For billions of years, Earth has experienced periodic glacial cycles governed by its orbit around the Sun. No force on the planet could disrupt this eons-long cycle\u2014until we came along.<\/p>\n
A new study published<\/a> in the journal Science suggests that a new glacial period was beginning some 2,000 years ago, but by the 1960s, greenhouse gas emissions had raised global temperatures enough to reverse it. Future emissions could forestall the next ice age for hundreds of thousands of years, altering one of Earth\u2019s most fundamental natural cycles.<\/p>\n \u201cIt was just getting started, everything was normal, the next glaciation was about to happen\u2014and yet it stopped,\u201d first author Nicol\u00e1s Young, a geochemist at Columbia University, said<\/a> in a statement.\u00a0\u201cIt\u2019s a very powerful example of how we have jolted the system.\u201d<\/p>\n The canceled glaciation <\/p>\n The glacial cycles within ice ages are governed<\/a> by Milankovitch cycles\u2014periodic changes in the shape of the planet\u2019s orbit, the tilt of its axis, or the direction of its axis rotation that influence how much sunlight 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 swaths of the planet covered by continental ice sheets and alpine glaciers.<\/p>\n That doesn\u2019t mean the climate remains invariable, however. Over the course of an ice age, shorter periods of warmer temperatures and glacial retreat (called interglacial periods) alternate with periods of colder temperatures and glacial advance (called glacial periods). Right now, we\u2019re in an interglacial period during the Late Cenozoic Ice Age (yes, we\u2019re technically living in an ice age), and the last glacial period ended about 11,700 years ago.<\/p>\n During the peak of that last glacial period, the Laurentide Ice Sheet covered much of Canada and the northern United States. A sole remnant of that ice sheet still survives today as the Barnes Ice Cap, which stretches across an area roughly the size of Delaware on Baffin Island in Nunavut, Canada.<\/p>\n Today, the Barnes Ice Cap is thinning and retreating<\/a>, but Young and his colleagues found that wasn\u2019t the case not too long ago. The glacier\u2019s shrinking allowed them to analyze newly exposed vegetation that was long preserved beneath the ice cap. By dating those plants, they constructed a timeline of its climatic history, finding that it reached its minimum extent about 3,000 years ago and then started expanding again 2,000 years ago.<\/p>\n The researchers argue that this growth marked the onset of Earth\u2019s next glacial period, but it derailed before it could really get underway. That change came in the 1960s, when global temperatures had warmed so much that the Barnes Ice Cap began losing more ice in the summer than it added in winter. That trend has continued ever since, and thus the next glaciation remains on hold.<\/p>\n An uncertain future <\/p>\n Climate models suggest<\/a> the Barnes Ice Cap is likely to disappear completely within the next 300 years. For eons, the Laurentide ice sheet has anchored the growth of continent-spanning ice. If this last remnant vanishes, it could change the dynamics of future glaciations and ice ages.<\/p>\n As humanity continues to heat the planet, experts can\u2019t be sure when the next glaciation might occur. In a perspective piece accompanying the new study, glaciologist and University of Wisconsin-Madison professor Shaun Marcott states<\/a> that future greenhouse gas emissions could forestall the next ice age for hundreds of thousands of years.<\/p>\n