About 12,900 years ago, the world got much cooler rather abruptly. In the span of just a few decades—the blink of an eye in geologic time—the temperature at the summit of Greenland, for example, dropped to 15 degrees Fahrenheit cooler than it is today. During this era, larger animals like mastodons and saber-toothed tigers became less common. Some scientists have even suggested that this quick shift to a cooler climate was instrumental in the human transition from hunting and gathering to farming, as large animals and their meat became less plentiful.
This period, called the Younger Dryas Cold Event, shows that sudden climate change has happened before. It and other events of rapid cooling or warming—like the greenhouse-gas-fueled one currently plaguing our planet—might help us understand just how sensitive our planet is to sudden temperature fluctuations.
But what, precisely, caused the Younger Dryas in the first place?
It depends who you ask or what research you consult, but a new study suggests it may have at least partially been caused by something from above—meaning a meteorite (or several meteorites). The study, published this week in the journal Proceedings of the National Academy of Sciences, analyzed the chemical composition of rock fragments thought to have landed on the ground in what is now Pennsylvania. The study suggests that the fragments, or spherules, were formed and kicked into the atmosphere by a high-temperature impact of a meteorite in present day Quebec, said Dartmouth researcher Mukul Sharma, a co-author of the study.
"The Younger Dryas cooling impacted human history in a profound manner," Sharma said in a statement. "Environmental stresses may also have caused Natufians in the Near East to settle down for the first time and pursue agriculture."
"By examining the mineralogy and chemistry of the spherules, [we found they] formed at temperatures exceeding 2000 degrees Celsius, and many formed in the absence of oxygen," Sharma wrote in an email. Both of these are indicative of meteorite impact. The study also measured the mix of isotopes of the elements strontium and neodymium. (Isotopes are different forms of the same element with differing numbers of neutrons.) The particular mixture of these isotopes allowed them to suggest the impact occurred in southern Quebec, although they haven't yet found a matching impact crater.
There is one large known impact crater in Quebec, but that isn't likely to have been formed at this time, Sharma said. But the study should help guide a future search for the location of the crater.
The leading theory suggests that the Younger Dryas was caused by the breakup of a massive glacial lake known as Lake Agassiz. This influx of freshwater led to a slowing of the Atlantic meridional overturning circulation, a "conveyor belt" in the ocean that brings warm water from the equator north and heats up North America and Europe.
The meteorite impact could have helped destabilize the ice sheet that held this glacial lake in place, leading to its collapse, Sharma implied. Meteorites can also cool the climate by suspending particles in the air that block out sunlight, leading to global cooling. This is one hypothesis for what led to the extinction of the dinosaurs.
If the Younger Dryas was caused by an influx of freshwater, that could have implications for today—with warmer temperatures, more ice could melt and make water in the North Atlantic less saline and thus less dense. Some research has suggested that this could slow down the "oceanic conveyor," which is powered in part by the sinking of this dense water in the North Atlantic. But whether or not that could happen, and what exactly the consequences would be, is unclear.
The exact cause of the Younger Dryas Cold Event remains quite controversial, and some researcher have voiced doubt about this study.
"At this point, the pro-impact literature is fringe science being promoted by a single journal," Nicholas Pinter, a geologist at Southern Illinois University in Carbondale not involved in the PNAS research, told LiveScience. Sharma also told the science site that the spherules had not been exactly dated, and may be thousands of years old. "We are assuming they are Younger Dryas, but [dating] is one of the things that should be done better," Sharma said.