Global temperatures will rise
further in the future than
previous studies have indicated,
according to new research from
two scientific teams.
They both used historical
records to calculate the likely
amplification of warming as
higher temperatures induce
release of CO2 from ecosystems.
They both conclude that current
estimates of warming are too
low, by anything up to 75%.
Their conclusion is backed up by
a new report from the Australian
government.
The Australian Greenhouse Office
says current estimates of
temperature rise are "being
challenged" by new research.
Higher temperatures will
probably cause trees to release
more carbon
Heightened sensitivity
The latest evidence comes in two
papers to be published in the
scientific journal Geophysical
Research Letters.
They challenge the consensus
view of the Intergovernmental
Panel on Climate Change (IPCC),
the global body charged with
collating and analysing climate
science.
|
|
 The
evidence for a warming
Earth is stronger and
the impacts of climate
change are becoming
observable in some cases
Australian Greenhouse
Office |
It predicts that the global
average temperature would rise
by between 1.5C and 4.5C if
human activities were to double
the amount of carbon dioxide
(CO2) in the atmosphere.
That figure, known as the
climate sensitivity, results
from a combination of two
factors:
-
the direct impact of rising
CO2 on the greenhouse effect
-
various "feedback"
mechanisms which amplify the
rate of warming, such as
changes in the Earth's
reflection of sunlight as
ice melts
The new research adds a third
component, by calculating the
likely contribution of carbon
dioxide released from natural
ecosystems such as soil as
temperatures rise.
This would add to the CO2
produced through human
activities, raising temperatures
still further.
Soil cycle
To calculate this extra warming,
both research groups have looked
back into the Earth's history.
Regularly, spells of relatively
high temperatures have produced
rises in atmospheric carbon
dioxide concentrations, which
have fallen again as colder
conditions took over.
The theory is that in warm
spells, ecosystems such as
soils, forests and oceans retain
less carbon.
As the Earth's surface is now
warming again, the process might
be expected to repeat itself,
with higher temperatures again
causing the biological world to
release CO2 into the atmosphere,
complementing the gas coming
from homes, factories and
vehicles.
To calculate the relationship
between temperature rise and
carbon release, the US study
examined a period of about
400,000 years using data from
the Vostok ice core of
Antarctica.
The European group worked on a
much shorter period, looking
back to the "Little Ice Age", a
period in the middle of the last
millennium when the northern
hemisphere experienced
relatively low temperatures.
"Our group used long time
periods, over entire glacial and
interglacial cycles, to get this
relationship between climate and
carbon," explained John Harte
from the University of
California, Berkeley.
"The European team looked at a
much more modern period, and
also used a different analytical
method," he told the BBC News
website.
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Several
studies looking at
climate sensitivity tend
to show higher figures
than we have been used
to
Martin Wild |
The European group calculates
that temperature rises in the
future have been underestimated
by between 15% and 78%; the US
team expresses its results in a
different way, giving a climate
sensitivity of between 1.6 and
6.0C.
"We don't get very different
answers," observed Professor
Harte.
"And using different periods is
very helpful, because we know
the results are more robust."
Supporting evidence
These are not the only recent
studies to suggest that climate
sensitivity may have been
underestimated.
The Australian Greenhouse Office
report cites research showing
that some forests which were net
absorbers of carbon may be
turning into net producers, an
effect anticipated as
temperatures rise.
|
Data
came from gas bubbles
trapped in the Vostok
ice core |
Martin Wild from the Institute
for Atmospheric and Climate
Science (IACETH) in Zurich, a
specialist on climate feedback,
believes there are other reasons
to revisit the current picture.
"The point is that there are
several studies coming from
several directions looking at
climate sensitivity, and they
tend to show higher figures than
we have been used to," he told
the BBC News website.
"There is some evidence on the
uptake of heat in the deep
ocean, for example, which could
make it higher."
On the current carbon studies,
he concluded: "If this
additional carbon feedback is
proven to be realistic, than
that would raise the climate
sensitivity up by a certain
amount."
Future reflections
Both scientific teams admit
their work is not as precise as
they would like, and that
uncertainties remain.
One particular issue is whether
the past accurately reflects the
future. Do forests and soils
behave now, in an era of vast
deforestation and widespread
fertiliser use, just as they did
100,000 or even 1,000 years ago?
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We
have, in fact, been
conservative on several
points
Marten Scheffer |
That remains unproven; and
climate "sceptics" will
undoubtedly seize on this as
evidence that the new research
is flawed, though they will have
to admit that it is
substantially grounded in data
and not computer models, often
the target of their ire.
The researchers counter that
they have not found reasons why
carbon feedback mechanisms
should be different in the
future. And even if differences
do arise, they say, future
feedback could be stronger as
easily as it could be weaker.
"We have, in fact, been
conservative on several points,"
said Marten Scheffer from
Wageningen University in the
Netherlands, leader of the
European group.
"For instance, we do not account
for the greenhouse effect of
methane, which is also known to
increase in warm periods."
Currently the IPCC is reviewing
its latest major study, the
Fourth Assessment Report, which
will be released next year.
The first draft, of which BBC
News has seen a leaked copy,
suggests it has not radically
changed its projections for
temperature rise since its last
report in 2001.
A climate sensitivity of up to
4.5C translates to a maximum
likely temperature rise of about
5.8C by the end of this century;
whereas in these two studies,
the US team calculates up to
7.7C, with the European group's
maximum value even higher.
"In view of our findings,"
observes Marten Scheffer,
"estimates of future warming
that ignore these [carbon
feedback] effects may have to be
raised by about 50%."
Richard.Black-INTERNET@bbc.co.uk
