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Global climate
change is "very likely" to have a human cause, an influential group of
scientists has concluded. The Intergovernmental Panel on Climate
Change (IPCC) said temperatures were probably going to increase by 1.8-4C
(3.2-7.2F) by the end of the century. It also projected that sea levels were
most likely to rise by 28-43cm, and global warming was likely to influence the
intensity of tropical storms. The findings are the first of four IPCC
reports to be published this year. "We can be very confident that the net
effect of human activity since 1750 has been one of warming," co-lead author Dr
Susan Soloman told delegates in Paris. The report, produced by a team tasked
with assessing the science of climate change, was intended to be the definitive
summary of climatic shifts facing the world in the coming years.
IPCC
PROJECTIONS
Probable temperature rise between 1.8C and 4C Possible temperature rise between 1.1C and 6.4C Sea level most likely to rise by 28-43cm Arctic summer sea ice disappears in second half of century Increase in heatwaves very likely
Increase in tropical storm intensity likely
The agency said that it would use stronger language to assess humanity's influence on climatic change than it had
previously done. In 2001, it said that it was "likely"
that human activities lay behind the trends observed at various parts of the
planet; "likely" in IPCC terminology means between 66% and 90% probability.
Now, the panel concluded that it was at least 90% certain that human emissions of greenhouse gases rather than natural
variations are warming the planet's surface. They projected that temperatures would probably rise by between 1.8C and 4C, though increases as small as 1.1C (2F) or
as large as 6.4C (11.5F) were possible. In 2001, using different methodology,
the numbers were 1.4 (2.5F) and 5.8C (10.4F). How computers model climate
On sea level, there has been a more fundamental debate. Computer models of climate
generally include water coming into the oceans as ice caps and glaciers melt.
But the potentially much larger contribution of "accelerated melting", where the
disintegration of ice shelves and lubrication of glaciers by meltwater speeds up
the flow of ice into the oceans, is much harder to model. So the IPCC had to decide whether to
exclude this from its calculations, or to estimate the effect of a process which
scientists do not understand well but which could have a big impact. They used the former, more conservative
approach, projecting an average rise in sea levels globally of between 28 and
43cm. The 2001 report cited a range of nine to 88cm. As for climate change influencing the
intensity of tropical storms in some areas of the world, the IPCC concluded that
it was likely - meaning a greater probability than 66% - that rising
temperatures were a factor. 'Unequivocal' Dr Rajendra Pachauri, the IPCC chairman, said: "It is extremely encouraging in that the science has moved on from what
was possible in the Third Assessment Report. "If you see the extent to which human
activities are influencing the climate system, the options for mitigating
greenhouse gas emissions appear in a different light, because you can see what
the costs of inaction are," he told delegates in Paris. Achim Steiner, executive director of the
United Nations Environment Programme (Unep), said the findings marked a
historical landmark in the debate about whether humans were affecting the state
of the atmosphere. THE FINDINGS
"It is an unequivocal series of evidence [showing that] fossil fuel burning and land use change are affecting the climate
on our planet." He added: "If you are an African child
born in 2007, by the time you are 50 years old you may be faced with disease and
new levels of drought." But a study published on the eve of the IPCC report suggested that the international body's previous reports may have
actually been too conservative. Writing in the journal Science, an
international group of scientists concluded that temperatures and sea levels had
been rising at or above the maximum rates proposed in the last report, which was
published in 2001. The paper compared the 2001 projections on temperature and sea level change report with what has actually happened.
The models had forecasted a temperature
rise between about 0.15C-0.35C (0.27-0.63F) over this period. The actual rise of
0.33C (0.59F) was very close to the top of the IPCC's range. A more dramatic picture emerged from the
sea level comparison. The actual average level, measured by tide gauges and
satellites, had risen faster than the intergovernmental panel of scientists
predicted it would. The IPCC's full climate science report
will be released later in the year, as will other chapters looking at the
probable impacts of climate change, options for adapting to those impacts, and
possible routes to reducing emissions of greenhouse gases.
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