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Data Reveal Synchronous Abrupt Climate Events in Last Glacial Cycle

cave speleothem
Cave formations can help precisely date abrupt climate changes during the last glacial cycle. | James Van Gundy/ Flickr

Abrupt climate warming events written in Greenland ice cores during the last glacial cycle occurred contemporaneously with other rapid climate change events observed in paleoclimate records from across the globe, according to a new study.

The results, published in the August 21 issue of Science, reveal a near-synchronous connection in climate events spanning the hemispheres. The abrupt and rapid warming periods in the Arctic occurred very closely in time to similarly abrupt changes in precipitation observed across the lower latitudes and spanning both hemispheres 115,000 to 11,700 years ago.

According to lead author Ellen Corrick, a researcher at the University of Melbourne, Australia, the results confirm a long-held assumption in paleoclimatology and provide important insights about the potential impacts of abrupt climate events in our own uncertain future.

The ancient ice of Greenland's glaciers contains a detailed record of Earth's past climate spanning hundreds of thousands of years. Tiny bubbles of air, water molecules and other particles that became trapped, as annual blankets of accumulating snow formed compressed layers of glacial ice, offer a unique, long-term window on the planet's changing climate and environmental conditions.

For example, the chemical composition of each pocket of ancient air preserves previous levels of greenhouse gases like carbon dioxide, and the layers of volcanic ash or dust can reveal powerful volcanic eruptions or extended global dry spells. Like fossils, they can be used to determine the date that particular layer of ice formed. Each ice core — long, vertical rods drilled from the deep, thick ice of glacier and ice caps — can be read like a history of Earth's complex and dynamic climate system.

Even the oxygen atoms that comprise the water molecules of each layer of ice core tell an important story. Paleoclimatologists use these oxygen isotopes like a thermometer and, measuring the variations in isotopic ratios across an ice core, derive a record of atmospheric temperature change.

The climate records from Greenland's ice cores cover Earth's last glacial cycle and reveal a series of abrupt climate fluctuations between warm and cold conditions. During these oscillations, also known as Dansgaard-Oeschger (DO) events, annual temperatures in the region warmed rapidly — sometimes by as much as 46 degrees Fahrenheit over a few decades — before gradually returning to a cooler climate state.

"[DO events] occurred repeatedly throughout the last glacial period," said Corrick. "Profound changes in temperature and rainfall occurred in many locations around the world, often within a human lifespan."

Outside of Greenland, similar abrupt climate change events during the last glacial period have also been identified in a host of other paleoclimate records from far-away tropical and sub-tropical regions across the globe, particularly in the Asian Monsoon, South American Monsoon and European-Mediterranean regions. While the processes responsible for the abrupt transitions aren't totally understood, they are thought to be linked through Earth system-wide climate mechanisms.

According to Corrick, however, a major sticking point in testing this assumption is determining if Greenland's DO events occurred in synchrony with the abrupt changes observed in lower latitudes.

"To test synchrony, we need many, very precisely dated records from a range of locations and it is difficult to date many last glacial climate records, such as ice cores and marine sediments, at the required level of precision," said Corrick.

Corrick and the team of researchers addressed this problem by compiling data from 63 high-resolution and precisely dated climate records from across the globe and derived from the thin layers of sedimentary cave rocks called speleothems, like stalactites and stalagmites.

Speleothems grow slowly, layer by layer, as the minerals and molecules delivered by seeping, dripping water slowly accumulate on the cave's surface. Similar to ice cores, the isotopic composition of each layer reflects changes in the temperature and precipitation of the climate surrounding the cave.

"The key advantage of speleothems is that we can date them much more precisely than many other materials by using a radiometric technique called uranium-thorium dating," said Corrick. "Together, this enables us to make precise comparisons of the timing of climate events between different regions of the Earth."

The study found that abrupt climate changes during the last glacial period — from the Arctic to the Southern Hemisphere — occurred within just a few decades of one another, suggesting that abrupt Arctic warming events likely triggered rapid climate disruptions on a global scale.

"Given the magnitude of the temperature changes observed in Greenland ice cores, it seems intuitive that there would be associated climate changes beyond the Arctic region," said co-author Russell Drysdale, also from the University of Melbourne. "However, there was no consensus in the paleoclimate community about this."

While it's widely thought that DO events are, in some way, associated with variations in the strength of a massive network of ocean currents that is responsible for redistributing large amounts of Earth's atmospheric heat and regulating climate in the Northern Hemisphere, a complete understanding of the underlying mechanics involved remains elusive.

According to Drysdale, the findings clearly demonstrate the high degree of connectivity of the climate system across very large geographic regions.

"They show us that the climate system is very connected — rapid warming in the Arctic occurred when monsoons weakened in the Southern Hemisphere," said Drysdale.

What's more, the observations largely agree with what paleoclimate models have suggested Earth's climate was like during during the last glacial period, adding confidence to modeled predictions of how our future under climate change may look.

With the Arctic warming nearly twice as quickly as the rest of the planet — and perhaps accelerating — understanding how abrupt warming events in the high latitudes affected global climate is particularly felicitous. Corrick and Drysdale said the logical next step is to extend this research further south, to the southern limits of South America, Africa and Australia.

"We currently know very little about how the climate of these regions responded to Arctic warming events," said Drysdale. "High-quality cave records such as the ones we used in our study will be critical."

[Credit for associated image: James Van Gundy/ Flickr]