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Why It's Bad News That Previous Estimates of Rates of Sea Level Rise May Have Been Too High

Rate of sea-level rise larger than thought, new analysis finds

The rate of global sea level rise is greater than previously estimated, according to a new analysis. (Photo: looseends/flickr/cc)

The rate of global sea level rise is greater than previously estimated, a new study has found.

Previous calculations indicated that sea level rise from 1900-1990 was between 1.5 and 1.8 millimeters annually, but Harvard researchers Carling Hay and Eric Morrow found that the rate was closer to 1.2 millimeters a year, meaning the previous rate could be over-estimated by as much as 30 percent. Their analysis found that estimates for the rise between 1993 and 2010 were consistent were with previous estimates for that time range.

But that doesn't give an optimistic scenario. Rather, it points to a faster rise in sea level.

"If we've been over-estimating the sea-level change during that period," Morrow, a recent PhD graduate of the Department of Earth and Planetary Sciences (EPS), said, "it means that these models are not calibrated appropriately, and that calls into question the accuracy of projections out to the end of the 21st century."

The problem, said Hay, a post-doctoral fellow in the EPS, and Morrow, is that the previous calculations were made using tide gauges, and those provided inadequate data.

Explaining the inadequacies, Hay stated, "Tide gauges are located along coasts; therefore, large areas of the ocean aren't being included in these estimates. And the records that do exist commonly have large gaps."

Speaking to the BBC, Hay explained the approach he and Morrow took: "What we have done, which is a bit different from past studies, is use physical models and statistical models to try to look for underlying patterns in the messy tide gauge data observations."

"Each of the different contributions actually produces a unique pattern, or fingerprint, of sea-level change. And what we try to do is model these underlying patterns and then use our statistical approach to look for the patterns in the tide gauge observations. That allows us to infer global information from the very limited records," he said.

"We know the sea level is changing for a variety of reasons," Hay said in a statement. "There are ongoing effects due to the last ice age, heating and expansion of the ocean due to global warming, changes in ocean circulation, and present-day melting of land-ice, all of which result in unique patterns of sea-level change. These processes combine to produce the observed global mean sea-level rise."

And the assessment they gleaned from the findings provides an a unsettling outlook.

"Unfortunately," Hay stated, "our new lower rate of sea-level rise prior to 1990 means that the sea-level acceleration that resulted in higher rates over the last 20 years is really much larger than anyone thought."

The findings were published Wednesday in the journal Nature.

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