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.
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.
"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," said Lonnie Thompson, University Distinguished Professor of Earth Sciences at Ohio State University.
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 ice caps are sentinels of the earth's overall climate," Thompson 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," he added.
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 centuries.
"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. (ANI)