Ice sheets may be more resilient than thought

September 3, 2015, Stanford University
A map of the Earth with a 6-meter sea level rise represented in red. A new Stanford study says that the sea level rise associated with a warming world may not be as high as predicted. Credit: NASA

Sea level rise poses one of the biggest threats to human systems in a globally warming world, potentially causing trillions of dollars' worth of damages to flooded cities around the world. As surface temperatures rise, ice sheets are melting at record rates and sea levels are rising.

But there may be some good news amid the worry. Sea levels may not rise as high as assumed.

To predict changes, scientists look to Earth's distant past, when climate conditions were similar to today, and investigate how the planet's ice sheets responded then to warmer temperatures brought on by increased in the atmosphere.

In a recently published study in the journal Geology, PhD students Matthew Winnick and Jeremy Caves at Stanford School of Earth, Energy & Environmental Sciences explored these very old conditions and found that sea level might not have risen as much as previously thought - and thus may not rise as fast as predicted now.

To better understand rise, Winnick and Caves analyzed the middle Pliocene warm period, the last time in Earth's history, approximately 3 million years ago, when carbon dioxide levels in the atmosphere were close to their present values (350-450 parts per million).

"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," said Winnick.

These similarities are why the Intergovernmental Panel on Climate Change (IPCC), the group responsible for global projections, focuses on the mid-Pliocene warm period to inform their computer models.

Previous studies of the mid-Pliocene warm period used oxygen isotope records to determine the volume of Earth's ice sheets and, by proxy, sea level. Effectively, the oxygen isotope records act as a fingerprint of Earth's ice sheets. By combining the fingerprint with models of ice sheet meltwater, many previous researchers thought that sea level was likely 82 to 98 feet (25 to 30 meters) higher during the Pliocene.

Such high sea level would require a full deglaciation of the Greenland Ice Sheet and the West Antarctic Ice Sheet, and as much as 30 percent of the East Antarctic Ice Sheet - enough to cover New York City under 50 feet of water. But these estimates arose because the researchers assumed that the Antarctic ice of the Pliocene had the same isotopic composition, that is, the same fingerprint, as it does today - an assumption that Winnick and Caves challenge in their new report.

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 , they were able to better refine the fingerprint of the Antarctic ice millions of years ago. In re-thinking this critical assumption, and by extending their analysis to incorporate ice sheet models, Winnick and Caves recalculated the global sea level of the Pliocene and found that it was 30 to 44 feet (9 to 13.5 meters) higher, significantly lower than the previous estimate.

"Our results are tentatively ," Winnick said. "They suggest that global sea level is less sensitive to high atmospheric carbon dioxide concentrations than previously thought. In particular, we argue that this is due to the stability of the East Antarctic Ice Sheet, which might be more resilient than previous studies have suggested." However, a rise in global sea level by up to 44 feet (13.5 meters) is still enough to inundate Miami, New Orleans and New York City, and threaten large portions of San Francisco, Winnick cautioned.

While the study helps refine our understanding of Pliocene sea level, both Winnick and Caves point out that it's not straightforward to apply these results to today's planet. "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 models to predict future changes in sea level."

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3.9 / 5 (11) Sep 03, 2015
We are only at that level of CO2 temporarily. In 20 to 40 years we'll be at 500 PPM of we don't do something about it.
4.4 / 5 (13) Sep 04, 2015
Keep in mind that the Pliocene was a reduction in CO2 from the Miocene and continued down with weathering & continental changes into the Pleistocene and able to drop CO2 below 200-ppm to a glacial period.

The Pliocene isn't an analog for CO2 and sea-level for us.

It was stable for a 1/2-million years near 400-ppm somewhere in the middle of it & some 25m higher sea-level, we're jetting through to 600-ppm before anything changes, eh? Ski jump back to the Oligocene why bother with the Miocene we'll be at 750-ppm too fast to bother?

We're retaining heat at a constantly increasing rate as nature never does and the Totten Glacier can empty ice 160-km inland being below sea-level, has shown acceleration too similar to Western Antarctica's situation so I'm skeptical this means anything to our need to drop CO2 emissions immediately, and, seems dated on these points with recent findings by NASA considered.
Eddy Courant
1.4 / 5 (9) Sep 04, 2015
Oh dear. Not that run-a-way trace gas again?
1.9 / 5 (13) Sep 04, 2015
If nothing else, this article unsettles once again the "settled science" argument. Science is continually evolving our understanding of the environment. Nothing is settled.
1.4 / 5 (9) Sep 04, 2015
If nothing else, this article unsettles once again the "settled science" argument. Science is continually evolving our understanding of the environment. Nothing is settled.

1.5 / 5 (8) Sep 05, 2015
What this study shows is the science of CO2 and melting is not settled. Geology want 25 bucks for a one day viewing of the article which may be worth it to the brain dead who insist the science of everything is settled. The problem with the study is the same problem with all the studies. They refuse to look at the great big heat lamp in the sky which has a varying output outside of the solar cycle. It is helpful to look at actual ocean rise. NOAA studies those things. Johnston Atoll 3 inch ocean rise trend in 100 years. Sydney Australia, zero same with Marshall islands. That means any melting going on is very small so the claims of many meters in thirty years are simply junk science.
4.6 / 5 (10) Sep 05, 2015
If nothing else, this article unsettles once again the "settled science" argument. Science is continually evolving our understanding of the environment. Nothing is settled.

How the ice sheets melt was never part of the settled science, so this has exactly no affect on the settled science. So relax, the settled science bits of climate science are still settled. ;)
1.4 / 5 (9) Sep 05, 2015
I hope no one tells this guy
4.1 / 5 (9) Sep 06, 2015
If nothing else, this article unsettles once again the "settled science" argument. Science is continually evolving our understanding of the environment. Nothing is settled.

How? Please explain how this has any impact on the role of increasing greenhouse gases in the determination of the amount of energy in the climate system?
4.4 / 5 (7) Sep 06, 2015
If nothing else, this article unsettles once again the "settled science" argument. Science is continually evolving our understanding of the environment. Nothing is settled.

No, it doesn't do that at all. It IS settled - increasing the CO2 content of the atmosphere will cause global climate temperatures to rise.

Or were talking about some other settled science? Like whether the Earth is flat perhaps?
5 / 5 (1) Sep 09, 2015
I hope no one tells this guy

Too late you've already been exposed ;)

all your sockpuppets in the wash today ?

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