Sea-level rise is a central element in detecting, understanding, attributing and correctly projecting climate change. During the 20th century, the oceans have stored well over 80 per cent of the heat that has warmed the earth. The associated thermal expansion of the oceans, together with changes in glaciers and ice caps, will likely dominate 21st century sea level rise. However, there is increasing concern that the contribution from melting of the ice sheet may be larger than previously estimated, and on longer time scales, the ice sheets of Greenland and Antarctica have the largest potential to contribute to significant changes in sea level.
During the 21st century, sea level will continue to rise due to warming from both past (20th century and earlier) and 21st century greenhouse gas emissions. The most robust projections of 21st century sea-level rise are the Assessments of the Intergovernmental Panel on Climate Change (IPCC) of 2001 and 2007.
In its 2007 assessment of global warming, the Intergovernmental Panel on Climate Change (IPCC) projected that global mean sea level is expected to rise between 0.18 to 0.59 meters (0.6 and 2 feet) in the next century.
Estimates of the ocean thermal expansion are made with coupled climate models for the range of SRES greenhouse gas emission scenarios. Recent estimates indicate that non-polar glaciers and ice caps may contain only enough water to raise sea level by 15 to 37 centimetres (Lemke et al. 2007). The largest contribution is from large glaciers in regions with heavy precipitation, such as the coastal mountains around the Gulf of Alaska, or Patagonia and Tierra del Fuego in South America. Many of these glaciers flow into the sea or large lakes and melt quickly because the ice is close to melting temperature.
For Greenland, both glacier calving and surface melting contribute to mass loss. Over the last few decades surface melting has increased and now dominates over increased snowfall, leading to a positive contribution to sea level during the 21st century. For the majority of Antarctica, present and projected surface temperatures during the 21st century are too cold for significant melting to occur and precipitation is balanced by glacier flow into the ocean. (Lemke et al. 2007).
In addition to these surface processes, there are suggestions of a potential dynamical response (sliding of the outlet glaciers over the bedrock) of the Greenland and Antarctic ice sheets. In Greenland, there was a significant increase in the flow rate of many of the outlet glaciers during the early 21st century. One potential reason for this is increasing surface melt making its way to the base of the glaciers, lubricating their flow over the bed rock, consistent with increased glacier flow rates. Another effect which may be becoming more important is that, as the ice shelves around Antarctica and Greenland melt or break up (e.g. Larsen B) they allow the glaciers behind them to flow faster, leading to increased flow into the ocean.
When projecting future sea-level rise, we need to recognise that local trends related to decadal variability will be superimposed on the slowly increasing global-mean sea level. At this stage there is no agreed pattern for the longer-term regional distribution of projected sea-level rise. There are, however, several features that are common to most model projections – for example a maximum sea-level rise in the Arctic Ocean and a minimum rise in the Southern Ocean south of the Antarctic Circumpolat current.
Figure 8 The multi-model mean of the departure of the projected regional sea-level rise from the global-averaged (SRES A1B) projections for 2030 and 2970 (source: http://www.cmar.csiro.au/sealevel)
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)
It is difficult for scientists to be more precise with sea-level projections because there are a number of uncertainties:
- Greenhouse gas concentrations
While scientists agree that the levels of greenhouse gases are rising, future increases depend on many factors, including population growth, energy use and the development of new technologies.
- Climate sensitivity
Climate sensitivity is the amount of atmospheric warming that results from a doubling of atmospheric carbon dioxide concentrations. It depends on the presence of greenhouse gases, and on feedback processes from clouds, water vapour and ice. This is a significant source of uncertainty in projections of long-term climate change.
- Ocean heat exchange
Heat moves between the atmosphere and the ocean’s surface. The temperature at the surface at any one time is influenced by what is going on in the ocean. Quite small changes in the transport of heat or salt can have large effects on surface temperature, and ultimately on climate. Ocean models have developed rapidly over the last two decades but accurately representing the most important ocean features remains a challenge.
- Ice
There is uncertainty about the response of the ice sheets in Greenland and Antarctica to hundreds of years of warmer temperatures. Scientists are concerned that there could be a rapid disintegration of the West Antarctic ice sheet, causing a rapid rise in sea level.
(source: http://science.org.au/nova/082/082key.htm).