The world's oceans may rise up to 140 cms (4 ft 7 in) by 2100 due to global
warming, a faster than expected increase that could threaten low-lying coasts
from Florida to Bangladesh, a researcher said on Thursday.
"The possibility of a faster sea level rise needs to be considered when planning
adaptation measures such as coastal defenses," Stefan Rahmstorf of the Potsdam
Institute for Climate Impact Research wrote in the journal Science.
His study, based on air temperatures and past sea level changes rather than
computer models, suggested seas could rise by 50-140 cms by 2100, well above the
9-88 cms projected by the scientific panel that advises the United Nations.
A rise of one meter might swamp low-lying Pacific islands such as Tuvalu, flood
large areas of Bangladesh or Florida and threaten cities from New York to Buenos
"The computer models underestimate the sea level rise that has already
occurred," Rahmstorf told Reuters of a rise of about 20 cms since 1900. "There
are aspects of the physics we don't understand very well."
Sea level changes hinge on poorly understood factors such as the pace of the
melt of glaciers and of ice sheets in Greenland and Antarctica. Water also
expands as it gets warmer but the rate of penetration of heat to the depths is
"My main conclusion is not that my forecast is better but that the uncertainty
is much larger because of the different results you get with reasonable
methods," he said.
Almost all climate scientists reckon the world is warming because of emissions
of greenhouse gases from human use of fossil fuels in factories, power plants
and cars. Rising temperatures could bring more droughts, floods and heatwaves.
Rahmstorf likened his approach to predicting the height of tides along a coast,
largely based on past observations.
He said seas were 120 meters below present levels during the last Ice Age 20,000
years ago and 25-35 meters higher than the present in the Pliocene epoch 3
million years ago.
In the Ice Age temperatures were 4-7 Celsius (7.2-12.6 Fahrenheit) cooler than
today and 2-3 C (3.6-5.4F) warmer in the Pliocene. That suggested sea levels
change 10-30 meters per rise or fall per degree Celsius (1.8F), over thousands
The U.N. climate panel has projected temperatures will rise by 1.4-5.8C
(2.5-10.4F) by 2100, mainly because of human influences.
"Sea level is a very slow component of the climate system so what we see by the
year 2100 is just a small percentage of the total we are causing," Rahmstorf
There was still time for the world to cut greenhouse gas emissions but he said
the slow pace of U.N. talks on extending the U.N.'s Kyoto Protocol beyond 2012
"gives you the impression that governments are not very well aware of how urgent
the whole problem has become."
Coastal cities in the North Atlantic -- from New York to London -- could be
especially vulnerable because a possible slowdown of ocean currents could also
raise sea levels in the North Atlantic and lower them in the southern
"Any time you change ocean currents you change the sea surface...if you slow
down the North Atlantic current you get a rise in the North Atlantic," Rahmstorf