快猫短视频

NOT for the first time, the human race may be about to take a dangerous short
cut. Next week, governments from around the world will meet in Bonn, the former
German capital, to complete plans for creating forests to soak up the greenhouse
gas carbon dioxide. They see planting trees as a partial alternative to cutting
emissions of the gas from power stations and vehicle exhausts.

Unfortunately, just two weeks ago, the UN鈥檚 Intergovernmental Panel on
Climate Change (IPCC) put the finishing touches to a report that shows this
strategy to be based on a dangerous delusion. In reality, say its scientists,
planned new forests, called 鈥渃arbon sinks鈥�, will swiftly become saturated with
carbon and begin returning most of their carbon to the atmosphere, temporarily
accelerating global warming. Peter Cox of the Hadley Centre, part of Britain鈥檚
Meteorological Office, shares the UN panel鈥檚 conclusions. 鈥淭his is not something
that may or may not happen as the world warms鈥攊t is more or less
inevitable,鈥� he says.

The result will be no overall reduction in CO₂ levels. Despite this,
the US and other major CO₂ producers will cite the 鈥渟ink鈥� process as
justification for their continuing tardiness in cutting CO₂
emissions.

The US Environmental Protection Agency would not comment when approached by
快猫短视频. But other governments are concerned. Britain鈥檚
environment department says: 鈥淭he UK emphasises that the main action should be
reducing actual emissions. Sinks are much less secure than carbon in fossil
fuels left unburnt.鈥�

The discovery that forests are not a panacea for global warming only emerged
after they were given a central role in the Kyoto Protocol, the treaty signed
two years ago by most of the world鈥檚 governments in a bid to stem the greenhouse
effect. 鈥淛ust a couple of years ago, the issue of sink saturation was barely
known,鈥� says Will Steffen of Sweden鈥檚 Royal Academy of Sciences, who chairs the
International Geosphere-Biosphere Programme, which has pioneered research into
the global carbon cycle. The first public warning came in the March issue of the
IGBP newsletter. And this month, the IPCC incorporated its analysis into a
forthcoming report on land use change and forestry.

Each year, CO₂ emissions from human activity pour just over 6
billion tonnes of carbon into the atmosphere. Around a third is absorbed by the
world鈥檚 forests. The discovery of this large carbon sink encouraged policy
makers to believe that CO₂pollution could be cut by planting more
trees. But now it seems the sink is a recent phenomenon, and a temporary one. In
fact, the suggestion that planting trees means less atmospheric CO₂
ignores simple logic.

Before the large-scale development of industry, mature forests were in
equilibrium with the atmosphere. Photosynthesis, the process that creates plant
matter, absorbs CO₂ from the air. But trees also release CO₂
back into the air when plant matter breaks down the sugars they make during
photosynthesis. This process is called respiration. Much the same happens in
forest soils, which absorb carbon from trees and release CO₂ as
microorganisms break down plant matter.

This equilibrium has been increasingly upset by the higher concentrations of
CO₂in the atmosphere from burning fossil fuels. Usually, the low level
of CO₂ is what limits photosynthesis. Higher CO₂ levels promote
鈥淐翱₂ fertilisation鈥�, accelerating both forest growth and the
accumulation of carbon in forest soils. And as the forests grow faster, they
absorb more CO₂, helping to stave off climate change鈥攆or a while.

Until a few months ago, researchers had assumed that as long as CO₂
levels in the air went on rising, the forest sink would continue to grow. The
IPCC鈥檚 last assessment, published in 1996, concluded that forests would soak up
around 290 billion tonnes of carbon next century, even without extra planting.
But this now seems highly unlikely. Experts such as Bob Scholes of the South
African government鈥檚 research agency, CSIR, argue that CO₂
fertilisation may already have peaked and that respiration may be about to
accelerate. Early in the next century, forests planted to protect the planet
from global warming could be contributing to it.

How did researchers get it so wrong? Scholes, a leading light in the IGBP鈥檚
Global Carbon Project, says that the confusion was caused by a time-lag. CO₂
fertilisation is an instantaneous process. But respiration increases in
response to temperature rises triggered by the CO₂. That warming has a
built-in delay of about fifty years, caused largely by the thermal inertia of
the oceans. So the extra outpouring of CO₂ from the world鈥檚 forests
would not yet be apparent. 鈥淒uring this delay there is an apparent carbon sink,鈥�
he says.

At the Hadley Centre, Cox has just finished modelling the likely future
carbon cycle. He warns that we are on a 鈥渟aturation curve鈥�, where extra CO₂
has an ever-smaller effect on plant growth. Respiration, on the other
hand, continues to increase with temperature. Soil respiration in particular
goes up exponentially with temperature, at least for a time, says Cox. So if
CO₂ levels in the air continue to rise, fertilisation rates will
flatten out while respiration rates soar. He predicts that by 2050, forests will
have released much of what they have absorbed. The overall reduction in CO₂
levels will therefore have been small.

鈥淭he timing is uncertain but we are pretty certain it will happen,鈥� he says.
Wolfgang Cramer of the Potsdam Institute in Germany has recently reached a
similar conclusion. Neither study is yet published.

The effect of accelerated respiration on the atmosphere could be even more
dangerous if, as predicted by some scientists, the heat and drought caused by
global warming degrade tropical forests at the same time.

This isn鈥檛 to say planting trees is in itself a bad thing. Whether they are
absorbing or releasing the gas, they will always be keeping some CO₂ out
of the atmosphere and providing other ecological benefits. But forests are an
insecure way of storing carbon out of harm鈥檚 way,鈥� says Steffen.

The real danger, he says, arises when countries use plans to plant forests as
a justification for not cutting their CO₂ emissions from burning fossil
fuels. And next week, the politicians and climate negotiators meeting in Bonn
will be doing just that. They are meeting to agree rules for implementing the
1997 Kyoto Protocol, which give the planting of forest sinks equal value to
emissions cuts as a way to meeting its targets.

Many countries, including the US, which produces about a quarter of the
world鈥檚 CO₂ emissions, are relying to a large degree on the supposed
benefits of tree planting to meet their targets. And dozens of forestry
companies are already planning to join the anticipated global market in
certificated carbon sinks. Typical is a new project, announced by a Norwegian
forestry company last month, to plant fast-growing pine and eucalyptus trees on
150 square kilometres of grassy plain in southwest Tanzania. The company, called
Treefarms, promises that by 2010 the Kilombero Forest will store more than a
million tonnes of carbon. But will it?

Such claims are based on models of CO₂ accumulation that assume
current rates of CO₂ fertilisation will continue. But Scholes believes
the carbon sink will start to decline within the next few decades. This would
make the certificates for carbon stored in forests such as Kilombero
worthless.

Ultimately, says Steffen, we will only save the world from catastrophic
climate change by cutting emissions. 鈥淣ew forests are temporary reservoirs that
can buy valuable time to reduce industrial emissions, not permanent offsets to
these emissions.鈥� But the Kyoto Protocol does not reflect that鈥攁nd nor
will next week鈥檚 negotiations. It could prove a devastating mistake.

鈥淭he carbon cycle has a very long equilibrium time,鈥� says Scholes. 鈥淭he
consequences of actions taken now will persist for many centuries.鈥�