Sea level from the year 200 to the year 2100. The rise in sea level of 1 m is calculated from global warming of 3 °C in this century. The dotted line indicates the IPCC prediction. The blue shade indicates the degree of uncertainty. Credit: Aslak Grinsted, Niels Bohr Institutet. Click to enlarge.

The great uncertainty in the projection of the future rise in the sea level lies in the uncertainty over how quickly the ice sheets on land will melt and flow out to sea.

The models of the melting of the ice sheets that are the basis for the Intergovernmental Panel on Climate Change’s predictions for the rise in sea level underpredict the actual rates of sea level rise between 1993–2006 by 40%.

Instead of trying to project what will happen with the melting of the ice sheets researchers in Denmark, the UK and Finland developed a model based on the direct relationship between the global temperature and the sea level over the past 2,000 years. A paper on their work was published online 6 January in the journal Climate Dynamics.

We use a physically plausible four parameter linear response equation to relate 2,000 years of global temperatures and sea level. We estimate likelihood distributions of equation parameters using Monte Carlo inversion, which then allows visualization of past and future sea level scenarios. The model has good predictive power when calibrated on the pre-1990 period and validated against the high rates of sea level rise from the satellite altimetry.

—Grinsted et al. (2009)

With the help of annual growth rings of trees and analysis from ice core borings researchers have been able to calculate the temperature for the global climate 2000 years back in time. For around 300 years the sea level has been closely observed in several places around the world and in addition to that there is historical knowledge of the sea level of the past in different places in the world.

For the sea level to rise between the predicted 0.9 and 1.3 meters so quickly means that the ice sheets will melt much faster than previously believed. But it has already been observed that the ice sheets react more quickly to increases in temperature than experts thought just a few years ago. Furthermore, studies from the 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 millimeters per year—equivalent to a meter in 100 years.

Resources

• Aslak Grinsted, J. C. Moore, S. Jevrejeva (2009) Reconstructing sea level from paleo and projected temperatures 200 to 2100 AD. Clim Dyn doi: 10.1007/s00382-008-0507-2