An analysis of North America's largest ice sheet suggests that human-caused climate change is now overriding the natural dynamics of Earth's climate. At the height of the last ice age about 20,000 years ago, the Laurentide ice sheet covered almost a third of North America - including most of what is now Canada.
Afterwards, the once kilometer-thick ice sheet shrank and reached its minimum around 3,600 years ago, as a research team led by Nicolás Young from Columbia University in Palisades, USA, writes in the specialist magazine “Science”. Accordingly, the ice sheet began to grow again around 2000 years ago. Young and his team see this as the gradual beginning of a new cold phase.
But from around 1960 the ice edge has retreated again by around 200 to 800 meters by 2022. Young attributes this primarily to significantly higher summer temperatures. This illustrates the consequences of global warming caused by the greenhouse gases emitted by humans. “The next ice age was imminent – and yet it never happened,” geochemist Young is quoted as saying in a statement from his university.
In fact, the question was still being discussed at the end of the 1970s as to whether the Earth was heading towards a new ice age or whether the increasing concentration of carbon dioxide in the atmosphere would actually trigger significant warming. The reason for the cooling thesis was, among other things, that global temperature series have shown cooling since the 1940s.
A possible cause for this trend was aerosols, which reflect sunlight and can therefore have a cooling effect. At the same time, however, measurements have already shown an increase in carbon dioxide in the atmosphere, and climate models have indicated warming due to the greenhouse effect. In 1958, US researchers began continuously measuring CO2 in the air at the Mauna Loa weather station in Hawaii.
Looking back, the controversy at the time is sometimes shortened and presented as a warning from science about an imminent new ice age. However, there was no scientific consensus on this.
However, an analysis of the literature published between 1965 and 1979 found that papers predicting warming significantly outnumbered those predicting cooling. However, some popular publications portrayed the threat of an approaching ice age very dramatically, helping to cement this idea in the public's mind.
Against this historical background, the new study has a special punchline: It shows that the idea of an impending long-term cooling due to natural climate cycles was by no means fundamentally wrong. According to Young and his colleagues' data, the Laurentide ice sheet had actually begun to grow again around 2,000 years ago.
“We shook the system”
However, the natural evolution towards colder conditions has been overshadowed by human-caused warming. “It’s a very powerful example of how we shook up the system,” Young says.
The research group reconstructed the temporal development of the ice sheet through analyzes in the area of the Barnes Ice Cap on Baffin Island in northeastern Canada. This approximately 6,000 square kilometer ice cap forms the remainder of the once mighty ice sheet - including layers of different ages - that has survived to this day.
The team reconstructed how the ice sheet grew and shrank on the rocks, primarily using the rare beryllium isotope 10. This isotope is created when the rock is exposed to cosmic radiation - and thus provides information about when the ice sheet retreated. In addition, the carbon isotope 14 (C-14) reveals when vegetation was covered by ice again.
With the analysis, the study provides an important benchmark for how ice sheets react to climate changes, writes geoscientist Shaun Marcot from the University of Wisconsin in Madison, who was not involved in the study, in a “Science” commentary. The study shows “how quickly the glacier reacts to changes in summer temperature,” emphasizes the expert.
“Humans have now become a geological agent capable of thwarting natural glacial cycles that have occurred over millions of years and unbalancing the Earth's climate for hundreds of thousands of years to come,” writes Marcot.
The lack of future glacial cycles will have implications for other biological and physical Earth systems that evolved along with the ice ages. The study offers an opportunity to “think about what an ice-free world could mean.”
The researchers themselves also assume this: “As atmospheric CO2 concentrations continue to rise, a return to glacial conditions is extremely unlikely,” they emphasize. According to simulations by the researchers, the remnants of the Laurentide ice sheet on Baffin Island are likely to disappear completely in the coming centuries.
