Future Sea Level Changes

Higher temperatures are expected to raise sea level by:

• expanding ocean water,
• melting mountain glaciers and small ice caps,
• causing portions of the coastal section of the Greenland and Antarctic ice sheets to melt or slide into the ocean.

Higher temperatures are also likely to increase the amount of snowfall over central Greenland and Antarctica. The higher snowfall is likely to offset part of the sea level rise from other factors because the additional snow is composed of water that would otherwise be in the ocean.

Past and projected global average sea level. The gray shaded area shows the estimates of sea level change from 1800 to 1870 when measurements are not available. The red line is a reconstruction of sea level change measured by tide gauges with the surrounding shaded area depicting the uncertainty. The green line shows sea level change as measured by satellite. The purple shaded area represents the range of model projections for a medium growth emissions scenario (IPCC SRES A1B). For reference 100mm is about 4 inches. Source: IPCC (2007)

Considering all of these influences, the Intergovernmental Panel on Climate Change (IPCC) estimates that the global average sea level will rise by 7.2 to 23.6 inches (18-59 cm or 0.18- 0.59m) by 2100 (see Figure 1) relative to 1980-1999 under a range of scenarios.

Note that these estimates assume that ice flow from Greenland and Antarctica will continue at the same rates as observed from 1993-2003. The IPCC cautions that these rates could increase or decrease in the future. For example, if ice flow were to increase linearly, in step with global average temperature, the upper range of projected sea level rise by the year 2100 would be 19.2 to 31.6 inches (48-79 cm or 0.48-0.79 m). But current understanding of ice sheet dynamics is too limited to estimate such changes or to provide an upper limit to the amount by which sea level is likely to rise over this century.

According to the IPCC, current model projections indicate substantial variability in future sea level rise between different locations. Some locations could experience sea level rise higher than the global average projection, while others could have a fall in sea level. The same factors that currently cause sea level to rise more rapidly along the Mid-Atlantic and Gulf Coasts, and less rapidly in parts of the Pacific Northwest, are likely to continue. Changes in winds, atmospheric pressure and ocean currents will also cause regional variations in sea level rise - but those variations cannot be reliably predicted.

Over time, more substantial changes in sea level are possible due to the vulnerability of the West Antarctic and Greenland Ice sheets. However, there are significant uncertainties about the magnitude and speed of future changes (IPCC, 2007):

• The West Antarctic Ice Sheet contains enough ice to raise sea level by 5-6 meters (17-20 feet). Possible instabilities in the ice sheet could allow it to slide into the oceans after a sustained warming, or if other factors raised sea level (IPCC, 2007). There is a small chance the collapse of this ice sheet could occur within a few centuries, but the response of the ice sheet to future climate change is uncertain and a subject of debate (IPCC 2007, NRC 2002).
• The Greenland ice sheet contains enough ice to raise sea level about 7 meters (23 feet). Although it is already contributing to sea level rise (from melting), it does not contain the same instabilities as Antarctica that could result in a rapid collapse. Most model projections suggest a gradual melting over millennia related to sustained climate warming (IPCC, 2007).