How to Make Sense of 'Alarming' Sea Level Forecasts

You may have read recent reports about huge changes in sea level, inspired by new research from James Hansen, NASA’s former Chief Climate Scientist, at Columbia University.  Sea level rise represents one of the most worrying aspects of global warming, potentially displacing millions of people along coasts, low river valleys, deltas and islands.

The Intergovernmental Panel on Climate Change, the UN’s scientific climate body, forecasts rises of approximately 40 to 60 cm by 2100.  But other studies have found much greater rises are likely.

Hansen and 16 co-authors found that with warming of 2C sea levels could rise by several meters.  Hansen’s study was published in the open-access journal Atmospheric Chemistry and Physics Discussion, and has not as yet been peer-reviewed.  It received much media coverage for its “alarmist” findings.

So how should we make sense of these dire forecasts?
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Looking to the past
The IPCC estimates stand in sharp contrast to projections made by some climate scientists, in particular James Hansen who pointed out in 2007 and in his and his colleagues' latest study of the effects of ocean warming on the ice sheets.

The IPCC reports did not take into account rates of dynamic ice sheet breakdown, despite satellite gravity measurements reported in the peer-reviewed literature by other scientists.
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Could it be worse?
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Warming of 2-4C implies a rise in sea level by several to many meters.  Future sea level rise, once it reaches equilibrium with temperature rise of about 2C above pre-industrial temperature, could reach levels on the scale of the Pliocene (pre-2.6 million years ago) around 25+/-12 meters.  Temperature rise of 4C higher than pre-industrial would be consistent with peak Miocene (about 16 million years ago) equilibrium sea levels of about 40 meters.

We don’t know how long it would take for seas to rise that high with rising temperatures. However the extreme rise rate of atmospheric greenhouse gas concentrations, higher than 2 ppm CO2 per year, if continues, threatens an accelerating rate of sea level rise.

Read (much) more at How to Make Sense of 'Alarming' Sea Level Forecasts