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eu No. 63 | APRIL 2010
SPECIAL REPORT
CLIM
A
T
E
IPCC (
1
) Vice-Chair
Jean-Pascal van Ypersele
presents the current status of
scientific knowledge of global
warming. Not forgetting the
sometimes surprising political
reading of the facts…
“We have done terribly little
compared with the immensity
of the problem.”
INTERVIEW
There now seems to be a consensus that the
world is warming, but is it certain that human
activities are responsible?
The level of confidence in attributing this
phenomenon to human activity is very high
and increasing by the year. In 1995, the IPCC
wrote that “a range of elements suggests that
there is a perceptible influence of human
activities on climate”. In 2007, the conclusion
was that the greater part of the global warming
of the past 50 years is “very probably due” to
greenhouse gases of human origin, which
translates to a probability of above 90 %.
This assurance is based on many arguments.
There are certainly the climate models, which
have improved greatly. But also the particular
form this global warming is taking: a cooling of
the upper atmosphere, as greenhouse gases trap
part of the heat of the lower atmosphere that is
heating up quickly. If the warming were due to
increased solar activity, for example, it would
be uniform or even more pronounced in the
upper atmosphere. Similarly, we are seeing that
the poles are heating up more quickly than the
tropics and that is again in line with greenhouse
gases being the cause.
What are the principal effects to be feared?
The latest IPCC report devotes hundreds of
pages to synthesising impacts that range from
falling agricultural yields to various health
problems. I should like to stress the importance
of hydrological changes: the models predict
a significant drought problem in a number of
densely populated regions, including the
Mediterranean Basin where we are already
seeing significant water access problems.
Another aspect is the melting of the glaciers
in the Andes and the Himalayas that act as
a reservoir for hundreds of millions of people
for whom there is only rain during a few weeks
or months of the year. The rest of the year it is
the glaciers that feed the rivers and their
programmed disappearance is therefore very
worrying.
Then there are the rising sea levels. All
the European coastlines could be affected by
this, but especially low-lying coasts as in the
Netherlands, Belgium and Germany. We will
see an increased rate of erosion, saltwater
invading the groundwater, increased storm
damage, etc. Then in the Nile Delta there are
10 million people living less than a metre
above sea level. The sea level will almost
certainly rise by at least 50 cm, and perhaps
a metre. Where will they go?
What about a 2°C temperature rise as the
‘danger threshold’?
The IPCC has never said that the temperature
rise should not exceed 2°C or that atmospheric
CO
2
concentrations should be kept at under
450 ppm (parts per million). Our job – and
the nuance is important – is to say that, for
a given emission scenario, we expect a certain
level of global warming and a given impact as
a result. It is for the public authorities to define
what impacts are acceptable as that supposes
value judgments and these are not the job
of scientists. Historically, the figure of 2°C
emerged in 1996 at a meeting of the EU Council
of Ministers. It was then in a sense validated
by the IPCC’s 2001 report that published
the famous ‘burning embers’ diagram that
synthesised the gravity of impacts for different
temperatures. Its colour code ranged from
white to red at around 2°C for the majority of
the impacts and this too helped fix this figure
in people’s minds when it was based on data
more than a decade old.
Are you saying that the latest scientific data
call into question this threshold?
We looked again in detail at these impacts,
at the request of politicians. The authors of the
2007 report, practically the same individuals as
in 2001, concluded that the impact thresholds
needed to be revised downwardly by around
0.5°C. Their new graph [See article ‘The tools
of diagnosis’ in this issue, editor’s note] was not
published in the report but subsequently, in
© Jack
y Delorme (UCL)
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