Oceans Gaining Ground at Unusually Rapid Rate
A comprehensive summary of data collected on sea levels around the world finds the evidence converging on one conclusion. Global warming has recently accelerated the pace of rising sea water.
Scientists have examined clues from ancient Roman fish tanks, new satellite altimeter measurements and plenty of other sources to trace the history of sea levels. They've discovered that ocean levels remained steady over the last 2000 years, until 1870. Then sea water around the globe began to rise.
At first the water level climbed by about one millimetre (0.04 inches) a year. Since 1993 though, the rate oceans are rising has tripled to over three mm annually. Between 1870 and 2001, seas worldwide rose by nearly 20 centimetres (8 inches).
While that amount may seem trivial, the overall impact is getting amplified in several ways. For one, storms have a lot more water to throw around. In Australia that effect has been documented as a threefold increase in the frequency of severe waves and storm surges over the 20th century. During the same period, 70% of the world's beaches have eroded.
Rising waters also become exacerbated in river deltas, where due to human activities land is sinking. Oil, gas and water extracted from underground and river dams that prevent influxes of sediments have caused many heavily built-up areas to settle. Millions of people live on subsiding coastal land, particularly in the Bengal and Mekong deltas.
Global Warming Blamed
Several factors are working together to raise sea levels, and they all can be traced back to global warming. Melting ice sheets near the poles are currently the smallest contributors. Greenland and Antarctic each supply a net 0.2 mm per year gain in the planet's sea level.
More melting is taking place on glaciers and ice caps in mountains, particularly coastal regions of North and South America. Since 2001 these have shed enough water to raise oceans by a millimetre each year. Their recent melting rate is more than double the flow from 1961 to 1990.
By far the largest contributor to sea levels creeping upwards is not additional water, but warmer ocean temperatures. As water heats, it expands. On a global scale, the extra room taken by warming water has pushed sea levels an estimated 1.2 mm higher per year since 1993. Some regions are heating up more than others, and so sea levels are actually rising faster in the western Pacific than in the eastern Pacific.
Even if the factors triggering global warming were turned off now, the momentum behind rising oceans would continue for centuries, likely amounting to gains of several metres. The Third Assessment Report of the Intergovernmental Panel on Climate Change (IPCC) published in 2007 forecasts a 18 to 79 centimetre increase in ocean levels over the 21st century.
Mountain glaciers would at most supply 37 cm before they entirely disappeared. But there's huge uncertainty around how fast Antarctic and Greenland ice sheets will disintegrate. Some scientists calculate that these will release enough meltwater to raise ocean levels by as much as 1.4 metres before 2100.
John A. Church, Neil J. White, Thorkild Aarup, W. Stanley Wilson, Philip L. Woodworth, Catia M. Domingues, John R. Hunter and Kurt Lambeck. 2008. Understanding global sea levels: past, present and future. Sustainability Science. In press.