Global warming
A new tree line
Apr 12th 2007From The Economist print editionA climate model suggests that chopping down the Earth's trees would help fight global warming
TREES are good. Good enough to hug. Trees have a nifty biochemical strategy called photosynthesis that enables them to take carbon dioxide in through their leaves, and swap that nasty gas for oxygen, a nice one. They use the carbon thus sequestered to make molecules like cellulose, and thus more tree.
That is why some rich people who love to burn things containing carbon, such as petrol and aircraft fuel, have recently started paying others to plant trees on their behalf. Burning adds oxygen to carbon, making carbon dioxide. And carbon dioxide makes the world warmer. A warmer world will mean higher sea levels. So if people burn things without offsetting the carbon dioxide thus produced, their holidays in the Maldive islands will disappear, along with the islands themselves.
This chattering-class environmental picture is not necessarily wrong, but it does include many assumptions. One of them, that planting trees will make the world cooler than it would otherwise be, is the subject of a newly published study by Govindasamy Bala, of the Lawrence Livermore National Laboratory, in California, and his colleagues. Dr Bala has found, rather counter-intuitively, that removing all of the world's trees might actually cool the planet down. Conversely, adding trees everywhere might warm it up.
Clearcut cooling
The reason for this is that trees affect the world's temperature by means other than the carbon they sequester. For instance forests, being generally green and bristly things, remain quite a dark shade even after a blizzard. They are certainly darker than grasslands smothered in snow, and thus they can absorb more of the sun's heat than vegetation which might otherwise cover the same stretch of land. That warms things up.
Transpiration—the process by which plants suck up groundwater and evaporate it into the atmosphere—is another and opposite matter. Woodlands are usually better than other ecosystems at getting water vapour into the air. In warm places this tends to make things cloudier, and those clouds, in turn, reflect the sun's heat back into space. That cools things down.
Dr Bala and his colleagues took such effects into account using a computer model called the Integrated Climate and Carbon Model. Unlike most climate-change models, which calculate how the Earth should absorb and radiate heat in response to a list of greenhouse-gas concentrations, this one has many subsections that represent how the carbon cycle (photosynthesis and its consequences) works, and how it influences the climate. Thus, Dr Bala's model can be told to replace all the world's forests with shrubby grasslands, and left alone to work out how such a change would alter greenhouse-gas concentrations and how that, in turn, would influence the temperature in different places.
When Dr Bala ordered global clearcutting, the model calculated that the atmosphere's carbon-dioxide levels would roughly double by 2100. This is a much greater increase than happens in a business-as-usual simulation, but it would, paradoxically, make for a colder planet. That is because brighter high latitudes would reflect more sunlight in winter, cooling the local environment by as much as 6°C. The tropics would warm up, since they would be less cloudy, but not by enough to produce a net global heat gain. Overall, Dr Bala's model suggests that complete deforestation would cause an additional 1.3°C temperature rise compared with business as usual, because of the higher carbon-dioxide levels that would result. However, the additional reflectivity of the planet would cause 1.6°C of cooling. A treeless world would thus, as he reports in the Proceedings of the National Academy of Sciences, be 0.3°C cooler than otherwise.
No one, of course, would consider chopping down the world's forests to keep the planet cool. But having made their point, Dr Bala and his colleagues then went on to look at the nuances of forest growth and loss at different latitudes.
In Russia and Canada, cutting trees down led mostly to local cooling. The carbon dioxide this released into the atmosphere, though, warmed the world all over. Around the equator, by contrast, warming acted locally (as well as globally), so a tropical country would experience warming that it, itself, created by cutting down trees.
Whether that will be enough to entice those countries to prefer rainforests to ranches is another matter. One thing that might persuade them would be if rich people with a fondness for burning things started paying them to do so. Carbon-offset outfits should take note of Dr Bala's paper. Planting trees in convenient places such as Europe and North America may actually be counterproductive. Instead, in an environmental two-for-one, it is the rainforests that need bolstering.
