NEW SCIENTIST FEATURE ON CLIMATE CHANGE
NOV 25/2000
Washed off the map
Better get that ark ready, because the sea levels are gonna
rise
GLOBAL warming could be on the verge of triggering a rise in sea
levels that would flood huge swathes of the Earth's most densely
populated regions, says an unpublished report from the world's
top climate scientists.
The report, due to be published next May by the UN's
Intergovernmental Panel on Climate Change (IPCC), is being read
by the world's governments. The final draft seen by New Scientist
suggests that dozens of the countries meeting this week to agree
on global warming limits through the Kyoto Protocol may face
being wiped off the world map.
Four years ago, the IPCC forecast that sea levels could rise by
half a metre in this century and by a maximum of between 1.5 and
3 metres over the coming 500 years. The new assessment suggests
an eventual rise of 7 to 13 metres is more likely. This is enough
to drown immense areas of land and many major cities. These rises
will occur even if governments succeed in halting global warming
within the next few decades, the report says.=20
Two factors are causing the rise: the slow spread of heat to the
ocean depths and the destabilising of major ice sheets. It will
take about a thousand years for warming in the atmosphere to
reach the bottom of the oceans. The resulting thermal expansion
"would continue to raise sea levels for many centuries after
stabilisation of greenhouse gas concentrations". Even if global
warming is halted within a century, thermal expansion will
eventually raise the oceans by between 0.5 and 4 metres.
Even more alarming is the fate of the ice that covers Greenland.
Among all of the world's ice sheets, this is now thought to be
"the most vulnerable to climatic warming". It contains enough
snow and ice to raise sea levels by about 7 metres if it melts.
And this looks increasingly likely to happen.
Models show that after any warming above 2.7 , "the Greenland
ice sheet eventually disappears". Nearly all predictions show
Greenland warming more than this, says the report, and the faster
the warming, the faster the melting. An extra 5.5 would cause
sea levels to rise by 3 metres over a thousand years. An 8 =B0C
warming would cause a 6-metre rise in sea levels in the same
time.
The report's authors are not allowed to discuss their findings
until publication. But Jonathan Gregory of Britain's Hadley
Centre for Climate Prediction and Research in Bracknell, who
co-authored the chapter on sea level, told New Scientist recently
that once under way, the disintegration of the Greenland ice
sheet would be "irreversible this side of a new ice age".=20
The fate of the West Antarctic ice sheet, which is perched on =
submerged islands, remains controversial, says the report. If it
melted, it would raise sea levels by a further 6 metres. Some
experts quoted in the report predict that the sheet could
entirely disappear within 700 years. Others, supported by the
authors, expect that the sheet will contribute "no more than 3
metres" to sea level in that time.
If sea levels were 10 metres higher than today by the year 3000,
it would cause the inundation of a total area larger than the US,
with a population of more than a billion people and most of the
world's most fertile farmland.
Fred Pearce
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