8.4 Longer-term projections

For the next few decades, the rate of sea-level rise is partly locked in by past emissions, and will not be strongly dependent on early 21st century greenhouse gas emission. However, sea level projections closer to and beyond 2100 are critically dependent on future greenhouse gas emissions, with both ocean thermal expansion and the ice sheets potentially contributing metres of sea-level rise over centuries for higher greenhouse gas emissions.
Present day contributions from the Greenland come from both surface melting and iceberg calving and for the Antarctic ice sheet from iceberg calving only. The contribution from the ice sheets is poorly understood at the moment and is an active area of research.

In the case of the Greenland Ice Sheet, if global average temperatures cross a point that is estimated to be in the range of 1.9°C to 4.6°C above pre-industrial values, surface melting is likely to exceed precipitation (Gregory and Huybrechts, 2006). The inevitable consequence of this is an ongoing shrinking of the Greenland Ice Sheet over a period of centuries and millennia. Total melting of the Greenland ice sheet alone would increase global mean sea level by around 7 metres. This conclusion is consistent with the observation that global sea level in the last interglacial, when temperatures were in this range, was several metres higher than it is today. This threshold (of melting exceeding precipitation) could potentially be crossed late in the 21st century.

Dynamic responses of the Greenland and/or West Antarctic Ice Sheets (sliding of the ice sheets over bedrock) could lead to a significantly more rapid rate of sea level rise than from surface melting alone. There is increasing evidence that this dynamic response may be occurring. (http://www.cmar.csiro.au/sealevel)