Study Suggests ‘Tentatively Good News’ for Ice Sheets and Sea Level Rise

Melting ice caps and rising sea levels are one of the hallmark consequences of a warming climate, according to scientists, but a new study says that it might not be as bad as projected.

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Stanford University Ph.D. students researched how Earth's ice sheets from 3 million years ago might have responded to an environment with carbon dioxide levels similar to present day and discovered "global sea level is less sensitive to high atmospheric carbon dioxide concentrations than previously thought," Matthew Winnick said in a statement.

"Our results are tentatively good news," he said.

The Intergovernmental Panel on Climate Change, which makes global sea level rise projections for the future, has based some of its estimates on data from mid-Pliocene, a point in the epoch characterized by a warmer climate.

"The Pliocene is an important analogue for today's planet not only because of the related greenhouse gas concentrations, but because the continents were roughly where they are today, meaning ocean and climate circulation patterns are comparable," Winnick said.

Previous studies have estimated that during this time period Earth's sea level was 82 to 98 feet higher than today, basing this estimate on oxygen isotope records that determined ice sheet volume and, thus, allowed researchers to calculate approximate sea level. The oxygen isotope records scientists are analyzing come from protozoa.

Winnick and his colleague Jeremy Caves though challenge this estimate, which they say was based on the assumption that the isotopic composition of oxygen then was the same as it is now.

Here's where they went from there:

To understand the isotopic composition of Pliocene ice, Winnick and Caves began in the present day using well-established relationships between temperature and the geochemical fingerprint. By combining this modern relationship with estimates of ancient Pliocene surface temperatures, they were able to better refine the fingerprint of the Antarctic ice millions of years ago.

According to their new estimates, sea level in the middle Pliocene, compared to present day, was 30 to 44 feet higher. This sea level rise, the study authors pointed out, would still put Miami, New Orleans and New York City under water.

Based on that, the researchers wrote that they believe the "East Antarctic Ice Sheet is less sensitive to radiative forcing than previously inferred from the geologic record."

They caution that while they think their research better explains Pliocene sea level, they're not sure how exactly it would translate for the present day environment.

"Ice sheets typically take centuries to millennia to respond to increased carbon dioxide, so it's more difficult to say what will happen on shorter time scales, like the next few decades," Winnick said. "Add that to the fact that CO2 levels were relatively consistent in the Pliocene, and we're increasing them much more rapidly today, and it really highlights the importance of understanding how sea level responds to rising temperatures. Estimates of Pliocene sea level might provide a powerful tool for testing the ability of our ice sheet models to predict future changes in sea level."

The research was published in the journal Geology.

The latest estimate from the IPCC was that the seas could rise by 11 to 38 inches by 2100. According to the National Oceanic and Atmospheric Administration, sea levels have been rising since 1900 at a rate of .04 to 0.1 inches per year.

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