Committing to several meters of sea-level rise?

Projected sea-level rise of 5 meters in western Europe. Source: Rowley et al. (2007)

New research indicates we could be heading for 6 meters of sea-level rise

Researchers part of the international Past Global Changes project, have analysed sea levels during several warm periods in Earth's geological past when global average temperatures were similar to or slightly warmer than today (~1C above pre-industrial levels) (Dutton et al. 2015). The team concluded that during the last interglacial, a warm period between ice ages 125, 000 years ago, the global average temperature was similar to the present and this was linked to a sea-level rise of 6-9 meters, caused by melting ice in Greenland and Antarctica. And 400,000 years ago sea-levels rose 6-13 meters. 

What is scary about these two periods is that carbon dioxide in the atmosphere remained around 280 parts per million (ppm). The research group also looked at sea levels during the Pliocene, 3 million years ago, when carbon dioxide levels reached around 400 ppm, similar to today's levels. According to the scientists, sea levels were at least 6 meters higher than today. This could happen to us, but surely it would take a long time, right?

Risk of rapid sea-level rise

Well, in another recent study a group of 17 scientists describes a scenario where the world oceans rise much faster than models have predicted (Hansen et al. 2015). The study basically points out that a 2C global average rise in temperature, a political limit to induced warming, would result in a rise of the world's oceans to dangerous levels. The team looked at what happened during the Eemian period when atmospheric temperatures were approximately 1C warmer than they are now and found that ocean levels were much higher than they should have been based on modern climate models. The explanation for this could be that even a small climate forcing could set in motion reinforcing feedback loops in the climate system. In this case, warming led to a small amount of ice sheet melt, which changed ocean currents, which melted more ice. Such complex dynamics are not well incorporated into modern climate models.

Sea-level rise is speeding up. Source: Hansen et al. 2015

Hansen and colleagues conclude that humanity faces near certainty of eventual sea level rise of at least Eeemian proportions, some 5-9 meters, if fossil fuel emissions continue on current trajectory. This would mean that coastal cities and low-lying areas such as Bangladesh, European lowlands, and large portions of the United States eastern coast and northeast China plains could be completely lost or almost impossible to protect. If reinforcing feedbacks kick in then rapid sea level rise could beigin sooner than most models assume. If the Southern Ocean subsurface warming of the Antarctic ice sheets continues to grow we will probably not be able to avoid sea level rise of several meters. And it could happen over decades, not centuries. But this is highly uncertain. What we do know is that we are on a very dangerous climate trajectory and time is running out to change course.


Out of the ashes into the fire

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