Rising sea level is one of the greatest threats to low-lying islands and coasts. This century it could go up by more than a metre. If ice sheets collapse, the sea could rise by several metres over a few centuries.
Today the sea is rising at more than 3 millimetres per year, as warming water expands and as glaciers melt. Predictions for 2100 vary, but the rise could be up to about 1.5 metres if emissions stay high. This could combine with growing storm surges, [Flood] to threaten coastal settlements with flooding and fresh water pollution. By mid-century millions of people may be displaced. According to a World Bank/OECD study, global economic losses could reach a trillion dollars per year by 2050 if defences are not improved.
The Greenland and West Antarctic ice sheets may reach tipping points where collapse over a few centuries becomes almost inevitable. Greenland holds enough to raise global sea level 7 metres; the West Antarctic 3 to 5 metres. To predict future rises, scientists can add up the thermal expansion of the sea and the melting of land ice. In the IPCC’s latest report, this approach is used to project 0.53 to 0.97 metres rise this century for high emissions (RCP8.5), and 0.28 to 0.60 metres for low emissions (RCP2.6). But this “process-based” method may not include the full effect of glaciers draining ice from Greenland and Antarctica. Alternative “semi-empirical” models, based on past observations, produce higher projections – from about 0.7 to 1.65 metres for RCP 8.5.
Sea level will be uneven because of changes in ocean currents and the shifting masses of ice. Among the fastest-rising sea levels will be along the US East Coast and near the equator. In Southeast Asia, tens of millions could be displaced by 2100. Some low-lying island states may be obliterated – such as Tuvalu, which lies in a region of rapid sea-level rise. Among industrial nations, many large coastal cities are at risk. The World Bank study names Guangzhou, Miami and New York as the three cities with the highest projected economic losses.