Plants buy Earth more time as CO2 makes them grow

The Sunday Times
April 5, 2009
Jonathan Leake, Environment Editor

TREES and plants are growing bigger and faster in response to the billions of tons of carbon dioxide released into the atmosphere by humans, scientists have found.
The increased growth has been discovered in a variety of flora, ranging from tropical rainforests to British sugar beet crops.
It means they are soaking up at least some of the CO2 that would otherwise be accelerating the rate of climate change. It also suggests the potential for higher crop yields.
Some researchers believe the phenomenon is strong enough to buy humanity some extra years in which to try to reverse the growth of greenhouse gas emissions. However, few dispute that this will provide anything more than a temporary reprieve.
“There is no doubt that the enrichment of the air with CO2 is increasing plant growth rates in many areas,” said Professor Martin Parry, head of plant science at Rothamsted Research, Britain’s leading crop institute. “The problem is that humans are releasing so much that plants can remove only a fraction of it.”CO2 Plants survive by extracting CO2 from the air and using sunlight to convert it into proteins and sugars.
Since 1750 the concentration in the air has risen from of CO2 278 parts per million (ppm) to more than 380ppm, making it easier for plants to acquire the CO2 needed for rapid growth.
One of the most convincing confirmations of this trend, recently published in the science journal Nature, came from a team at Leeds University.
Simon Lewis, a fellow of the Royal Society, led the study that measured the girth of 70,000 trees across 10 African countries and compared them with similar records made four decades ago.
“On average, the trees were getting bigger faster,” Lewis said. He found that each hectare of African forest was trapping an extra 0.6 tons of CO2 a year compared with the 1960s.
If this is replicated across the world’s tropical rainforests they would be removing nearly 5 billion tons of CO2 a year from the atmosphere. Humans, however, generate about 50 billion tons of the gas each year.
Scientists have been looking for a similar impact on crop yields and have carried out experiments where plants growing in the open are exposed to extra CO2 released upwind of the site.
The experiments generally suggest that raised CO2 levels, similar to those predicted for the middle of this century, would boost the yields of main-stream crops, such as maize, rice and soy, by about 13%. Some niche crops, such as lavender, would similarly benefit.
In one study conducted at Rothamsted, Mikhail Semenov, a researcher, found that sugar beet exposed to increased CO2 could boost yields from 11 tons a hectare to 15 tons by 2080.
Researchers in Germany recently discovered that wheat grown in similar conditions would produce up to 16% more grain.
However, scientists have warned against drawing false comfort from such findings. They point out that although levels will boost plant growth, other factors asso-rising CO2 ciated with climate change, such as rising temperatures and drought, are likely to have a negative effect.
Semenov said: “Global warming will increase the frequency and magnitude of extreme weather events which may have a significant impact on crop productivity.”
Steve Long, a British professor of crop sciences at the University of Illinois, believes that even in tropical rainforests any increase in tree size would be shortlived.
“There are other factors that , limit plant growth besides CO2 such as shortages of potassium, phosphorus or nitrates,” said Long.
“These forests will grow fast for a while but then run out of nutrients and slow down again.”
What research does confirm, however, is the importance of preserving the world’s remaining tropical forests.
This is because the forests and the soils the trees grow in have accumulated billions of tons of carbon that would be released should they be destroyed.
Lewis said: “Once a forest is cut down, the carbon it contains returns to the atmosphere and it would take hundreds of years to recover the carbon it once stored.”