Could elevated levels of carbon dioxide be making the planet greener?

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With temperatures already pushing 50°C weeks before their typical August peak, we look set for a blistering summer.

To step outside even for a moment is to wilt, and the idea of even higher temperatures through global warming hardly bears contemplating. But now there’s a glimmer of good news.

Evidence is emerging that even nature has had enough, and is recruiting a green ally to help dial down the heat – vegetation.

After decades of being told that the science of global warming is settled, it can come as a surprise to learn this is not the case.

While few doubt that greenhouse gases such as carbon dioxide warm the planet, exactly when, where and by how much remains deeply controversial.

And one of the biggest areas of uncertainty is over the influence of vegetation – trees, shrubs and the like – on the climate.

As every schoolchild knows, vegetation mops up carbon dioxide, and so can help slow the rate of the global warming. At least, that’s the theory.

Yet for decades we’ve also been told that deforestation, drought and urban sprawl has undermined this natural remedy, and now threatens to turn our planet into a dustbowl.

Again, it is not true. While some parts of the world have indeed suffered dramatic reductions in vegetation cover, globally the picture is far less bleak.

A recent paper in the journal Nature Climate Change highlighted an irony of environmental science – the combination of more carbon dioxide, plus more nitrogen-rich soil and changes in land use has actually boosted leaf coverage in about 25 to 50 per cent of the world’s verdant areas.

In contrast, less than 4 per cent of the globe shows signs of declining cover.

The greening effect is especially marked in Africa. This year, an international team of researchers reported that 36 per cent of the continent has become greener in the course of the last 20 years – three times the proportion that has lost trees and bushes.

This is more than just an aesthetic effect. Trees not only lock up carbon dioxide but also affect the exchange of moisture and energy between the ground and the atmosphere. The result can be an overall cooling effect.

Exactly how big the effect is remains subject to intense research, but there’s growing evidence that regions such as the Emirates might benefit most.

Last month, the journal Science published research linking vegetation coverage with local temperature change. The results showed that the areas with increasing vegetation typically experienced 14 per cent less warming – with arid desert regions benefiting the most.

On a global scale, the results suggest the greening of the planet is producing a cooling effect that outstrips the overall warming due to deforestation.

Just how big the greening effect will become remains unclear. But the idea of natural processes kicking in to keep the planet habitable has resonances with one of the most controversial theories in modern science – the so-called Gaia Hypothesis.

Proposed more than 50 years ago by the English chemist James Lovelock, it argues that living organisms do not passively accept their environment and adapt to it, but actively modify it to increase its chances of survival.

Lovelock came up with his idea while working on a Nasa programme to search for life on other planets.

Rather than second-guess what lifeforms might exist, Lovelock focused on finding the most general test for the processes of life.

At their most basic, all organisms extract nutrient from their environment, chemically change them and then excrete the waste products back out into the environment. One consequence of this is a complex atmosphere made up of reactive gases such as oxygen and methane.

Lovelock realised that this opened up the possibility of being able to detect the presence of life on other planets without visiting them. Simply examining their atmospheres for tell-tale reactive gases would be enough.

The idea did not go down well with Lovelock’s paymasters at Nasa. Observations of both Mars and Venus – top targets for Nasa’s search for alien life – showed both planets have unreactive atmospheres, suggesting any life has long since gone extinct – a conclusion that still stands today.

Lovelock persisted with his idea, however, and began to look for other ways in which life could alter the planet it inhabits. This led to what is now called the Gaia Hypothesis, after the Greek goddess of the Earth.

Understandably perhaps, the idea of life adapting the planet to suit itself was initially dismissed as bizarre. But over the years Lovelock and other have found support for the idea.

One striking example is the link between global warming, clouds and marine plankton.

These tiny organisms are well-known for providing food for marine life. Less well-known is their influence on the atmosphere.

Using sunlight to stay alive, they emit huge amounts of a sulphur-rich gas known as dimethylsulphide (DMS) which floats off into the sky to trigger cloud formation. That, in turn, affects the amount of sunlight that reaches the surface – and thus the amount of sunlight available to the plankton.

If the levels of sunlight creep up, the plankton respond by emitting more DMS, triggering more cloud formation, which brings the level of sunlight back down again. The result is a natural stabilising influence on an overheating earth.

As with the “greening” effect of vegetation, quite how big the influence of plankton is on global warming levels remains unclear. What is certain, however, is that current climate predictions have yet to include such subtle effects – casting doubt over their long-term reliability.

So are the “climate deniers” right all along ?

Not at all – but neither are those who insist the science of climate change is settled.

In the face of such uncertainty, the way forward lies not in apathy but in taking “no regret” action which brings benefits regardless of what the future holds.

And the evidence on the climatic effects of trees and shrubs suggests it is time to stop thinking of “going green” as merely a metaphor.

Robert Matthews is Visiting Professor of Science at Aston University, Birmingham, UK

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