About 115,000 years ago, Homo sapiens they continued to live in bands of hunter-gatherers, largely confined to Africa. We continue to share the world with the Neanderthals, although it is not clear that we have found them yet.
And although these several hominids did not know it, the Earth was reaching the end of an important warm period. It was one that was quite close to our current climate, but with an important discrepancy: the seas at that time were 20 to 30 feet (6 to 9 meters) higher.
During this ancient period, sometimes called Eemian, the oceans were as warm as they are today.
And last month, a new intriguing research emerged that suggests that glaciers in the northern hemisphere have already retreated as much as they did in Eemian, driven by dramatic warming in the Arctic regions.
The finding came when a team of researchers working on Baffin Island, in northeastern Canada, made a sample of the remains of ancient plants that had emerged from rapidly retreating mountain glaciers.
And they discovered that the plants were very old and that they had probably grown for the last time in these places around 115,000 years ago.
It is the last time that the areas were not covered by ice, scientists believe.
"It's very hard to find another explanation, except that at least in that area we're working on … the last century is as warm as any other century in the past 115,000 years," said Gifford Miller, a geologist. at the University of Colorado at Boulder, who led the research on Baffin Island.
But if Miller is right, there is a big problem. We have geological records of the Eemian sea levels. And the oceans, according to scientists, were 20 to 30 feet (6 to 9 meters) higher.
Probably, some additional water came from Greenland, whose ice currently contains more than 20 feet (6 meters) of potential sea level rise. But it could not have been just in Greenland, because the entire ice sheet did not melt at that time.
That's why researchers also suspect a collapse of the most vulnerable part of Antarctica, the ice sheet of West Antarctica. This region could easily supply another 10 feet (3 meters) of sea level rise, or more.
"There is no way to get dozens of meters of sea level rise without getting tens of meters of sea level rise from Antarctica," said Rob DeConto, an Antarctic expert at the University of Massachusetts.
Trying to understand how Antarctica will fall.
Scientists are now intensively debating precisely what processes might have taken place at that time, and how soon they will reappear. After all, Western Antarctica has already demonstrated, once again, that a withdrawal is beginning.
Some researchers, including DeConto, believe they have found a key process, called the collapse of a sea ice cliff, that can cause a large rise in sea level from West Antarctica in a hurry.
But they are being challenged by another group, whose members suspect that the changes in the past were slow, and will be again.
To understand the dispute, consider the vulnerable environment of West Antarctica itself.
Essentially, it is a huge block of ice submerged mainly in very cold water. Its glaciers lean against the ocean in all directions, and towards the center of the ice sheet, the seabed tilts rapidly downward, even as the surface of the ice sheet grows much thicker, to a total of two miles.
As much as a mile and a half of that ice rests below sea level, but there is still a lot of ice on top of it.
So, if the glaciers at the entrance gate begin to move backward, particularly the glacier called Thwaites, the largest of them, the ocean will quickly have access to a much thicker ice.
The idea is that during the Eemian, this whole area was not at all a block of ice, but a sea without a name. Somehow, the ocean came in, knocking down the outer glacial defenses, and gradually left all of western Antarctica afloat and on the way to melting.
DeConto, along with his colleague David Pollard, built a model that looked at the Eemian and another ancient warm period called the Pliocene, to try to understand how this could happen.
In particular, they included two processes that can eliminate glaciers. One, termed "sea ice instability," describes a situation in which a partially submerged glacier becomes deeper and thicker as it moves toward its center.
In this configuration, hot water can cause a glacier to move back and down, exposing increasingly thick ice to the ocean, and thicker ice flows out faster.
So the loss feeds on itself.
The instability of the sea ice layer is probably already under way in West Antarctica, but in the model it was not enough. DeConto and Pollard also added another process that they say is currently taking place in Greenland, on a large glacier called Jakobshavn.
Jakobshavn is moving backwards along a slope of an underwater hill, just in the way that it is feared that the much larger Thwaites will be diverted. But Jakobshavn is also doing something else. It is constantly breaking thick pieces in its front, almost like a loaf of bread, falling one slice after another.
This is because Jakobshavn no longer has an ice shelf, a floating extension that used to grow on the ocean at the front of the glacier and stabilize it. The platform collapsed when Greenland became hot in the last two decades.
As a result, Jakobshavn now presents a steep vertical front to the sea. Most of the glacier ice is under water, but more than 100 meters (330 feet) extend over it, and for DeConto and Pollard, that is the problem. That is too much to be sustained.
Ice is not steel. Breaks. And it breaks. And it breaks.
This additional process, called "collapse of the sea ice cliff", causes a total disaster if applied to Thwaites. If one day Thwaites loses its own ice shelf and exposes a vertical front to the ocean, it will have ice cliffs hundreds of meters above the surface of the water.
DeConto and Pollard say that such cliffs would fall continuously into the sea. And when they added this calculation, they not only recreated Eemian's rise in sea level, but they also considerably increased their projection of how much ice Antarctica could produce in this century: more than three feet.
As there are other factors that drive sea level rise, such as Greenland, this meant we could see up to six feet in total in this century, roughly twice as many previous projections. And in the next century, the loss of ice would worsen even more.
"What we pointed out was that if the type of births we see today in Greenland starts to be activated in analogous environments in Antarctica, then Antarctica has much thicker ice, it's a much larger ice sheet, the consequences would potentially be very monumental ". for sea level rise, "said DeConto.
In addition, the process, he argues, is essential to understand the past and, therefore, how we can replicate it.
"We can not recreate six meters of sea level rise early in the Eemian without noticing some fragile fracture in the ice sheet model," said DeConto.
A massive debate about the sea ice cliffs.
Tamsin Edwards is not convinced. Glaciologist at Kings College, London, is the lead author, along with other Antarctic experts, of a study published on Wednesday in Nature (the same magazine published by DeConto and Pollard in 2016) that disputes its model, in great detail.
Using a statistical technique to examine the results, Edwards and his collaborators find that falling ice cliffs are not necessary to reproduce the past warm periods, after all.
They also present lower possibilities of sea level rise from Antarctica in this century. If they are right, the worst case is to measure back 40 centimeters, or a little more than one foot, instead of three or four feet.
"Things may not be as utterly terrible as the last study predicted," Edwards said. "But they're still bad."
She is a new science, he said, and without further models it is not clear how the ice cliffs will finally affect the rise in sea level.
But then, what happened in the Eemian? Edwards believes it only took a long time to lose West Antarctica. That was not fast. After all, the entire geological period was thousands of years.
"We are an impatient group, humans, and the ice caps do not respond in a decade, they are slow beasts," he said.
DeConto says he learned something from criticism.
"The Edwards study illustrates the need for more detailed statistics than we originally applied to our exit from the 2016 model, but the models are evolving rapidly and have already changed considerably since 2016," he said in a written statement.
But it is not receding on the sea ice cliffs. The new criticism, said DeConto, implies that "these processes are not important for the future rise in sea level, and I think that is a dangerous message."
It certainly has its allies. Richard Alley, a well-known glaciologist at Penn State University who has published with DeConto and Pollard, wrote in an email that "cliff removal is not a strange and unexpected physical process, it is happening now in some places, it has happened in the past, and high temperatures are expected to occur in the ocean or the air around the ice that flows into the ocean. "
The Eemian – but worse?
There is one important thing to consider: the Eemian happened without humans emitting many greenhouse gases.
The atmospheric carbon dioxide was much lower than today. Instead, the event was driven by changes in the Earth's orbit around the sun, which led to more sunlight falling on the northern hemisphere.
The big difference, this time, is that humans are heating things up much faster than is believed to have happened in the geological past.
And that makes a key difference, said Ted Scambos, an Antarctic researcher who heads the US side of a multimillion-dollar international mission to study the Thwaites glacier, and who is a principal investigator at the National Snow and Ice Data Center in Colorado.
"The current pace of climate change is very fast," Scambos said, and the rate of warming could cause glaciers to behave differently than they did in the past.
Accordingly, Scambos says he considers the current debate fruitful, "it is the discussion that must happen", but that it does not diminish its concern for the fate of the Thwaites glacier if it retreats sufficiently.
"There is no model that says the glacier will not accelerate if it reaches those conditions," Scambos said. "You just have to do it."
Humans were not close to Antarctica in Eemian, and never, in the modern period, have we seen a glacier as large as the Thwaites retreat. It is possible that something happens for which we have no precedent or prediction.
Last week, for example, scientists reported that a large cavity opened beneath a part of the glacier, something they said the models could not predict.
Now there is a massive involvement involved at least in trying to figure out what could happen, before it actually happens. It will help determine if humans, now organized and industrialized and fossil fuel masters, are prepared to drive a repetition of our own geological history.
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This article was originally published by The Washington Post.