Daylife/AP Photo by Luigi Costantini

Global sea level is rising, and faster than expected. We need to honestly discuss this risk rather than trying to play it down.

Measurements from tide gauge stations around the world show that the global sea level has risen by almost 20cm since 1880. Since 1993, global sea level has been measured accurately from satellites; since 1993 figures have shown levels rising at a rate of 3.2cm per decade.

The two main causes of this rise are extra water entering the ocean from melting land-ice and the expansion of ocean water as it gets warmer. Both are inevitable physical consequences of global warming. Both contributions can be estimated independently from satellite and other data, and their sum is consistent with the observed rise. Depending on the time period considered, 50% to 80% of the rise is due to melting ice.

Despite knowing the causes, we cannot predict future sea level rise very well. Particularly uncertain is how ice sheets will respond to warming, as this involves complex flow processes. For example, warming ocean waters destroy the floating tongues of ice that form when glaciers meet the sea. These ice tongues are pinned to rock outcrops and hold back the glacier behind them. When the ice tongue goes, the glacier speeds up its flow. This has happened to the Jakobshavn Isbrae and other glaciers in Greenland as well as many outlet glaciers on the Antarctic Peninsula.

The latest report by the Intergovernmental Panel on Climate Change found that sea level has been rising 50% faster since 1961 than its computer models predict…

Even after we have stopped global warming, sea level rise set in motion by our emissions of the coming decades will continue for centuries. Such is the inertia in the response of the deep ocean and the ice sheets to warming. While we can bail out banks, there is no way to turn back sea level — our only chance is to stop the warming soon enough to keep it within manageable limits.

Hopefully, with full understanding and cooperation – by the end of the century or sooner.