This undated handout photo provided by NASA shows the Thwaites Glacier in West Antarctic. (AP Photo/NASA)

It’s one of the most important questions on the planet: How much are the seas ultimately going to rise, thanks to what we’re doing to the atmosphere with all our cars and power plants? Scientists are still struggling to find a clear answer to it.

But a new scientific analysis, just out in the journal Science by researchers led by Andrea Dutton of the University of Florida — and including a large team of scientists from the United States, Britain, and Germany — gives a pretty clear sense of what’s at stake. The new assessment compares the current state of the planet with three other warm periods from the Earth’s deep past that are, to varying degrees, comparable with where we may now be steering things.

And the punchline is that in each of these periods within the last 3 million years or so, the researchers estimate that sea levels eventually rose some 6 meters — equivalent to nearly 20 feet — higher than they are right now.

“We looked at these three different warm periods, because there’s no one time period that’s going to be a perfect analogue,” said Dutton. “We looked at several of the warmest interglacials, and for each of them, we’re finding at least 6 meters worth of sea level rise.”

It’s important to emphasize that the researchers are not saying we’re committed to this much long term sea level rise yet — just that if current emissions and warming continue, we could get there.

This may seem odd, given that while we read regular headlines about how the planet is losing ice — from mountain glaciers around the world, and from the great ice sheets of Greenland and Antarctica — the consequent sea level increase seems terribly small, just a few millimeters per year.

Yet current sea level rise may be deceptive — the main factors behind it aren’t the really big ones. The current top drivers are thermal expansion of sea water as it heats up and the loss of glaciers around the world — including 75 billion tons of ice loss yearly from the Alaska region alone. That’s enough to cause sea level upticks of a millimeter or so per year, but when you think about the grand pageant of planetary history, it’s not the real story.

Far more consequential are the slow moving ice sheets of Greenland and Antarctica, which contain enough frozen water to raise sea levels by 20 feet and 200 feet, respectively. They have started to awaken — but only barely. Antarctica, for instance, is currently contributing well under a millimeter per year to rising seas. Greenland might be contributing one millimeter — but only one.

In past eras of planetary history, though, we know that the pair must have lost ice amounts measured in meters, not millimeters.

“Sea level is a relatively slow responding part of the climate system, and those 20 centimeters of sea level rise that we have seen in the last 100 years or so are clearly just a very small beginning of a much larger sea level rise, which will inevitably unfold,” said Stefan Rahmstorf, one of the study authors who is based at the Potsdam Institute for Climate Impact Research in Germany. “Because it takes a long time to heat up the oceans into the depths, and also it takes a long time, thousands of years, to melt big ice sheets.”


This NASA Earth Observatory image released May 3, 2012 shows a photo captured during an IceBridge flight, of a glacier in eastern Greenland flowing through a long and narrow valley—a fjord—carved by the movement of ice.  AFP PHOTO/NASA/HANDOUT

So which past periods are a good guide to the present in this respect? The researchers picked three analogues — each clearly imperfect, because none of them featured billions of fossil fuel burning humans. The current era is unique because unlike in the past, we’re in the driver’s seat. Still, past analogues — featuring climates that were the result of natural planetary changes and cycles — can be illuminating.

The first and most ancient analogue considered in the study is the so-called “Mid-Pliocene warm period,” about 3 million years ago – which contained several “interglacials,” or warm periods in between ice ages. Then, the researchers also considered two much more recent interglacials with less than exciting names: “Marine Isotope Stage 11,” which occurred around 424,000-395,000 years ago, and “Marine Isotope Stage 5e,” the most recent, around 129,000-116,000 years ago.

The state of the planet during these various periods has been painstakingly inferred based on a wide range of evidence, ranging from temperature records preserved in corals and sediments to climate and ice sheet models.

So what did these eras look like? Here are the crucial figures (all of which are characterized, not surprisingly, by considerable uncertainty):

Mid-Pliocene warm era: Average temperatures were some 2 to 3 degrees Celsius warmer than pre-industrial levels. Atmospheric carbon dioxide levels were quite close to today at around 400 parts per million. Sea levels were at least 6 meters (2o feet) higher than current levels — a “lower bound” — but could have been dramatically higher than that.

Marine Isotope Stage 11: Average temperatures were between 1 and 2 degrees Celsius higher than pre-industrial levels. Atmospheric carbon dioxide was less than 300 parts per million. Sea levels were 6 to 13 meters higher than present.

Marine Isotope Stage 5e: Average temperatures were 1 degree Celsius warmer than pre-industrial levels (pretty close to today). Atmospheric carbon dioxide levels were around or below 300 parts per million. Sea levels were 6 to 9 meters higher than present.

So what should one make of these possible analogues?

In some ways, it’s tempting to liken our situation to Marine Isotope Stage 5e – sometimes also called the “Eemian” period —  given the similarity in average temperatures. But the analogy has a critical flaw — the period was the way it was because of planetary orbital cycles, not carbon dioxide emissions, and the poles were considerably hotter than they are now (the temperature measurement most relevant to the fate of ice sheets).

“Apparently the Arctic at that time was a couple of degrees warmer than today (due to orbital forcing) which would have contributed to the Greenland mass loss,” explains Rahmstorf by email.

As a result, he thinks that currently, the mid-Pliocene is a better analogy for where we could be headed, given the comparable carbon dioxide levels. “In the Pliocene, global temperatures 1 – 2 °C warmer than present came with at least 6 m of rise,” Rahmstorf wrote. Thus, while we may not currently be committed to raising seas as much as occurred in these past periods, if we don’t get global warming under control, that could change.

Rahmstorf has previously published results suggesting that if we hold global warming to 1.5 degrees Celsius above pre-industrial levels, we might thereby hold sea levels to 1.5 meters above where they were in the year 2000 by the year 2300. Warm things up by 2 degrees C, though, and you’d get a 50 percent chance of more than 2.7 meters, the study suggested, by 2300.

Other recent research, meanwhile, has similarly implied that for every degree Celsius that we warm the planet, we’re could be committed to 2.3 meters of sea level rise.

Much like Rahmstorf, Dutton agreed that we’re not committed to 6 meters — yet. However, she said, “we’re getting very close to where we see, repeatedly in the paleo record, something like 6 meters from that type of temperature change.”

There remains much uncertainty to this kind of analysis – especially when it comes to how fast sea level rise will happen going forward. “Rates of sea-level change for previous warm periods when sea level was higher than present range from highly uncertain to completely unconstrained depending on the time period, yet this is perhaps the most societally relevant information the paleorecord can provide for predicting and adapting to future sea-level change,” noted the authors.

It’s a question that people around the world will depend on — and scientists still have a long way to go in answering it. But their current results tell you one big thing: While we may not yet be destined to see the seas of these past eras, what’s troubling is that we’re even making the comparison.

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