An investigation of what is causing climate change and one of its major impacts – the rising level of the sea.
The level of the sea is constantly changing everywhere on Earth as tides rise and fall in predictable patterns. Both the timing and height of high tides are forecast accurately because they depend on the relative positions of the Earth, the Moon, and the Sun. However, tides are just variations on average water levels – they are not changes in average water levels.
Less widely understood is that sea levels have changed much more dramatically in the past as the Earth has moved in and out of ice ages. In cold periods the sea has been low and in warm periods the sea has been high. Thus the sea has risen and fallen by over a hundred metres many times in the history of the Earth. But over the last few thousand years the climate has been relatively stable and the sea level has varied only a little.
However, since about 1900, sea levels have risen by about 20 centimetres. There is a strong consensus among scientists that rising sea levels are largely a consequence of increasing concentrations of carbon dioxide and other greenhouse gases in the atmosphere raising global temperatures.
The Intergovernmental Panel on Climate Change (IPCC) currently expects sea level to rise a further 30 to 100 centimetres by 2100, and to continue rising for several centuries.5 How much the sea actually rises by 2100 and beyond will depend on what action is taken to reduce greenhouse gas emissions in the coming decades.
Three processes drive this rise in sea level – seawater expands as it warms, glaciers melt and retreat, and ice sheets shrink. Although expanding seawater and melting glaciers can be modelled with high confidence, there are still big questions around how the massive ice sheets that cover Antarctica and Greenland will react.
Sea level rise is not the only consequence of a warming world, but it is a particularly sobering one since many millions live just above the high tide mark.6 New Zealanders are not as vulnerable as those who live on low-lying islands and in river deltas, but as a coastal nation the impact on our beaches, buildings, roads and other infrastructure, and on our communities will be considerable.