False claim Earth is 4 degrees cooler than 'medieval times' | Fact check

The claim: Earth has cooled 4 degrees Celsius since 'medieval times' and warmed only 0.7 degrees Celsius since 1850

A June 22 Facebook post (direct link, archive link) shows an edited excerpt from an interview with Ian Plimer, a prominent skeptic of human-driven climate change.

"If you started measuring in the 1850s, there's been a general warming of about 0.7 degrees Celsius," reads part of the post. "If you started to measure from the medieval times, we've had a cooling of about four degrees Celsius. So if you're telling me that the planet's warming, my question is, since when?"

The post was shared more than 2,000 times in five weeks.

More from the Fact-Check Team: How we pick and research claims | Email newsletter | Facebook page

Our rating: False

Earth has warmed, not cooled, since "medieval times." The planet has warmed around 1.36 degrees Celsius (2.45 degrees Fahrenheit) compared to the average temperatures between 1850 and 1900, according to NASA.

Earth warmer now than in 'medieval times'

Earth has not cooled 4 degrees Celsius since "medieval times," Michael Mann, a climate scientist at the University of Pennsylvania, told USA TODAY.

"It’s an absurd statement that stands in contradiction to every peer-reviewed study on this topic of the past two decades," he said in an email, noting that the most comprehensive studies show recent warming is unprecedented in tens of thousands of years.

For instance, researchers at the University of Arizona reported that modern average temperatures are higher than they have been in at least 24,000 years (the timeframe of the study).

Their work "also suggests that the speed of human-caused global warming is faster than anything we've seen in that same time," Jessica Tierney, a co-author of the study, said in a press release.

More: Can we count on renewable energy? Four ways wind, solar and water can power the US

The Intergovernmental Panel on Climate Change reports the average temperature of the Earth's surface between 2011 and 2020 was higher than at any other time in the last 100,000 years.

The Facebook post may have been referencing the "Medieval warming period" ? a time during the Middle Ages when certain regions of Earth warmed. However, on a global level, it is currently warmer than it was during this timeframe, Mann said.

The Facebook post also understates how much warming has occurred since 1850. While the post claims global temperatures have risen 0.7 degrees Celsius since 1850, the National Oceanic and Atmospheric Administration reports warming of slightly more than 1 degree Celsius (nearly 2 degrees Fahrenheit) since 1850.

NASA reports warming of 1.36 degrees Celsius (2.45 degrees Fahrenheit) compared to average temperatures between 1850 and 1900. Other climate agencies report similar amounts of warming.

Modern climate change is caused by increased levels of greenhouse gases in the atmosphere due to human activity.

USA TODAY reached out to Plimer and the Facebook user who shared the post for comment but did not immediately receive a response.

Reuters and AAP also debunked the claim.

Our fact-check sources:

• Michael Mann, July 25, Email exchange with USA TODAY

• NASA Vital Signs of the Planet, accessed July 25, Causes

• NASA Vital Signs of the Planet, accessed July 25, Methane

• NASA Vital Signs of the Planet, accessed July 25, Carbon dioxide

• NASA Vital Signs of the Planet, accessed July 25, Global temperature

• NOAA, Jan. 18, Climate Change: Global Temperature

• Intergovernmental Panel on Climate Change, Dec. 13, 2022, Climate Change 2021: Summary for All

• The Conversation, July 21, 2023, Is it really hotter now than any time in 100,000 years?

• The Conversation, April 20, 2021, Climate explained: what was the Medieval warm period?

• Copernicus, accessed July 25, Climate indicators: Temperature

• Science, Nov. 27, 2009, Global Signatures and Dynamical Origins of the Little Ice Age and Medieval Climate Anomaly

Thank you for supporting our journalism. You can subscribe to our print edition, ad-free app or e-newspaper here.

USA TODAY is a verified signatory of the International Fact-Checking Network, which requires a demonstrated commitment to nonpartisanship, fairness and transparency. Our fact-check work is supported in part by a grant from Meta.

This article originally appeared on USA TODAY: Earth warmer than during 'medieval times' | Fact check