For billions of years, the Earth has experienced periodic glacial cycles governed by its orbit around the Sun. No force on the planet could interrupt this eon-long cycle, until we came along. A new study published in the journal Science suggests that a new glacial period began about 2,000 years ago, but by the 1960s greenhouse gas emissions had raised global temperatures enough to reverse it. Future emissions could anticipate the next ice age by hundreds of thousands of years, disrupting one of Earth’s most fundamental natural cycles. “It was just beginning, everything was normal, the next ice age was about to happen, and yet it stopped,” first author Nicolas Young, a geochemist at Columbia University, said in a statement. “It’s a very powerful example of how we’ve shaken up the system.” Canceled glaciation Glacial cycles within the ice ages are governed by Milankovitch cycles: periodic changes in the shape of the planet’s orbit, the inclination of its axis, or the direction of rotation of its axis that influence the amount of sunlight that reaches certain parts of the planet. During an ice age, which can last millions to tens of millions of years, Earth is relatively cold, with large areas of the planet covered by continental ice sheets and alpine glaciers. However, that does not mean that the climate remains unchanged. Over the course of an ice age, shorter periods of warmer temperatures and retreating glaciers (called interglacial periods) alternate with periods of colder temperatures and advancing glaciers (called glacial periods). Right now, we are in an interglacial period during the Late Cenozoic Ice Age (yes, we technically live in an ice age), and the last glacial period ended about 11,700 years ago. During the peak of that last glacial period, the Laurentide Ice Sheet covered much of Canada and the northern United States. A single remnant of that ice sheet still survives today as the Barnes Ice Sheet, which extends across an area roughly the size of Delaware on Baffin Island in Nunavut, Canada. Today, the Barnes Ice Sheet is thinning and retreating, but Young and his colleagues discovered that wasn’t the case not long ago. The shrinking of the glacier allowed them to analyze the newly exposed vegetation that was long preserved beneath the ice sheet. By dating those plants, they built a timeline of its climate history and found that it reached its minimum extent about 3,000 years ago and then began expanding again 2,000 years ago. Researchers argue that this growth marked the start of Earth’s next glacial period, but was derailed before it could really get underway. That change came in the 1960s, when global temperatures had warmed so much that the Barnes Ice Sheet began losing more ice in the summer than it added in the winter. That trend has continued since then and, therefore, the next ice age remains on hold. An uncertain future Climate models suggest that the Barnes Ice Sheet is likely to disappear completely within the next 300 years. For eons, the Laurentide Ice Sheet has anchored continent-spanning ice growth. If this last remnant disappears, it could change the dynamics of future ice ages and glaciations. As humanity continues to warm the planet, experts can’t be sure when the next ice age might occur. In a perspective paper accompanying the new study, glaciologist and University of Wisconsin-Madison professor Shaun Marcott says future greenhouse gas emissions could anticipate the next ice age by hundreds of thousands of years. It is amazing that a single species can so fundamentally alter the planet. With global temperatures set to rise more than 2.7 degrees Fahrenheit (1.5 degrees Celsius) above pre-industrial levels in the coming years, Earth is entering a whole new chapter in its climate story, one shaped by us.
