Humans accidentally canceled the next ice age — but it’s no reason to celebrate, scientists say

NASA Earth Observatory/Lauren Dauphin

A shrinking ice cap in Arctic Canada tells a striking story of how humans may have halted the planet’s next ice age by burning fossil fuels , according to new research.

Around 20,000 years ago, the once vast Laurentide Ice Sheet sprawled from the Arctic Circle as far south as Illinois and was up to two miles thick in parts. All that remains of it now is the Delaware-sized Barnes Ice Cap on Baffin Island in Canada’s Nunavut territory.

Roughly 2,000 years ago, this ice cap started to slowly and steadily re-expand. Its growth marked the start of a new period of glaciation, commonly called an ice age, according to the new study published in the journal Science.

But around 1960, as human-driven climate change pushed up summer temperatures, the ice cap abruptly began to shrink.

“It was just getting started, everything was normal, the next glaciation was about to happen — and yet it stopped,” said Nicolás Young, a study author and a geochemist at the Lamont-Doherty Earth Observatory, part of the Columbia Climate School. “It’s a very powerful example of how we have jolted the system.”

For millions of years the planet has moved in and out of ice ages, driven by shifts in Earth’s orbit, and the Laurentide Ice Sheet often serves as a starting point.

Young and a group of scientists wanted to understand whether the ice sheet had ever been smaller than it currently is and then regrown. It’s a tough task as geological clues are often erased as ice regrows.

The Barnes Ice Cap, however, is frozen to its bedrock and doesn’t slide, meaning it has preserved the plants it entombed as it grew. The scientists gathered long-dead mosses near the ice cap’s edge and analyzed them to figure out when they were last alive. “We can actually track the expansion of the ice margin as it’s progressively covering mosses and killing (them),” Young said.

They also examined nearby rocks to find out how long they had been ice-free by testing for a rare, radioactive isotope that accumulates once a rock is exposed to air.

The techniques helped them reconstruct a picture of the ice’s past. They found the ice sheet reached its minimum size about 3,000 years ago, then started to regrow roughly 2,000 years ago in response to cooler temperatures.

This regrowth should be considered the start of a new ice age, but it never got past its infancy before it reversed course in the 1960s, the study authors argue. “We think that it’s no coincidence that that’s at the same time as we’ve pumped an extraordinary amount of greenhouse gases into the atmosphere,” Young said. Climate pollution has “essentially overpowered” the other mechanisms that “want to make that region cool,” he added.

Meredith Kelly, an earth sciences professor at Dartmouth College, who was not involved with the study, said the research applies techniques including “novel radiocarbon dating of plants” to set out a good picture of the ice cap’s advance and retreat.

The conclusions that the advance of the Barnes Ice Cap suggests the start of an ice age could be right, she added, but a comparison with glaciers in other regions that were also advancing at a similar time would help to understand more about the mechanisms at play.

Scientists knew the Doomsday Glacier was melting. They didn’t realize how ‘aggressively’

Andrey Ganopolski, a physicist at the Potsdam Institute for Climate Impact Research, said there has been plenty of research into whether carbon pollution may significantly delay the onset of the next ice age.

But, he added, other modeling studies, including his own, conclude the next ice age wouldn’t be expected for another 50,000 years or so — even without global warming. The growth of the Barnes Ice Cap “is insufficient to support the conclusion that a new glacial cycle had already begun and was subsequently aborted,” he said.

Young and co-authors, however, argue the first regrowth of the Laurentide Ice Sheet counts as the start of an ice age and we don’t need to wait for the formation of enormous ice sheets to declare one has begun.

The findings also throw new light on a controversial period in the 1970s, when some scientists suggested the world might be entering a new ice age, Young said. This idea was picked up by the media, including a now infamous article in Newsweek with the headline “The Cooling World,” which warned of a food crisis as the world chilled.

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It’s a period climate change skeptics and deniers often reference to discredit scientists as alarmist and incorrect. But Young believes the new research suggests they were right, that “a new glaciation actually had started; it was just getting going, and now it’s stopped.”

Young cautioned that the study should not be interpreted as suggesting global warming has “saved humanity from an ice age.”

Periods of glaciation happen over very long time frames. It would have taken thousands of years before the ice cap would have approached a size where it would have affected people’s everyday lives, he said.

Instead, the findings reveal just how profoundly humans have pushed Earth’s systems out of whack. Not only have we avoided this ice age, Young said, but “we’ve probably put off any chance of any other future glaciation for a very, very long time.”

美国宇航局地球观测站/劳伦·多芬

最新研究表明，加拿大北极地区冰盖的缩小讲述了一个令人震惊的故事，说明人类燃烧化石燃料可能已经阻止了地球的下一个冰河时代。

大约两万年前，曾经广袤无垠的劳伦泰德冰盖从北极圈向南延伸至伊利诺伊州，部分区域厚度达两英里。如今，它只剩下位于加拿大努纳武特地区巴芬岛上、面积与特拉华州相当的巴恩斯冰盖。

大约2000年前，这片冰盖开始缓慢而稳定地重新扩张。根据发表在《科学》杂志上的这项新研究，它的增长标志着新一轮冰川作用的开始，通常被称为冰河时代。

但大约在 1960 年，随着人类活动导致的气候变化推高了夏季气温，冰盖突然开始缩小。

“一切才刚刚开始，一切都很正常，下一次冰期即将到来——然而它却停止了，”该研究的作者之一、哥伦比亚气候学院拉蒙特-多尔蒂地球观测站的地球化学家尼古拉斯·杨说道。“这有力地证明了我们是如何扰乱了整个系统的。”

数百万年来，地球在地球轨道变化的影响下，经历了冰河时期和冰河时期的交替，而劳伦泰德冰盖通常被视为冰河时期的起点。

杨格和一群科学家想要了解冰盖是否曾经比现在小，然后又重新增长。这是一项艰巨的任务，因为随着冰盖的重新增长，地质线索往往会被抹去。

然而，巴恩斯冰盖冻结在基岩上，不会滑动，这意味着它保存了生长过程中被其掩埋的植物。科学家们在冰盖边缘附近采集了早已死亡的苔藓，并对其进行分析，以确定它们最后一次存活的时间。“我们实际上可以追踪冰缘的扩张过程，它正逐步覆盖苔藓并导致它们死亡，”杨说道。

他们还检查了附近的岩石，通过检测一种稀有的放射性同位素来判断这些岩石已经无冰状态了多久。这种同位素在岩石暴露于空气中时就会积累。

这些技术帮助他们重建了冰川的过去图景。他们发现，冰盖在大约3000年前达到最小规模，然后大约在2000年前由于气温下降而开始重新增长。

研究作者认为，这种植被再生应该被视为新冰河时代的开始，但它在20世纪60年代就逆转了，此前它还只是处于萌芽阶段。“我们认为，这绝非巧合，因为我们当时向大气中排放了数量惊人的温室气体，”杨格说道。他还补充说，气候污染“基本上压制了”其他“旨在使该地区降温”的机制。

达特茅斯学院地球科学教授梅雷迪思·凯利（Meredith Kelly）并未参与这项研究，她说，这项研究应用了包括“植物新型放射性碳测年法”在内的技术，以清晰地描绘冰盖的进退情况。

她补充说，巴恩斯冰盖的推进表明冰河时代开始的结论可能是正确的，但将其与同时期其他地区推进的冰川进行比较，将有助于更好地了解其中的机制。

科学家们知道末日冰川正在融化，但他们没有意识到融化的速度有多么“迅猛”。

波茨坦气候影响研究所的物理学家安德烈·加诺波尔斯基表示，已经有很多研究探讨碳污染是否会显著推迟下一个冰河时代的到来。

但他补充说，其他模型研究，包括他自己的研究，都得出结论：即使没有全球变暖，下一个冰河时代预计也要再过5万年左右才会到来。他表示，巴恩斯冰盖的增长“不足以支持新的冰期循环已经开始但随后中止的结论”。

然而，杨格及其合作者认为，劳伦泰德冰盖的第一次再生就标志着冰河时代的开始，我们不需要等到巨大的冰盖形成才能宣布冰河时代已经开始。

杨格表示，这些发现也为上世纪70年代一段备受争议的时期提供了新的视角。当时一些科学家提出，世界可能正在进入一个新的冰河时代。媒体纷纷报道了这一观点，其中包括《新闻周刊》一篇如今臭名昭著的文章，标题为《正在变冷的世界》，该文章警告说，随着全球气温下降，粮食危机将会出现。

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这是气候变化怀疑论者和否认者经常用来诋毁科学家、指责他们危言耸听、错误的时期。但杨格认为，新的研究表明他们的观点是正确的，“新的冰期实际上已经开始；它刚刚起步，现在却停止了。”

杨格警告说，这项研究不应被解读为全球变暖“使人类免于冰河时代”。

他说，冰期持续时间非常长。冰盖需要数千年时间才能达到足以影响人们日常生活的规模。

相反，研究结果揭示了人类活动对地球系统造成的破坏程度之深。杨格表示，我们不仅避免了这次冰河时代，而且“我们可能已经将未来再次发生冰川作用的可能性推迟了很长时间”。