Sea-Level Rise Will Be Worse for Some, We Just Don’t Know Who

Jess McNally Wired Science 16 Jul 10;

The Seychelles could see up to 10 percent more sea-level rise than the global average. Or the sea level around the islands could drop. It depends on who you ask.

The fact that oceans will rise in a warming world is well established, but depending on how wind patterns change, climate change could mean quick inundation or more beach space for different coastlines. Wind patterns maintain height differences between different regions of the ocean, and if altered or intensified, they would push water from one part of the ocean to another.

The resulting sea-level changes could be up to 30 percent more, or less, than the global average in some regions, says oceanographer Axel Timmermann.

The problem is that scientists are just beginning to understand what will happen to climate at scales smaller than entire continents or ocean basins.

The 5th Intergovernmental Panel on Climate Change report, slated for 2011, will have an entire chapter devoted to the subject of regional differences in climate change and sea level. But so far, the estimates of regional sea level rise are preliminary at best, and in some cases completely contradict each other.

Two studies released this year on patterns of sea-level rise in the Indian Ocean are a case in point. One, published July 11 in Nature Geoscience predicts future patterns of sea-level rise using a combination of recorded changes and climate modeling to simulate the last 50 years of change.

The other, in press at The Journal of Climate, used a different ocean and climate model to look at how changing wind patterns have influenced past and future sea-level rise.

The scientists agree on one thing: The Indian Ocean has heated by about 1 degree Fahrenheit over the past 50 years, partially due to human-generated greenhouse gases (also reported by the 2007 IPCC report). But in their estimations of how wind patterns will change due to climate change, they come to almost completely opposite conclusions.

“I was really, really amazed when Han [lead author of the Nature Geoscience paper] said that the Seychelles will see no sea-level rise,” said Timmermann, lead author of The Journal of Climate paper, from the University of Hawaii.

“We’re saying they should be really worried.”

The cause of the disagreement is a discrepancy between the wind pattern changes that most climate models, including Timmermann’s and the 22 different models used in the IPCC report, predicted for the last 50 years in the Indian Ocean, and the actual observed changes. The models show more warming near the equator and on the western side of the ocean basin. But the observed warming has been on the eastern side.

There are two possible reasons the models’ predictions don’t match reality. Timmermann suspects the actual changes in sea surface temperature are due to natural variability in the ocean that canceled out the human-caused changes predicted by the models.

The other possibility is that the climate models don’t do a good job for this part of the ocean, and that we should expect the future to be more in line with the real changes we’ve seen so far, as oceanographer Weiqing Han from the University of Colorado suspects.

“It has been really challenging to simulate atmospheric wind-pattern change over the Indian Ocean because the system is very, very complicated because of the land,” Han said.

Both Timmermann and Han agree that the key to solving the differences will be understanding the natural variability in the ocean at finer scales so that the human-caused and natural changes can be teased apart from one another. To do that, researchers need to continue reconstructing past ocean temperatures using coral reefs and ocean sediments in different parts of the ocean to help models do a better job.

Until that happens, inhabitants of low-lying islands shouldn’t count on the wind to keep them dry.