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regions, says an unpublished report from the world's top climate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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