A Climate Debateby Bill Chameides | September 29th, 2010
posted by Erica Rowell (Editor)
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Ice cores, like the one shown here being collected by a scientist in East Antarctica, are used to determine things like temperature from ages past. What such proxies hold can be a matter of debate. (icedrill.org)
Yes, climate scientists really do disagree about some things.
There are lots of things about which climate scientists agree, including certain issues that leave no room for legitimate debate. For example, scientists have concluded that “global warming is unequivocal” (e.g., see reports by the Intergovernmental Panel on Climate Change and the National Research Council). But there are also lots of issues where debate is lively. A case in point: the cause of the 1,300-year event in the Earth’s climate past referred to as the Younger Dryas.
A Mysterious, Sudden Cooling
It is said that the past is the key to the future. And understanding the Earth’s climate in the past (referred to as paleoclimate) can provide important insights into how the climate system operates today and how it might change in the future.
Studying paleoclimate is challenging however — there were no climate scientists around thousands of years ago taking temperature measurements and no satellites orbiting the planet gathering global data. Fortunately, scientists have found that the Earth has stored other types of data that can serve as proxies for global temperatures, precipitation, ice cover and the like. For example, ice cores from the Greenland and Antarctic ice sheets have proved to be a veritable treasure trove of paleoclimate proxies.
But while we’ve learned an incredible amount from these proxies, we’ve also stumbled upon some perplexing puzzles.
Over the past two million years or so, a period geologists refer to as the Pleistocene, the Earth’s climate has been in a cold, icy state. During most of this period, large swaths of the Earth’s surface have been covered by ice. But for the last million years every 100,000 years or so, the Earth warms up and the ice retreats. These warm periods are called interglacials and typically last about ten thousand years, although longer periods lasting tens of thousands of years or so occur as well. Eventually the interglacials end and the climate plunges back into an ice age.
These swings between ice ages and interglacials are understood to be triggered by subtle changes in the Earth’s orbit about the Sun and amplified by feedbacks involving, among other things, changes in carbon dioxide (CO2) and other greenhouse gases.
The last ice age began its decline some 12,000 years ago. But then something strange happened. Suddenly the climate reversed itself — temperatures plunged, and ice began to expand anew. After about 1,300 years of this unexpected icy period, the climate rapidly warmed, bringing us to the conditions of the interglacial period we enjoy today.
The sudden, 1,300-year cold period is called the Younger Dryas, after the wildflower Dryas octopetala, which thrived in the cold conditions found throughout Europe during this period and whose pollen is used as a proxy.
But What Caused the Younger Dryas Cooling?
Climate scientists have been trying to answer that question for decades. Theories abound. A popular one holds that a huge input of meltwater from the North American continent shut down the so-called ocean conveyor belt circulation, including the Gulf Stream which brings warm water to the North Atlantic, and the loss of this warm flow triggered the cold conditions of the Younger Dryas. (See my post. The movie The Day After Tomorrow is a very fanciful and unrealistic depiction of how this scenario might play itself out today.)
Another theory is that the Younger Dryas was triggered by an Earthly collision with a huge comet or meteorite. Such an impact, the theory goes, would have caused widespread fires that would have injected huge quantities of sun-blocking particles into the atmosphere, causing temperatures to plunge.
The extra-terrestrial impact theory is feasible, but did it happen? To answer that question, scientists have focused on an extremely tiny diamond called lonsdaleite that occurs naturally in meteorites and impact craters.
If it can be shown that these nanodiamonds are uniquely present in ice or sediments formed just before the Younger Dryas, then the extra-terrestrial impact theory becomes a lot more credible. And that is where the debate on the Younger Dryas has heated up. Some scientists claim to have seen nanodiamonds in the appropriate layers, and others say no.
What Lies Beneath: Seeing ‘Rocks’ in the Ice
Illustrative are two papers that have recently appeared in the peer-reviewed literature. Andrei Kurbatov of the University of Maine, Orono, and coauthors report [pdf] in the Journal of Glaciology the presence “in the Greenland ice sheet of a discrete layer of free nanodiamonds in very high abundances.”
To support their conclusion, they show extensive data to back up their identification of the material in the layer as nanodiamonds. These data, along with their findings that the presence of the material they identify as nanodiamonds is unique to this one layer, lead the authors to conclude that the material was “most likely formed during a cosmic impact” that “occurred after the last glacial episode.”
Not Seeing ‘Rocks’ in the Ice
Almost simultaneous with the Kurbatov et al paper came another paper, this one published in the Proceedings of the National Academy of Sciences by Tyrone Daulton of Washington University in St. Louis and coauthors. They argue that nanodiamonds do not occur in Younger Dryas sediments.
Their data indicate that these so-called nanodiamonds, previously identified as such from one location, are really more mundane forms of carbon (such as graphene) that are ubiquitous in sediments from the late Pleistocene forward that have simply been misidentified. They write: “Our results cast doubt on one of the last widely discussed pieces of evidence supporting the [Younger Dryas] impact hypothesis.”
So who’s right? I don’t know. Part of the problem is that the Daulton et al paper re-examines a different set of data published last year by the other group, and so the two new papers on nanondiamonds don’t examine data from the same location. Perhaps the nanodiamonds discovered in Greenland will prove to be just that. I expect it will be revisited in the peer-reviewed literature soon.
In the meantime the Younger-Dryas puzzle continues to puzzle.filed under: climate change, faculty, global warming, Planetary Watch, temperatures, Younger-Dryas
and: climate, climate science, global temperatures, ice age, ice cores, interglacial, nanodiamonds, paleoclimate, paleoclimate proxies, temperature proxies, temperature records