Rising Ocean Levels A Long Term Problem

A new study has shown that not only does melting ice contribute more to rising sea levels than thermal expansion, but that ocean levels are likely to continue rising well after the warming of the atmosphere stabilises.


If sea levels rose to where they were during the Last Interglacial Period, large parts of the Gulf of Mexico region would be under water (red areas), including half of Florida and several Caribbean islands. (Photo illustration by Jeremy Weiss)

The University of Arizona-led study focused on analysing paleoceanic records of sea surface temperatures during the warmest 5,000 year period of the Last Interglacial Period, a warm period that lasted from 130,000 to 120,000 years ago.


The researchers then compared their data to results of computer based climate models that were set out to simulate ocean temperatures during a 200 year snapshot in the middle of the Last Interglacial to calculate the contributions to global sea level rise of the waters thermal expansion.

What they found was sea surface temperatures of about 0.7 degrees warmer than today, and that even if the oceans warmed all the way down to 2,000 metres in depth – a highly unlikely scenario – the thermal expansion would only have accounted for no more than 30 centimetres.

This allowed the authors of the study to calculate that 4.1 to 5.8 metres of sea level rise during the Last Interglacial Period was derived from the Antarctic Ice sheet.

“This means that even small amounts of warming may have committed us to more ice sheet melting than we previously thought. The temperature during that time of high sea levels wasn’t that much warmer than it is today,” said Nicholas McKay, a doctoral student at the UA’s department of geosciences and the paper’s lead author.

“Even though the oceans are absorbing a good deal of the total global warming, the atmosphere is warming faster than the oceans. Moreover, ocean warming is lagging behind the warming of the atmosphere. The melting of large polar ice sheets lags even farther behind.”

“As a result, even if we stopped greenhouse gas emissions right now, the Earth would keep warming, the oceans would keep warming, the ice sheets would keep shrinking, and sea levels would keep rising for a long time,” McKay added.

Jonathan Overpeck, co-director of the UA’s Institute of the Environment and a professor with joint appointments in the department of geosciences and atmospheric sciences, who is also McKay’s doctoral advisor and a co-author of the study, said: “This study marks the strongest case yet made that humans – by warming the atmosphere and oceans – are pushing the Earth’s climate toward the threshold where we will likely be committed to four to six or even more meters of sea level rise in coming centuries.”

“Unless we dramatically curb global warming, we are in for centuries of sea level rise at a rate of up to three feet per century, with the bulk of the water coming from the melting of the great polar ice sheets – both the Greenland and Antarctic Ice Sheets.”

Source: University of Arizona

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