Monday, May 27, 2013

Sea level rise: Drowning in numbers

Michael Le Page in New Scientist: Imagine your job is to protect London from surging seas...The stakes are enormous. Building new defences will cost tens of billions and involve decades of planning and controversy before construction even begins. Get it wrong, and storm surges could kill thousands and displace millions. So all around the world, planners are clamouring to know how fast the seas will rise as the planet warms.

Until recently, scientists could not give them any reliable numbers. There were no computer models capable of simulating the melting of the world's ice sheets and glaciers.

The 2007 report of the Intergovernmental Panel on Climate Change (IPCC) handled this uncertainty really badly. It acknowledged that we don't know how fast all the ice will melt, but then gave some numbers anyway – between 18 and 59 centimetres of sea level rise by 2100 – based on highly dubious assumptions such as glaciers continuing to flow at the same rate and the Antarctic ice sheet growing larger. The numbers also assumed a maximum warming of 5.4 °C, even though the report's highest projection was 6.4 °C. Unsurprisingly, many people wrongly took 59 cm of sea level rise to be the worst case.

Now we have some more numbers. A European-funded project called ice2sea has developed computer models of glaciers and ice sheets. Earlier this month it announced that melting ice would contribute between 4 and 37 cm to global sea level by 2100. Adding this to the other causes of sea level rise – the main one being the expansion of the oceans as they warm – gives figures of between 16 and 69 cm by 2100.

...Yet actual emissions today are much closer to the worst-case scenario, which some recent studies predict could lead to warming of 6 °C or more. And far from falling, annual global emissions are rising ever faster. With hundreds more coal-fired power stations being built and new sources of fossil fuels like tar sands being exploited, there is good reason to think emissions will continue to soar for many decades to come....

The London Eye and the Sea Life London Aquarium at night, shot by Ralf Roletschek (talk) - Fahrradtechnik auf, Wikimedia Commons, under the Creative Commons ‘Attribution-NonCommercial-NonDerivative 3.0 (US)’

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