Fact check: Contemporary, human-driven warming has different ramifications than past warming

·6 min read

The claim: The Arctic was warmer 6,000 years ago and 90% of glaciers were smaller or absent

A recent academic paper reported evidence that summer temperatures in the Arctic were warmer 8,000 to 10,000 years ago than they are today. It said this spurred glacial melting that peaked about 6,000 years ago.

One blog post called the existence of such past warm periods an "inconvenient fact" for "climate alarmists."

"New Study: Arctic Was Much Warmer 6000 Years Ago… 90% Of Glaciers, Ice Caps Smaller Than Present Or Absent," reads the blog link in an April 11 Facebook post.

The Facebook post received more than 200 interactions. Versions of the blog were also shared on Reddit and Twitter, though the Twitter account was later suspended.

But, the post is misleading. Paleoclimatologists, who study the earth's climate history, have documented periods of warming and cooling. Warm summer temperatures in one of the periods referenced in the paper – the early Holocene period – were caused by normal variation in the Earth's orbit around the sun, whereas today's warming trend is driven by human behavior.

In other words, there are different causes and different long-term ramifications for the two different periods of warming, according to researchers.

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USA TODAY reached out to social media users who shared the claim for comment. The blog author, who also posted the link on Twitter, could not be reached.

Past periods of warming useful for understanding modern and future climate patterns

Despite what the blog post implies, the existence of past warm periods does not contradict modern climate science, Laura Larocca, the study's lead author and postdoctoral fellow at Northern Arizona University, told USA TODAY.

"Evidence of past periods of warmth from paleoclimate archives is not an 'inconvenient fact,'" Larocca said in an email. "We are learning quite a lot from past warm periods about our rapidly warming world and about what we can expect in the future."

In fact, one of the main goals of Larocca's study was to place contemporary human-driven Arctic glacier retreat into a long-term context, she said.

Her paper provides evidence that Arctic summer temperatures roughly 2° Celsius higher than present-day ultimately caused the loss of a significant number of land terminating mountain glaciers and ice caps. Losses peaked about 6,000 - 7,000 years ago when more than 90% of the glaciers disappeared or were reduced in size.

Other Arctic ice masses such as the Greenland ice sheet and sea ice were not evaluated in the study.

Fact check: Warming varies across oceans and atmosphere, doesn't contradict climate change

Larocca emphasized that Arctic summer temperatures due to human-driven global warming are projected to be even warmer by the end of this century than they were thousands of years ago.

"Summer temperatures only moderately warmer ... than today drove major environmental change across the Arctic including the widespread decline and loss of small mountain glaciers," she said. "This is an important point because future, end-of-century temperatures are expected to exceed early Holocene values in most locations, portending the eventual loss of most of the Arctic's small glaciers."

Current warming has different causes and ramifications than past warming

Michelle Stirling, the communications manager for Friends of Science, who posted the claim on Facebook, told USA TODAY that contemporary warming is not "extreme or unusual" when examined in the context of Earth's geological record.

However, while the study affirms that Arctic summers were warmer in the the period referenced in the paper, that warming was caused by predictable changes in Earth's orbit which exposed the Northern Hemisphere to more solar radiation in the summer, Larocca said.

As Earth's orbit continued to change, Arctic summers cooled and glaciers began to advance. Currently, Earth's orientation to the sun in its orbit would support the expansion of glaciers, she said. However, warming driven by human behavior is causing a retreat.

Twila Moon, a research scientist at the National Snow and Ice Data Center, told USA TODAY that these disparate causes mean that ancient and contemporary periods of warming have different ramifications.

"Today’s loss of glacier ice is occurring for different reasons than the loss that occurred 6,000 years ago," Moon said in an email. "Unfortunately, that means that we cannot expect natural changes to shift us towards glacier stability or growth over coming centuries or millennia, not to mention the next decades."

Another difference is that contemporary human-driven warming is progressing at a much faster rate than in the past, Samantha Bova, a paleoclimatologist and assistant professor at San Diego State University, told USA TODAY in an email.

"Rapid rates of change mean, for example, that the biosphere (global ecosystem) doesn’t have very much time to adapt to new conditions," she said.

Additionally, modern glacier melt, which partially drives sea level rise, poses unique threats to modern human populations, said Larocca.

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Unlike thousands of years ago, millions of people live in coastal cities threatened by rising sea levels. Further, she said, millions of people rely on mountain glaciers for water.

However, Moon emphasized that human behavior can still influence future outcomes for Arctic glaciers.

"In the same way that human activities are causing today’s rapid ice loss and temperature rise, human activity can also shift to reduce the current causes of warming and limit ice loss into the future," she said.

Our rating: Missing context

Based on our research, we rate MISSING CONTEXT the claim that the Arctic was warmer 6,000 years ago and 90% of glaciers were smaller or absent. According to researchers, warm temperatures in the early Holocene period were caused by variation in Earth's orbit around the sun, whereas today's warming trend is driven by human behavior. Thus, the ramifications of the two periods of warming are different.

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This article originally appeared on USA TODAY: Fact check: Past climate records do not contradict climate science