by Chris Bentley
Oct 21, 2010
Earth’s last deep freeze gripped the Great Lakes about 13,000 years ago, but climate scientists need to explain how it happened as though it occurred last night.
Scientists are in a race against time to figure out how Earth can shift abruptly from warm weather to glacial conditions and back again. One big mystery to solve is how global climate change can affect the two hemispheres differently.
New dating techniques help explain how massive glaciers interact with greenhouse gases such as carbon dioxide to alter otherwise predictable climate patterns north and south of the equator.
“If you were to think about the present and the future situations, this has an important implication — there is an association between what CO2 is doing and what the glacier behavior in some areas is doing,” said Michael Kaplan of Columbia University’s Lamont-Doherty Earth Observatory.
At this year’s Comer Conference on abrupt climate change in rural Wisconsin, leading scientists presented new evidence that the Southern Hemisphere was actually warming thousands of years ago while glaciers returned in the north.
The mystery reignites debate over patterns and theories of climate change in past eras. Scientists agree that today’s Earth overall is warming at an escalating pace due to human combustion of fossil fuels. But what disparities could occur across the continents and at what pace?
Research presented at the conference, much of it published recently in journals such as Science and Nature, provides some of the strongest evidence yet that New Zealand’s ice sheets started melting rapidly while Greenland’s were making their biggest surge since the previous global Ice Age, which occurred about 20,000 years ago.
A compelling record of ice cores from Greenland show a sudden drop in air temperature during the Younger Dryas in North America. That cold snap, which occurred about 13,000 years ago, is named after a genus of white flower known to flourish with glacial weather conditions.
Kaplan and his colleagues used a sophisticated new method of chemical analysis to back up evidence obtained through the more time-tested approach: mapping the extent of glacial rock deposits called moraines. As glaciers retreat, they leave successive ridges of boulders that mark their boundaries, like a series of fingerprints.
“Glaciers are good climatologists,” said Richard Alley, a geoscientist and professor at Pennsylvania State University and a contributor to the 2007 report by the Intergovernmental Panel on Climate Change. The report predicts diminishing food and freshwater supplies, epidemics and extreme weather as a result of climate change.
But the moraines could be reflecting processes related to geology instead of climate, Alley added.
That’s where the new techniques come in. The bedrock in glacial valleys is exposed to cosmic rays — charged particles hurtling across the galaxy — only when there is no glacier covering it. Reactions with cosmic rays turn some oxygen in silicon dioxide, a basic ingredient in bedrock, into the isotope beryllium-10. So by measuring the accumulation of Be-10, while correcting for erosion, scientists can date how long a particular moraine has been exposed to the open air.
University of Maine’s Aaron Putnam, who worked with Kaplan, showed that the glacial retreat patterns from New Zealand didn’t hold up in the Western United States. Using the improved precision of Be-10 dating, he showed a different rate of change for each hemisphere.The study implicates carbon dioxide in the warming and glacier retreat.
Carbon dioxide is considered a thermostat for global warming and is at much higher levels now than it has been over the past hundreds of thousands of years.
But other factors came into play. “One of the best ways to cause abrupt climate change is to change [ocean circulation],” said Ben Flower, a geochemist at the University of South Florida.
Flower said shifts in the sunlight hitting Earth caused an influx of melting freshwater into the North Atlantic shortly as the glaciers of 20,000 years ago retreated. This could have slowed routine ocean circulation of warm water moving north and shuffled wind patterns, changing rates of ocean upwelling — an important source of CO2. More sea-ice might have formed in the North Atlantic as a result, which could have delayed the warming effect of CO2 in Greenland.
“In kind of a funny way, you melt the [freshwater] ice and it leads to more sea-ice, which can cool the air temperature over Greenland,” Flower said. His research tracks glacial melt water from the Laurentide ice sheet — a massive glacial expanse that extended from the Hudson Bay south to Chicago during the most recent ice age.
When Flower’s team finds the chemical signature of glacial ice in the Gulf of Mexico, they can safely guess that the Laurentide was melting into the Mississippi River.
And in the gulf, shifting wind patterns kick-started the upwelling of CO2 in a positive feedback loop that eventually warmed Greenland.
“Our new records from the Gulf of Mexico show that the Laurentide ice sheet, that was basically next door to the Greenland ice sheet, was melting [between the last ice age and the onset of the Younger Dryas],” Flower said. “Greenland has been misleading us, in a sense, about the full story of northern hemisphere climate change.”
“CO2 in the past has not been the initial trigger, but it has been a strong feedback,” Flower said. “You needed CO2 to [force] the end of the last ice age.”
“It’s exciting how quickly it’s all coming together,” Alley said. “So many [papers] are just playing with the toes of the elephant.”
True to Alley’s analogy, paleoclimatology — the study of Earth’s ancient climate history — is a bit like trying to assemble an old jigsaw puzzle whose few remaining pieces have become waterlogged and deformed.
To collect evidence about temperature and greenhouse gas concentrations tens of thousands of years ago, scientists and technicians scour caves, corals and ocean sediments that have reacted predictably to certain climatic variations over time. Here, they measure ‘proxy data’ that bears the chemical mark of the ancient climatic conditions that produced it.
The ratio of rare oxygen-18 isotopes to the abundant oxygen-16 isotopes provide much of the compelling evidence for glacial advance and retreat in the first place. The relatively light oxygen-16 is preferentially evaporated and deposited into glaciers via precipitation, leaving the ocean with a markedly heavy isotopic signature of oxygen-18 when more of the Earth’s water is tied up in glaciers.
Of course, ancient oxygen is hard to find. To measure oxygen-18, an isotope that can indicate glacial advance when measured as a ratio against oxygen-16, scientists such as Flower collect ocean sediments. They locate remnants of ancient plankton, called foraminifera, that trap seawater oxygen when they build their calcium carbonate shells. When they die, their shells entomb the samples in fossils the size of sand grains.
Flower’s research team has compared their records of oxygen-18 with some of the proposed geological timelines for warm, interglacial periods. “It’s surprising how early things are changing in the deglacial period,” he said. “The ramp-up in forcing not a lot in terms of energy on the Earth’s surface.” While the dynamics remain complex, the results suggest that relatively small or local events can cause much larger and unpredictable change.