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Fred Pearce
New Scientist
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densely populated regions, says an unpublished report from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.
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| Photo: Stone |
Caused
in large part by the melting of Greenland's ice sheet, this
process would take a thousand years or more but would be
"irreversible" once under way.
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.
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 °C, "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 °C would cause sea levels to rise by 3
metres over a thousand years. An 8 °C 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".
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.
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