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Felled Trees Deal Double Blow to Global Warming
DYING forests could con tribute twice as much as previously thought to dmg to^a new *anatysto*being the American scientist who pioneered measurements of carbon dioxide in the air three decades ago.
In an interview Keeling claimed that deforestation could be sending 4 billion tonnes of carbon dioxide per year into the air today, not 2 billion tonnes as estimated. The practice of burning fossil fuels currently adds around 5.6 billion tonnes to this per year.
"Commercial logging destroys soils and prevents regrowth of trees." Keeling said. It's not just Brazil and Indonesia. I can see it in British Columbia." Keeling, who works at the Scripps Institution of Oceanography in Seattle. Washington State, continued "There is a fear in the timber industry that they the loggers may be stopped. So they are clear-felling larger patches of forests. There is a lumber war' in North America."
Carbon dioxide contributes more than any other gas to the Cnhouse effect, which is y to warm the planet by several degrees in the next few decades. The rate at which carbon dioxide accumulates in the air has increased by two-thirds over the past two years, according to measurements by Keeling at his observatory on the Hawaiian Island of Mauna Loa. The rise has seen the average concentration exceeding 350 parts per million for the first time, compared with a pre-industrial level of around 270 parts per million.
Keeling does not blame this surge primarily on deforestation. It is connected, he says, with the occurrence in 1987 and 1988 of an El Nino, a switch In ocean and wind currents in the Pacific Ocean that causes droughts. These release large amounts of carbon dioxide Into the atmosphere. The recent surge was "the largest oscillation in carbon dioxide levels that we've seen," said Keeling. It ceased In June this year, some months after the end of the El Nino event.
But Keelings latest analysis, to be published as a monograph of the American Geophysical Union, suggests that over the past 15 years, the burning and logging of tropical forests has become an increasing cause of the underlying rapid rise in carbon dioxide in the atmosphere.
The air today Is receiving roughly an extra billion tonnes of carbon from vegetation than it did 15 years ago. and a similar amount extra from the burning of fossil fuels, he said. Keelings conclusion is that the only thing stopping an even bigger release of carbon from forests is a counter-balancing "carbon dioxide fertilisation effect". The theory is that extra carbon dioxide In the air will encourage trees to grow faster, thus drawing extra carbon dioxide from the atmosphere as photo-synthesis intensifies. "Ecologists disagree, but most modellers believe this is happening" he said.
Keeling bases his conclusions on a-detailed investigation of how much of the year-by-year rise in carbon dioxide tn the air can be explained by burning fossil fuels alone, and whether other' sources or sinks are involved. He relies both on his own data and on recent analyses of ancient bubbles of air trapped In Ice nt pa
•The data show that there was a steady rise in airborne carbon dioxide in the early 1800s, before the Industrial Revolution was fully under way. This he attributed to the "pioneer effect" of forest clearing. mainly in Europe and North America
From around 1940. says Keeling, vegetation seems to have become a "net sink" for carbon dioxide, with the fertilisation effect causing it to absorb more than It released. "In the past 15 years, however." he says, "the balance has changed again.
To explain this sudden swing. Keeling proposes that the effects both of carbon dioxide fertilisation and of deforestation on. the chemical constitution of the atmosphere are much greater than previously supposed. This means that the extra destruction of forests is much more dangerous to the climate than assumed. But it also gives hope that If deforestation were halted, the green-house effect could be greatly reduced.
Then, the "fertilisation effect" of the carbon dioxide tn the atmosphere could allow the surviving forests to absorb a substantial proportion of the gas put there by burning fossil fuels. This would allow the world to "buy time" while it rebalanced the planet's atmosphere.— Fred Pearce