Sea levels to rise for centuries (…even with U.N. temperature limits)

Oceans will keep rising for centuries, perhaps reaching 1.5 to 4.0 metres (5-13 feet) above current levels by 2300 even if the world achieves a U.N. target for limiting global warming, according to a new study.

Melting ice from Greenland and Antarctica, together with the fact that water gets bigger as it warms, will keep pushing up sea levels, they said in this week’s edition of the journal Nature Climate Change.

If true, even the study’s “best estimate” of a 2.7 metre rise by 2300 would be disastrous for many coastal societies, cities and even entire nations — Tuvalu’s highest point is 4 metres above the Pacific Ocean.

As you’d expect, the paper has some giant caveats — “It remains open, however, how far the close link between global sea level and temperature found for the past will carry on into the future,” according to the scientists at German, Dutch and Finnish research institutes.

Still, it’s an interesting stab at the problem.

The authors note that more than 190 nations agreed in 2010 at a U.N. meeting in Mexico to limit temperature rises to below 2 degrees Celsius above pre-industrial times — viewing that as a threshold for “dangerous” changes such as more droughts, floods and rising sea levels. That 2C goal is ambitious since temperatures have already risen by about 0.8 degree C and world greenhouse gas emissions are rising.

Yet even 2 degrees C is too much for the oceans, the experts reckon.

“A 2 °C warming limit…would probably lead to many metres of sea level rise in the coming few centuries and would maintain rates of sea level rise higher than today for many centuries,” they wrote. Above the 2 degree Celsius limit, sea levels would rise even more.

A stricter limit of 1.5 degrees Celsius, advocated by small island states and the poorest nations, would help a lot, they reckon. And Bolivia’s left-wing government refused to sign up for the 2 degree target, saying that anything more than 1 was too much (a political paradox to have a landlocked country advocating a policy that would do most to help the oceans?).

The U.N. panel of climate scientists has given wide ranges for sea level rise, partly because the response of ice sheets is so hard to predict. It estimated in 2007 that oceans would rise by between 18 and 59 cms by 2100, reckoning even in its basic scenarios that Antarctica would withdraw water from the oceans because more snow would fall on the frozen continent.

The new study puts this century’s rise at about a metre (higher than the U.N. panel, in line with a lot of estimates since 2007). And a lot of the current warming of the atmosphere will take decades to affect the deep oceans, it says, based on past “semi-empirical” knowledge of the oceans’ response.

“Sea-level rise is a hard-to-quantify, yet critical risk of climate change,” Michiel Schaeffer of Climate Analytics and Wageningen University, lead author of the new study, says in a release by the Potsdam Institute for Climate Impact Research. “Due to the long time it takes for the world’s ice and water masses to react to global warming, our emissions today determine sea levels for centuries to come.”

(Photos: top – Coast of Maine, U.S., 2012; lower – Antarctica, near Norway’s Troll station, 2008)