July 6, 2009 — Two abrupt and drastic
climate events, 700 years apart and more than 45 centuries ago, are
teasing scientists who are now trying to use ancient records to
predict future world climate.
The events – one, a massive, long-lived drought believed to have
dried large portions of Africa and Asia, and the other, a rapid cooling
that accelerated the growth of tropical glaciers – left signals in ice
cores and other geologic records from around the world.
Lonnie Thompson, University Distinguished Professor of Earth
Sciences at Ohio State University, and researcher with the Byrd Polar
Research Center there, outlined the puzzle June 18 to colleagues at the
Chapman Conference on Abrupt Climate Change. The meeting was sponsored
by the American Geophysical Union and the National Science Foundation.
Thompson, who has led more than 50 expeditions to drill cores
through ice caps on some the highest and most remote regions of the
planet, believes that the records from the tropical zones on Earth are
the most revealing and that the last 1,000 years provides the best
“I would argue that the last 1,000 years are most critical from the perspective of looking at the future,” he said
The first of the two tantalizing events is apparent in an ice core
drilled in 1993 from an ice field in the Peruvian Andes called
Huascaran. Within that core, they found a thick band of dust
particles, most smaller than a micron in diameter, the concentration of
which was perhaps 150 times greater than anywhere else in the core.
That band dated back to 4,500 years ago.
“Dust that small can be transported great distance – the question is
where did it come from?” Thompson said. “I believe that record
accurately reflects drought conditions in Africa and the Middle East
and that the dust was carried out across the Atlantic Ocean by the
northeast trade winds, across the Amazon Basin and deposited on the
Huascaran ice cap.
Thompson said that other records, including an ice core taken from
glaciers atop Tanzania’s Mount Kilimanjaro, also show a dust event
dating to a time when there was substantive drying up of lakes in
Africa. He said that it is the only such huge event that the ice core
records show for the past 17,000 years.
The other mystery surrounds a major cooling event that Thompson
believes happened about 700 years earlier. During a 2002 expedition to
the Quelccaya ice cap in Peru, the largest tropical ice field in the
world, Thompson and colleagues discovered patches of ancient wetland
plants that had been exposed as the edge of the ice cap retreated.
When carbon-dated, the plants were shown to be 5,200 years old, meaning
that they had been covered, and preserved, by the ice for the last 52
Since then, recent expeditions have located similar patches of
plants revealed by the ice’s retreat. All date back to at least 5,200
years ago and some as much as 7,000 years ago.
“This means that sometime around 5,200 years ago, there was a rapid
cooling event in this region and the ice expanded shielding the plants
from damage and decay,” Thompson said.
Other records from around the world seem to support the idea of a
cooling event at this time. Divers in Lake Tahoe, Nevada, found nearly
two dozen ancient tree trunks preserved at the lake’s bottom. Wood
samples from the trunks date back 5,200 years and geologic records show
the current lake levels have remained steady since that point in time.
Thompson also pointed to the timing of past climate changes in South
America and the rise and fall of early cultures in the region.
Evidence from the ice cores from Quelccaya suggest that cultures
might have grown during wet periods in the Peruvian Highlands and waned
when the climate became drier. Conversely, cultures appeared to grow
in the country’s coastal regions when the climate became wetter and
were lost as drying increased.
“This suggests that there could have been persistent climate periods
that allowed these cultures to flourish under certain conditions and
fail under others,” he said.
Thompson leads a new expedition next week to two new sites in the
Andes in hopes of drilling cores that will show more detailed records
of both events.
The evidence that researchers have, both from ice cores and from the
rapid retreat of glaciers, show that high-altitude ice fields reflect
similar changes that are currently visible all across the temperate
portions of the globe.
“The ice caps are sentinels of the earth’s overall climate,” he
said. “And the data shows that at all of these sites, the rate at
which the ice is vanishing is accelerating.
“To me, these are indicators that these areas are already being adversely impacted by changes in our current climate.”
Source : Ohio State University