Sea level may rise one metre by 2100

16 January 2009

Evidence from the past suggests the Greenland and Antarctic ice sheets may melt faster than current predictions.

The rise is three times higher than predictions made by the Intergovernmental Panel on Climate Change (IPCC).

Sea level rise is one of the biggest climate uncertainties this century - many research teams in the UK and elsewhere are focusing efforts to reduce this uncertainty.

One issue is how the Earth's major ice sheets, Antarctica and Greenland, will respond to rising temperatures. Over the last few years both ice sheets have been losing ice. Best estimates suggest they are making a relatively small contribution to sea level rise of 0.1-0.8 mm a year -- the global average rise is around 3 mm/yr: the biggest cause of sea level rise is thermal expansion of the oceans.

Ice flow speeds have accelerated recently. Scientists have yet to establish whether these recent changes are part of a natural cycle or related to warming caused by human activities. For this reason, the IPCC authors have been cautious about including them in sea level predictions.

The team's reconstruction of sea level rise over the last 2000 years, and extrapolated to 2100. Compare this with the IPCC estimates. Note how the sea level rose during the Middle Ages warm period (12th century) and fell during the Little ice Age (17th century).

Researchers at the University of Copenhagen, Proudman Oceanographic Laboratory and the University of Lapland in Finland have taken a different approach.

'Instead of making calculations based on what one believes will happen with the melting of the ice sheets we have made calculations based on what has actually happened in the past,' says Dr Aslak Grinsted, a geophysicist at the Niels Bohr Institute, Copenhagen.

The team use actual measurements, tree growth rings and ice cores to piece together temperature fluctuations over the last 2000 years. Tide gauges and geological evidence gave sea level over this time.

The researchers found that during the Middle Ages warm period, around 12th century, sea level was approximately 20 cm higher than today. During the 'Little Ice Age' in the 18th century, when it was possible to skate on the Thames in winter, sea level was approximately 25 cm lower than it is today.

The team, who published their findings in the journal Climate Dynamics, say that present sea level is within 20 cm of the highest sea level for 110,000 years.

The IPCC predicts global average temperature will rise 2-4 ºC this century based on rapid economic growth, population peaking mid-century and energy demands met by a balance between fossil and non-fossil fuels.

When the team assumed a 3 ºC rise, the new model predictions indicate that the sea level will be between 0.9-1.3 m.

The research suggests projected sea level rise for the most optimistic IPCC scenario -- a two degree rise in temperature by the end of the century -- is around 80 cm.

To rise so much so quickly, the climate researchers say the ice sheets will melt much faster than previously believed.

The authors believe there is a precedent for such a bold statement. Studies from the last ice age show that ice sheets can melt quickly. When the ice age ended 11,700 years ago, the ice sheets melted so quickly that sea level rose 11 mm a year, or one metre in a century.

The IPCC Fourth Assessment Report published in 2007 says, 'Understanding of these effects is too limited...to provide a best estimate or an upper bound for sea level rise.'

So the IPCC does not rule out much greater rises than predicted. The challenge now is to find a consensus on how fast the ice sheets are responding to the warming.