A new study adds evidence that climate swings in Europe and North America during the last ice age were closely linked to changes in the tropics. The study, published this week in the journal Science, suggests that a prolonged cold spell that sent glaciers in Europe and North America creeping forward several hundred years ago may have affected climate patterns as far south as Peru, causing tropical glaciers there to expand too. Glaciers in both the tropics and North Atlantic region reached their most recent maximum extents during the so-called Little Ice Age, about 1650 AD to 1850 AD, according to the study, which employed a cutting-edge technique for dating glacial deposits.
By understanding how glaciers behaved in the past, scientists hope to predict how different parts of the world will react as the planet warms. Human civilization arose during the fairly stable temperatures that have prevailed since the end of the last ice age, about 12,000 years ago. But research shows that even during this time glaciers have fluctuated in large and sometimes surprising ways.
Most of the world’s glaciers are now retreating, as manmade greenhouse gas levels rise. The Intergovernmental Panel on Climate Change predicts that global temperatures may climb another 1.1˚ to 6.4˚ Celsius by this century’s end. “If the current dramatic warming projections are correct, we have to face the possibility that the glaciers may soon disappear,” said Joerg Schaefer, a geochemist at Columbia University’s Lamont-Doherty Earth Observatory, who coauthored the study with lead author Joseph Licciardi, a glacial geologist at the University of New Hampshire. In a warmer world, regions that depend on glaciers for drinking water, farming and hydropower will need to come up with strategies to adapt.
Below Peru’s Nevado Salcantay, early Holocene glacial moraine (foreground), Little Ice Age moraine (background).
Developments in surface exposure dating have allowed scientists to place far more precise dates on recent glacial fluctuations than was previously possible. When glaciers advance, they drag rocks and dirt with them. When they recede, ridges of debris called moraines are left behind, and the newly exposed deposits are bombarded by cosmic rays passing through Earth’s atmosphere. The rays react with the rock and over time form tiny amounts of the chemical isotope beryllium-10. By measuring the buildup of this cosmogenic isotope, scientists can calculate when the glacier receded. Using this technique, the authors showed that glaciers in southern Peru moved at similar times as glaciers in the northern hemisphere.
The global picture, however, is more complex. Schaefer recently published another study in Science showing that glaciers in New Zealand’s Southern Alps, 7,000 miles southwest of the study in Peru, expanded and contracted more frequently than northern glaciers, reaching their most recent maximum 6,500 years ago—long before the Little Ice Age.
“If we compare records—New Zealand, Europe, Peru--we can say that the tropical Andes look like Europe but not New Zealand,” said Licciardi. “What’s emerging is a more complicated picture of recent glaciations.”
On vacation in 2003, Licciardi was hiking to the ruins of the ancient Inca city of Machu Picchu when he noticed the massive, well-preserved moraines around him. Two years later, a friend and co-author of the study, David Lund, hiked the same trail and chiseled off samples of the salt-and-pepper colored granitic rock. Licciardi returned in 2006 to the nearby slopes of Nevado Salcantay, a 20,000-foot peak that is the highest in the Cordillera Vilcabamba range. Over the next two years, Licciardi and his graduate student Jean Taggart, also a co-author, collected more rock samples from the moraines and analyzed them using the beryllium isotope method, with the help of Schaefer.
The beryllium dating method was pioneered in the 1980s but only recently has it become precise enough to track the ebb and flow of glaciers over the last thousand years. “Until the last year or two, we had no way of dating the youngest deposits with the beryllium method,” said Licciardi. “Recent breakthroughs with this technique we’re using have allowed this story to emerge.”
In a related study, Lund and colleagues suggest that the northward flow of warm Gulf Stream water slowed during the Little Ice Age, possibly causing sea ice to expand and glaciers in Scandinavia and the Swiss Alps to creep forward. As the north grew colder, the Intertropical Convergence Zone—the place where trade winds from the north and south meet—was apparently pushed southward. This shift in winds may have brought increased snowfall to the high tropical Andes, causing the Salcantay glaciers to advance.
Now that Peru’s climate has been linked to northern Europe’s, the team hopes to expand their investigation to other parts of the South American tropics to establish a regional pattern of glacial advances and retreats that can be compared with other places, including New Zealand.
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Funding was provided by the U.S. National Science Foundation, UNH, Sigma Xi and the Geological Society of America.
Copies of the paper, “Holocene glacier fluctuations in the Peruvian Andes indicate northern climate linkages,” are available from the authors or Science.