Rising oceans: Too late to turn the tide?
By Daniel Stolte, University Communications
14 July 2011 Melting ice sheets contributed much more to rising sea levels than thermal expansion of warming ocean waters during the Last Interglacial Period, a UA-led team of researchers has found. The results further suggest that ocean levels continue to rise long after warming of the atmosphere levels off. Thermal expansion of seawater contributed only slightly to rising sea levels compared to melting ice sheets during the Last Interglacial Period, a University of Arizona-led team of researchers has found. The study combined paleoclimate records with computer simulations of atmosphere-ocean interactions and the team’s co-authored paper is accepted for publication in Geophysical Research Letters. As the world’s climate becomes warmer due to increased greenhouse gases in the atmosphere, sea levels are expected to rise by up to three feet by the end of this century. But the question remains: How much of that will be due to ice sheets melting as opposed to the oceans’ 332 million cubic miles of water increasing in volume as they warm up? For the study, UA team members analyzed paleoceanic records of global distribution of sea surface temperatures of the warmest 5,000-year period during the Last Interglacial, a warm period that lasted from 130,000 to 120,000 years ago. The researchers then compared the data to results of computer-based climate models simulating ocean temperatures during a 200-year snapshot as if taken 125,000 years ago and calculating the contributions from thermal expansion of sea water. The team found that thermal expansion could have contributed no more than 40 centimeters – less than 1.5 feet – to the rising sea levels during that time, which exceeded today’s level up to eight meters or 26 feet. At the same time, the paleoclimate data revealed average ocean temperatures that were only about 0.7 degrees Celsius, or 1.3 degrees Fahrenheit, above those of today. “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. McKay pointed out that even if ocean levels rose to similar heights as during the Last Interglacial, they would do so at a rate of up to three feet per century. “Even though the oceans are absorbing a good deal of the total global warming, the atmosphere is warming faster than the oceans,” McKay added. “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,” he explained. They are absorbing most of that heat, but they lag behind. Especially the large ice sheets are not in equilibrium with global climate,” 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, 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.” Overpeck, who is McKay’s doctoral advisor and a co-author of the study, added: “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.” […] “The message is that the last time glaciers and ice sheets melted, sea levels rose by more than eight meters. Much of the world’s population lives relatively close to sea level. This is going to have huge impacts, especially on poor countries,” McKay said. “If you live a meter above sea level, it’s irrelevant what causes the rise. Whether sea levels are rising for natural reasons or for anthropogenic reasons, you’re still going to be under water sooner or later.”
Rising Oceans – Too Late to Turn the Tide? via Richard Pauli
