Tag: greenland ice sheet

New Study Shows Ice ages Connected to Earth’s Tilt

Over the last two and a half million years the Earth has undergone more than 50 major ice ages, each having a profound effect on our planet’s climate. But what causes them and how do we predict when the next big ice age will hit?

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About 40 years ago, scientists realized that ice ages were driven by changes in the Earth’s orbit. But, as I recently argued in Nature, it’s not that simple. Scientists are still trying to understand how such wobbles interact with the climate system, particularly greenhouse gases, to push the planet in to or out of an ice age.

During the last ice age, only 21,000 years ago, there was nearly continuous ice across North America from the Pacific to the Atlantic Ocean. At its deepest over the Hudson Bay, it was over two miles thick and reached as far south as what would now be New York and Cincinnati. In Europe, there were two major ice sheets: the British ice sheet which reached as far south as what would now be Norfolk, and the Scandinavian ice sheet that extended all the way from Norway to the Ural mountains in Russia.

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In the Southern Hemisphere there were significant ice sheets on Patagonia, South Africa, southern Australia and New Zealand. So much water was locked up in these ice sheets that the global sea level dropped by over 125 meters – around ten meters lower than the height of the London Eye. In comparison if all the ice on Antarctica and Greenland melted today it would only raise sea level by 70 meters.

So what caused these great ice ages? In 1941, Milutin Milankovitch suggested that wobbles in the Earth’s orbit changed the distribution of solar energy on the planet’s surface, driving the ice age cycles. He believed that the amount of incoming solar radiation (insulation) just south of the Arctic Circle, at latitude 65°N, was essential. Here, insulation can vary by as much as 25%. When there was less insulation during the summer months, the average temperature would be slightly lower and some of the ice in this region could survive and build up – eventually producing an ice sheet.

But it wasn’t until 30 years later that three scientists used long-term climate records from analyzing marine sediments to put this to the test. Jim Hays used fossil assemblages to estimate past sea surface temperatures. Nick Shackleton calculated changes in past global ice volume by measuring oxygen isotopes (atoms with different numbers of neutrons in the nucleus) in calcium carbon fossil in marine sediments. John Imbrie used time-series analysis to statistically compare the timing and cycles in the sea surface temperature and global ice volume records with patterns of the Earth’s orbit.

In December 1976 they published a landmark climate paper in Science, showing that climate records contained the same cycles as the three parameters that vary the Earth’s orbit: eccentricity, obliquity and precession. Eccentricity describes the shape of the Earth’s orbit around the sun, varying from nearly a circle to an ellipse with a period of about 96,000 years. Obliquity is the tilt of the Earth’s axis of rotation with respect to the plane of its orbit, which changes with a period of about 41,000 years. Precession refers to the fact that both Earth’s rotational axis and orbital path precess (rotate) over time – the combined effects of these two components and the eccentricity produce an approximately 21,000-year cycle.

The researchers also found that these parameters have different effects at different places on our globe. Obliquity has a strong influence at high latitudes, whereas precession has a notable impact on tropical seasons. For example precession has been linked to the rise and fall of the African rift valley lakes and so may have even influenced the evolution of our ancestors. Evidence for such “orbital forcing” of climate has now been found as far back as 1.4 billion years ago.

Beyond wobbles
However, the scientists realized that there were limitations and challenges of their research – many of which remain today. In particular, they recognized that variations in the Earth’s orbit did not cause the ice age cycles per se – they rather paced them. A certain orbit of the Earth can be associated with many different climates. The one we have today is in fact similar to the one we had during the most intense part of the last ice age.

Small changes in insulation driven by changes in the Earth’s orbit can push the planet into or out of an ice age through the planet’s “climate feedback” mechanisms. For example when summer solar radiation in reduced it allows some ice to remain after the winter. This white ice reflects more sunlight, which cools the area further and allows more ice to build up, which reflects even more sunlight and so forth. Therefore, the researchers’ next step was to understand the relative importance of ice sheet, ocean and atmospheric feedbacks.

New Study Indicates Many Scientists Clueless to Cause of Climate Cycles

Now, two first-of-their-kind studies provide new insight into the deep history of the Greenland Ice Sheet, looking back millions of years farther than previous techniques allowed. However, the two studies present some strongly contrasting evidence about how Greenland‘s ice sheet may have responded to past climate change – bringing new urgency to the need to understand if and how the giant ice sheet might dramatically accelerate its melt-off in the near future.

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The two new studies were published in the journal Nature on December 8, including one led by University of Vermont geologist Paul Bierman. The other led by Joerg Schaefer of Lamont-Doherty Earth Observatory and Columbia University.

Bierman and four colleagues – from UVM, Boston College, Lawrence Livermore Laboratory, and Imperial College London – studied deep cores of ocean-bottom mud containing bits of bedrock that eroded off of the east side of Greenland. Their results show that East Greenland has been actively scoured by glacial ice for much of the last 7.5 million years – and indicate that the ice sheet on this eastern flank of the island has not completely melted for long, if at all, in the past several million years. This result is consistent with existing computer models. Since the data the team collected only came from samples off the east side of Greenland, their results do not provide a definitive picture of the Greenland ice sheet.

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The other study in Nature – led by Joerg Schaefer of Lamont-Doherty Earth Observatory and Columbia University, and colleagues – looked at a small sample of bedrock from one location beneath the middle of the existing ice sheet and came to what appears to be a different conclusion: Greenland was nearly ice-free for at least 280,000 years during the middle Pleistocene – about 1.1 million years ago. This possibility is in contrast to existing computer models.

“These results appear to be contradictory” UVM’s Bierman says. He notes that both studies have “some blurriness,” he says, in what they are able to resolve about short-term changes and the size of the ancient ice sheet. “Their study is a bit like one needle in a haystack,” he says, “and ours is like having the whole haystack, but not being sure how big it is.”

Both Studies Analyze Cosmic Ray Bombardment in Bedrock

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Both teams of scientists used, “a powerful new tool for Earth scientists,” says Dylan Rood, a scientist at Imperial College London and a co-author on the Bierman-led study: isotopes within grains of quartz, produced when bedrock is bombarded by cosmic rays from space. The isotopes come into being when rock is at or near Earth’s surface – but not when it’s buried under an overlying ice sheet. By looking at the ratio of two of these cosmic-ray-made elements – aluminum-26 and beryllium-10 caught in crystals of quartz, and measured in an accelerator mass spectrometer – the scientists were able to calculate how long the rocks in their samples had been exposed to the sky versus covered by ice.