Results of a new study add evidence that climate swings in the northern hemisphere over the past 12,000 years have been tightly linked to changes in the tropics.
The findings, published this week in the journal Science, suggest that a prolonged cold spell that caused glaciers in Europe and North America to creep 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 scientists conducting the research.
To make the discovery, they employed a cutting-edge technique for dating glacial deposits.
"The results bring us one step closer to understanding global-scale patterns of glacier activity and climate during the Little Ice Age," says lead author Joe Licciardi, a glacial geologist at the University of New Hampshire.
By understanding how glaciers behaved in the past, the geoscientists hope to predict how parts of the world will react as the planet warms.
Human civilization arose during fairly stable temperatures since the end of the last ice age, about 12,000 years ago. But research shows that even during this time glaciers 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 degrees 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 (LDEO) and co-author of the paper.
In a warmer world, regions that depend on glaciers for drinking water, farming and hydropower will need to come up with strategies to adapt.
Recent developments in a technique called surface exposure dating have allowed scientists to place far more precise dates on glacial fluctuations during recent times 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 rare chemical isotope beryllium-10. By measuring the buildup of this isotope in glacial rocks, scientists can calculate when the glaciers receded.
Using this technique, the authors showed that glaciers in southern Peru moved at times similar to those in the northern hemisphere.
"The results are based on the CRONUS-Earth Project, which aims to improve measurements of these isotopes so precise ages may be assigned to 'young' glacial moraines," says Enriqueta Barrera, program director in the National Science Foundation (NSF)'s Division of Earth Sciences, which funded the research.
"Through CRONUS-Earth, we hope to create a global map of recent glacial fluctuations."
The global picture is complex. Glaciers in New Zealand's Southern Alps, 7,000 miles southwest of this study area in Peru, for example, 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."
Licciardi first noticed the glacial deposits in 2003 while he was in Peru on vacation.
Hiking to the ruins of the ancient Inca city of Machu Picchu, he was struck by the massive, well-preserved moraines he encountered along the way.
Two years later, David Lund, a paleoclimatologist at the University of Michigan, hiked the same trail and collected rock samples there, which he sent to Licciardi. "That was the catalyst for turning our ideas into a project," says Licciardi.
Licciardi returned in 2006 to the nearby slopes of Nevado Salcantay, a 20,000-foot-high peak that's the tallest in the Cordillera Vilcabamba range.
Over the next two years, Licciardi and graduate student Jean Taggart, also a co-author of the paper, 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 this method," said Licciardi." Recent breakthroughs in the technique have allowed this story to emerge."
With Peru's climate now linked to northern Europe's, the scientists plan to expand their research to other parts of the South American tropics. They hope to establish a regional pattern of glacial advances and retreats that can be compared with other places worldwide.
Funding also was provided by UNH, Sigma Xi, and the Geological Society of America.
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