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http://www.spiegel.de/international/world/0,1518,662092,00.html 11/19/2009
03:47 PM Stagnating Temperatures Climatologists Baffled by Global Warming
Time-Out

By Gerald Traufetter

Global warming appears to have stalled. Climatologists are puzzled as to why
average global temperatures have stopped rising over the last 10 years. Some
attribute the trend to a lack of sunspots, while others explain it through
ocean currents.

At least the weather in Copenhagen is likely to be cooperating. The Danish
Meteorological Institute predicts that temperatures in December, when the
city will host the United Nations Climate Change
Conference,<http://www.spiegel.de/international/world/0,1518,661747,00.html>will
be one degree above the long-term average.

Otherwise, however, not much is happening with global warming at the moment.
The Earth's average temperatures have stopped climbing since the beginning
of the millennium, and it even looks as though global warming could come to
a standstill this year.

Ironically, climate change appears to have stalled in the run-up to the
upcoming world summit in the Danish capital, where thousands of politicians,
bureaucrats, scientists, business leaders and environmental activists plan
to negotiate a reduction in greenhouse gas emissions. Billions of euros are
at stake in the negotiations.

*Reached a Plateau*

The planet's temperature curve rose sharply for almost 30 years, as global
temperatures increased by an average of 0.7 degrees Celsius (1.25 degrees
Fahrenheit) from the 1970s to the late 1990s. "At present, however, the
warming is taking a break," confirms meteorologist Mojib Latif of the
Leibniz Institute of Marine Sciences in the northern German city of Kiel.
Latif, one of Germany's best-known climatologists, says that the temperature
curve has reached a plateau. "There can be no argument about that," he says.
"We have to face that fact."

Even though the temperature standstill probably has no effect on the
long-term warming trend, it does raise doubts about the predictive value of
climate models, and it is also a political issue. For months, climate change
skeptics have been gloating over the findings on their Internet forums. This
has prompted many a climatologist to treat the temperature data in public
with a sense of shame, thereby damaging their own credibility.

"It cannot be denied that this is one of the hottest issues in the
scientific community," says Jochem Marotzke, director of the Max Planck
Institute for Meteorology in Hamburg. "We don't really know why this
stagnation is taking place at this point."

Just a few weeks ago, Britain's Hadley Centre for Climate Prediction and
Research added more fuel to the fire with its latest calculations of global
average temperatures. According to the Hadley figures, the world grew warmer
by 0.07 degrees Celsius from 1999 to 2008 and not by the 0.2 degrees Celsius
assumed by the United Nations Intergovernmental Panel on Climate Change.
And, say the British experts, when their figure is adjusted for two
naturally occurring climate phenomena, El Niño and La Niña, the resulting
temperature trend is reduced to 0.0 degrees Celsius -- in other words, a
standstill.

The differences among individual regions of the world are considerable. In
the Arctic, for example, temperatures rose by almost three degrees Celsius,
which led to a dramatic melting of sea ice. At the same time, temperatures
declined in large areas of North America, the western Pacific and the
Arabian Peninsula. Europe, including Germany, remains slightly in positive
warming territory.

*Mixed Messages*

But a few scientists simply refuse to believe the British calculations.
"Warming has continued in the last few years," says Stefan Rahmstorf of the
Potsdam Institute for Climate Impact Research (PIK). However, Rahmstorf is
more or less alone in his view. Hamburg Max Planck Institute scientist
Jochem Marotzke, on the other hand, says: "I hardly know any colleagues who
would deny that it hasn't gotten warmer in recent years."

The controversy sends confusing and mixed messages to the lay public. Why is
there such a vigorous debate over climate change, even though it isn't
getting warmer at the moment? And how can it be that scientists cannot even
arrive at a consensus on changes in temperatures, even though temperatures
are constantly being measured?

The global temperature-monitoring network consists of 517 weather stations.
But each reading is only a tiny dot on the big world map, and it has to be
extrapolated to the entire region with the help of supercomputers. Besides,
there are still many blind spots, the largest being the Arctic, where there
are only about 20 measuring stations to cover a vast area. Climatologists
refer to the problem as the "Arctic hole."

The scientists at the Hadley Center simply used the global average value for
the hole, ignoring the fact that it has become significantly warmer in the
Arctic, says Rahmstorf. But a NASA team from the Goddard Institute for Space
Studies in New York, which does make the kinds of adjustments for the Arctic
data that Rahmstorf believes are necessary, arrives at a flat temperature
curve for the last five years that is similar to that of their British
colleagues.

Marotzke and Leibniz Institute meteorologist Mojib Latif are even convinced
that the fuzzy computing done by Rahmstorf is counterproductive. "We have to
explain to the public that greenhouse gases will not cause temperatures to
keep rising from one record temperature to the next, but that they are still
subject to natural fluctuations," says Latif. For this reason, he adds, it
is dangerous to cite individual weather-related occurrences, such as a
drought in Mali or a hurricane, as proof positive that climate change is
already fully underway.

"Perhaps we suggested too strongly in the past that the development will
continue going up along a simple, straight line. In reality, phases of
stagnation or even cooling are completely normal," says Latif.

*The Difficulties of Predicting the Climate*

Climatologists use their computer models to draw temperature curves that
continue well into the future. They predict that the average global
temperature will increase by about three degrees Celsius (5.4 degrees
Fahrenheit) by the end of the century, unless humanity manages to
drastically reduce greenhouse gas emissions. However, no one really knows
what exactly the world climate will look like in the not-so-distant future,
that is, in 2015, 2030 or 2050.

This is because it is not just human influence but natural factors that
affect the Earth's climate. For instance, currents in the world's oceans are
subject to certain cycles, as is solar activity. Major volcanic eruptions
can also curb rising temperatures in the medium term. The eruption of Mount
Pinatubo in June 1991, for example, caused world temperatures to drop by an
average of 0.5 degrees Celsius, thereby prolonging a cooler climate phase
that had begun in the late 1980s.

But the Mount Pinatubo eruption happened too long ago to be related to the
current slowdown in global warming. So what is behind this more recent
phenomenon?

*Weaker Solar Activity*

The fact is that the sun is weakening slightly. Its radiation activity is
currently at a minimum, as evidenced by the small number of sunspots on its
surface. According to calculations performed by a group of NASA scientists
led by David Rind, which were recently published in the journal *Geophysical
Research Letters*, this reduced solar activity is the most important cause
of stagnating global warming.

Latif, on the other hand, attributes the stagnation to so-called Pacific
decadal oscillation (PDO). This phenomenon in the Pacific Ocean allows a
larger volume of cold deep-sea water to rise to the surface at the equator.
According to Latif, this has a significant cooling effect on the Earth's
atmosphere.

With his team at the Leibniz Institute of Marine Sciences, Latif has been
one of the first to develop a model to create medium-term prognoses for the
next five to 10 years. "We are slowly starting to attempt (such models),"
says Marotzke, who is also launching a major project in this area, funded by
the Federal Ministry for Research and Technology.

Despite their current findings, scientists agree that temperatures will
continue to rise in the long term. The big question is: When will it start
getting warmer again?

If the deep waters of the Pacific are, in fact, the most important factor
holding up global warming, climate change will remain at a standstill until
the middle of the next decade, says Latif. But if the cooling trend is the
result of reduced solar activity, things could start getting warmer again
much sooner. Based on past experience, solar activity will likely increase
again in the next few years.

*Betting on Warmer Temperatures*

The Hadley Center group expects warming to resume in the coming years. "That
resumption could come as a bit of a jolt," says Hadley climatologist Adam
Scaife, explaining that natural cyclical warming would then be augmented by
the warming effect caused by anthropogenic greenhouse gas emissions.

While climatologists at conferences engage in passionate debates over when
temperatures will start rising again, global warming's next steps have also
become the subject of betting activity.

Climatologist Stefan Rahmstorf is so convinced that his predictions will be
correct in the end that he is willing to back up his conviction with a
€2,500 ($3,700) bet. "I will win," says Rahmstorf.

His adversary Latif turned down the bet, saying that the matter was too
serious for gambling. "We are scientists, not poker players."

*Translated from the German by Christopher Sultan*