Global Warming and Sea Level Rise

With the recent news that global average surface temperatures are now about 1°C above pre-industrial levels, reports that greenhouse gas emissions are still inexorably rising, increasing frequency of extreme weather events and attention focussing on Paris and the COP21, there has been a new flurry of articles about sea level rise in the newspapers and other media.

Here's an example from The Independent.

"Homes belonging to more than half a billion people could be submerged by rising sea levels if the current rate of global warming continues, scientists have said.

"A 2C increase in the Earth's temperature would result in houses occupied by 130 million people being left underwater by rising sea levels, according to an investigation by research group Climate Central. However, a 4C change - which would be the likely scenario at the current rate of increase - would impact more than 600 million."

The Internet is awash with interactive maps that show which parts of the world get flooded at different levels of global warming.

Some pictures mocked up to represent what various places would look like with flooding following either 2 or 4° of warming produced by Climate Central have been shared widely around the Internet. While they have undoubtedly done a great service in drawing people's attention to the problem, there is an implicit assumption that sea level will rise with a linear correlation to warming.  It's potentially much worse.

There is a fundamental error in the underlying thinking. Sea level rise is partly caused by the thermal expansion of the oceans' water as it warms, but this is trivial in comparison with the potential rise through the melting of the Greenland and Antarctic Ice Sheets.  The ice melt is only rather indirectly linked to a rise in global temperature.

Look at it this way.  Take two ice cubes out of the freezer, put one in a cool room and one in a warm room.  They both melt.  For sure one will melt faster than the other but they both melt.  As soon as the ice is out of the freezer it is no longer in equilibrium with its surrounding temperature and its eventual fate is sealed.

So it is with the polar ice sheets; as soon as the global climate has warmed to the point at which the ice is no longer in equilibrium they will melt.  And that's what has happened.  Of course it's a lot more complicated that an ice cube from the freezer.  Most of the Antarctic, and Greenland most of the year, has weather well below zero but the warming oceans are eroding the ice from around the edges and below.  The loss of ice is, partly, made up by snowfall, but the ice mass balance, the difference between snowfall gain and melting loss, is now negative.  The polar ice caps are melting and will continue to melt until there is none left, and that's just with 1°C of warming.

And here's the rub; all those articles that relate sea level rise to temperature are disingenuous. If we stop burning all fossil fuels by tea-time today and hold temperatures to just what's baked into the system by greenhouse gasses already released, all the ice will melt and land within about 60 metres of present sea level will be lost.

But keep in mind the melting ice cubes, one in a cool room and one in the warm.  The rate of melting will be different. The big uncertainty about the ice caps is the timing of their melting and the speed of the consequent sea level rise. Anything we can now do to reduce the amount of global warming will help slow the sea level rise, giving humanity more time to adapt to the new geography. Mitigation has the potential to improve, perhaps save, the lives of a billion people.