Dozens of the world’s cities, including London and New York,
could be flooded by the end of the century, according to research which suggests
that global warming will increase sea levels more rapidly than was previously
The first study to combine computer models of rising temperatures with records
of the ancient climate has indicated that sea levels could rise by up to 20ft
(6m) by 2100, placing millions of people at risk.
The threat comes from melting ice sheets in Greenland and Antarctica, which
scientists behind the research now believe are on track to release vast volumes
of water significantly more quickly than older models have predicted. Their
analysis of events between 129,000 and 116,000 years ago, when the Arctic last
warmed to temperatures forecast for 2100, shows that there could be large rises
in sea level.
While the Greenland ice sheet is expected to start melting as summer temperatures
in the Arctic rise by 3C degrees to 5C (5.4F-9F), most models suggest that the
ice sheets of Antarctica will remain more stable.
The historical data, however, show that the last time that Greenland became
this warm, the sea level rise generated by meltwater destabilised the Antarctic
ice, leading to a much higher increase than can be explained by Arctic ice alone.
That means that the models of sea-level rise used to predict an increase of
up to 3ft by 2100 may have significantly underestimated its ultimate extent,
which could be as great as 20ft.
Such a rise would threaten cities such as London, New York, Bombay and Tokyo.
Large parts of the Netherlands, Bangladesh and Florida would be inundated, and
even smaller rises would flood extreme low-lying areas, such as several Pacific
islands and New Orleans.
“Although the focus of our work is polar, the implications are global,”
said Bette Otto-Bliesner, of the US National Centre for Atmospheric Research
in Boulder, Colorado, who led the study. “These ice sheets melted before
and sea levels rose. The warmth needed isn’t that much above present conditions.”
Her colleague, Jonathan Overpeck, of the University of Arizona, said: “This
is a real eye-opener set of results. The last time the Arctic was significantly
warmer than the present day, the Greenland ice sheet melted back the equivalent
of two to three metres (6ft-10ft) of sea level. Contrary to what was previously
believed, the research suggests the Antarctic ice sheet also melted substantially,
contributing another 6ft to 10ft of sea level rise.”
The findings, which are published today in the journal Science, have emerged
from a study that used data from ancient coral reefs, ice cores and other natural
records to reconstruct the climate during the last gap between Ice Ages. In
this interglacial period, between 129,000 and 116,000 years ago, temperatures
in the Arctic were between 3C and 5C above present levels — a similar
level to that predicted for the end of this century.
The scientists found that meltwater from Greenland raised the sea level by
up to 11ft, but coral records showed that the total global rise was between
13ft and 20ft. Dr Overpeck said that the melting of Antarctic ice sheets was
the most likely explanation. As sea levels rose, the floating ice shelves off
the coast of the continent would have become more likely to break up. That in
turn would have allowed glaciers to dump more ice from the continent itself
into the sea.
He said that this was particularly worrying at present as the base of the West
Antarctic ice sheet lay below sea level, which would allow ice to escape to
the sea easily.
Several recent studies have indicated that the Greenland ice sheet, which contains
enough water to raise sea levels by 23ft, and the West Antarctic sheet, which
holds enough for a 20ft rise, are thinning. Both are expected to take several
centuries to melt completely, but could release substantial quantities of water
Dr Overpeck said that the results added to the urgency of measures to control
the greenhouse gas emissions contributing to global warming.