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There is clear evidence that climate is changing but what is the evidence concerning rising sea levels?
Richard Pickering, Market Harborough
Professor Mitchell: Sea level responds to climate change mainly through thermal expansion of sea water and the addition of water from the melting of land based ice. In measuring sea level rise, one also has to take account of the vertical movement which is a delayed recovering from the weight of the vast ice sheets of the last ice age (e.g. Scandinavia is rising whereas southeast England is sinking). Direct measurements of sea level from tidal stations indicate a global mean rise of 170mm since 1900 with some uncertainty due in part to distribution of the stations.
Measurements of sea surface and subsurface temperatures confirm that there is a significant contribution from thermal expansion. Most of the world's mountain glaciers have decreased in length over the last century. It is more difficult to estimate the contribution from land ice over Greenland, but it is estimated to contribute about 5% to the increase. Antarctica in the short term is expected to contribute to a decrease in sea level. This is because increased air temperatures are expected to lead to increased snowfall over Antarctica at the expense of moisture form the ocean. The future long term behaviour of the Antarctic Glaciers is problematic.
In future, uncertainties in the observed changes in sea level should reduce as the record of satellite altimetry becomes longer, and the measurement of the upper ocean temperatures improves through use of the network of ARGO floats as part of a WMO programme.
In future, uncertainties in the observed changes in sea level should reduce as the record of satellite altimetry becomes longer, and the measurement of the upper ocean temperatures improves through use of the network of ARGO floats as part of a WMO programme.
