theory developed by two Russian meteorologists.

Victor Gorshkov and Anastassia Makarieva of the St Petersburg Nuclear Physics Institute in Russia claim that vast forests generate winds that help pump moisture across continents, introducing a new theory dubbed the ‘biotic pump.’

Can forests generate wind?

Pearce (2009) describes how the Russians explain the wind generation by forests.

Water vapour from coastal forests and oceans condense quickly and form droplets and clouds. When gas turns liquid, it takes up less space, causing a drop in air pressure. Evaporation is stronger above the forest than above the ocean; therefore, the air pressure is lower above the forest, resulting in a rush of moist air towards the forest from the ocean. Wind thus generated drives moisture further inland, sometimes transporting moisture thousands of kilometres into the interior of a continent.

Conventional meteorology considers the differential heating of the atmosphere as the only driver of atmospheric motion. That is, warm air rises. What the Russians have ‘discovered’ is the pressure drop resulting from the turning of water vapour into liquid water.

Biotic-pump effect around the world

To back up their new theory, as Pearce (2009) reports, the Russians show how regions without coastal forests (e.g. West Africa) become exponentially drier inland. Likewise, in northern Australia, rainfall drops from 1600 millimetres a year on the coast to 200 millimetres some 1500 kilometres inland. They argue that the Aborigines’ burning of coastal forests and thereby shutting down the continent’s biotic pump is the reason why once forested Australia is a largely deserted continent today.

The Russians show that, on continents with large forests from the coast to the interior, rainfall is as strong inland as on the coast. For example, the Congo experiences an annual rainfall of 2000 millimetres both on the coast and inland. The same is true in the Amazon, the Siberian Arctic and the Mackenzie River basin in northern Canada.

New theory’s opponents and allies

Pearce (2009) reveals the opponents as well as the allies of the biotic pump theory, first published in 2007.

An opponent, Antoon Meesters of the Free University in Amsterdam, the Netherlands, recently described this theory as ‘an untenable result of confused principles,’ claiming that its effect is negligible.

An ally, Doug Sheil, a leading British forest scientist based in Uganda, says that the theory suggests that even localised clearing might ultimately switch entire continental climates from wet to arid, with rainfall declining by more than 95 per cent. He observes that the biotic pump may be the answer to a question that the current theory cannot explain – namely, ‘how do the lowlands in continental interiors maintain wet climates?’ To him, the biotic pump is ‘a most profound insight into the impact of forest loss on climate.’

For another ally, Andrew Mitchell, director of the Global Canopy Programme, Oxford, UK, the biotic-pump idea makes perfect sense and the implications are global. He says, if this theory is correct, it means that large forests help kick-start the global water cycle.

Some worries and good news

Summing up, Pearce (2009) ends with some serious worries punctuated by some good news.

Climatologists are already worried about the deteriorating state of the Amazon rainforest. The UK’s Met Office recently warned that a warming of the planet by 4 degrees will cause an 85 per cent of the forest dry out and die. If the Russians’ theory is right, the Amazon will be gone before warming kicks in. They predict that even modest deforestation could shut down the biotic pump, cutting rainfall in central Amazonia by 95 per cent. The same could happen in the world’s other large rainforest regions, such as central Africa.

But there is some good news, too. If natural forests can create rain, then planted forests can, too. If forests attract rain, Sheil says, then replanting deforested coastal regions could re-establish a biotic pump and bring back the rains. Once forests are established, the pump would be powerful enough to water them. He says, ‘Could we one day afforest the world’s deserts? Makarieva and Gorshkov’s hypothesis suggests we might.’

Reference

PEARCE, F. (2009) Keep the planet’s heart pumping. New Scientist, 4 April, pp.6-7.

The Prince’s Rainforests Project http://www.rainforestsos.org/

Rainforests are essential for the wellbeing of all: they are the world’s natural air-conditioning and thermostat systems, generate much of the world’s rainfall, and are undeniably linked to our ability to prevent catastrophic climate change. Their destruction contributes to rising temperatures and sea levels. Set up by the Prince of Wales in 2007, The Prince’s Rainforests Project aims to find a fair way to encourage rainforest nations to protect their forests, and to identify ways to fund that approach. It works closely with senior politicians, business leaders, non-profit organisations and other interested parties, from rainforest nations to developed countries.