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Ocean warming underestimated by half

Ocean warming underestimated by half JL-Pfeifer
02 Feb
2016
The expanding effect of warmer temperatures on sea water has been greatly underestimated say German scientists

Imagine for a moment that the Earth had no melting ice caps. Even in that bizarre scenario, global sea levels would be rising anyway. This is because warmer temperatures cause water to expand – like mercury in a thermometer – resulting in higher sea levels. A team of scientists at the University of Bonn are arguing that this expansion effect has contributed up to twice as much to the global sea level rise than had been reported over the past 12 years.

‘To date, we have underestimated how much the heat-related expansion of the water mass in the oceans contributes to a global rise in sea level,’ says Dr. Jürgen Kusche, co-author of the paper and Professor of Astronomical, Physical and Mathematical Geodesy at the University of Bonn. For the past 12 years it was thought that heat-related expansion contributed to a 0.7 to 1.0 millimetre rise in global sea levels every year. However, the new findings suggest it could be as much as 1.4mm per year. Now thought to be twice as large as the effect of melting ice from Greenland, it has become the leading factor in sea level rise.

Though 1.4 millimetres per year sounds like a small increase, every millimetre of water can add extra energy to storm surges. With all the warmer temperatures, these storms may become more frequent. Also, with time, millimetres add up. ‘No country will raise its levees because of a couple of millimetres,’ says Dr. Roelof Rietbroek, co-author of the paper and researcher at the University of Bonn, ‘however, these small amounts add up to several centimetres within decades. Under such conditions, the likelihood of a destructive storm surge could increase dramatically.’

Screen shot 2016 02 01 at 1Each number shows the mean annual sea level rise for its shaded region of the coast. The wedges show the different factors contributing to regional sea level rise, with water expansion or ‘steric’ shown in orange. Water expansion can be seen to vary across the globe (Image: Kusche et al.)

To make matters worse, expanding water does not have a uniform effect across sea levels, leaving some coasts more at risk than others. In this sense, the single, average figure for global sea rise can be misleading: sea levels are actually rising at different rates in different regions. ‘When heat, wind and melting ice impact the sea regionally, the ocean is simply not quick enough to compensate,’ says Rietbroek. ‘This prohibits it from spreading the water equally.’ The Philippines experiences the highest annual sea level rise at 14.7 millimetres per year – five times the global rate. According to Rietbroek, most of that is due to heat expansion, along with other factors such as El Nino cycles.

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