Mitigating climate change won’t stop ocean rises

Even with substantial cuts to the emissions of greenhouse gases, sea levels will continue to rise for centuries to come, according to researchers at the US National Center for Atmospheric Research.

Although sea-level rise cannot be stopped for at least the next several hundred years, with aggressive mitigation it could be slowed down, and this would buy time for adaptation measures to be adopted, the researchers say.

In a paper published in Nature Climate Change, the scientists point to a common misconception, that if significant reductions in emissions are made, global temperature increases could be stabilised and the most dangerous consequences of climate change avoided.

Their modelling shows that even with averaged global temperature increases stabilised either below 2°C or near 3°C above pre-industrial values, sea levels would continue to rise.

With little reduction, future sea-level rise would be large and continue unabated for centuries but “with aggressive mitigation it can be slowed down, and this would buy time for adaptation measures to be adopted”, the scientists say.

Several factors mean the oceans will continue to increase in size, the most quantifiable being thermal expansion of sea water.

As warming temperatures make their way deeper and deeper into the ocean through mixing processes in various ocean regions, an ever-increasing volume of water warms and expands, thus producing ongoing rises in sea level. This would lead to further melting of ice sheets and glaciers, which would contribute to further sea-level rise.

But calculating the actual magnitude of future sea-level rise has proved difficult to quantify, the researchers say. Recent estimates have been mixed, with some showing that ice melt could be accelerating, raising the spectre of more-rapid sea-level rise, and others indicating a possible slowing down of outlet glaciers and ice streams that accelerated in the 1990s.

“Furthermore, models are not sufficiently reliable to project future dynamic behaviour of ice sheets and, because observations of ice sheets indicate complex responses to recent climate changes, the credibility of extrapolations is difficult to assess,” the scientists write in their paper.

“Thus, owing to limitations of our current knowledge of the physics involved with ice-sheet stability, and the early stages of incorporating complex ice-sheet formulations in the climate models used to project climate change, various ways to estimate future sea-level rise have been devised. The most readily quantifiable involves computing thermal expansion of sea water from global climate models.”

They say recent estimates of sea-level rise from 1972-2008 indicate a contribution from thermal expansion of about 0.8 millimetres a year to a total sea-level rise of about 1.8 millimetres a year, with the rest accounted for mostly by melting of glaciers and ice sheets.

But there is also the possibility that groundwater mining may have contributed between 0.4 and 0.8 millimetres a year to rise in recent decades.

“Current in situ and satellite observations show that land-based ice is perhaps losing mass at an accelerated rate since 1990 and contributing to about one-third of the global sea-level rise while the mass loss of the combined Greenland and Antarctic ice sheets is estimated to make up another third, leaving the contribution of seawater thermal expansion for roughly the other third.”

But the scientists note that palaeoclimate evidence from the last interglacial period indicates a possible much larger role for ice-sheet melt from ice-sheet instability compared with thermal expansion for total sea-level rise.

They says the key uncertainty relates to timing: a few hundred years of warming that has occurred may not be sufficient to trigger a multi-metre sea-level rise, which took place over a much longer time period during the last interglacial period.

“In lieu of credible simulations of coupled ice-sheet dynamics in climate models, there are significant caveats associated with any estimate of future sea-level rise out to 2300. There is at present no way to reliably evaluate the upper limits on future sea-level rise.

“The most credible calculation from the models comes from the sea-level rise owing to thermal expansion, but that is almost certainly a low estimate given the likelihood of further contributions from ice-sheet melting,” the researchers say.