Earth has warmed in recent decades, but trends over past 4,000 years unclear | Fact check

Kate S. Petersen, USA TODAY
·6 min read
370

The claim: Earth has not warmed in the last 38 years, warmed by only 1.08 degrees in 150 years and has cooled for the last 4,000 years

A May 2 Facebook video (direct link, archive link) shows Ian Plimer, a skeptic of human-driven climate change, talking about purported long-term temperature trends.

"We have been cooling down for the last 4,000 years," Plimer says. "If we look at the last 38 years, there has been no change in temperature. If we look at the last 150 years, we've had three warming periods and three cooling periods with a total warmth of about 0.6 degrees Celsius (1.08 degrees Fahrenheit)."

The video also shows a chart labeled "Greenland GISP2 Ice Core - Last 10,000 Years Interglacial Temperature," which depicts a general temperature decline over 10,000 years prior to about 1855.

The video was shared nearly 3,000 times in a month.

Follow us on Facebook! Like our page to get updates throughout the day on our latest debunks

Our rating: Partly false

The Earth has warmed over the last 38 years, and it warmed more than 1.08 degrees Fahrenheit over the last 150 years, according to National Oceanic and Atmospheric Administration data. While there is some evidence to suggest a cooling trend over the 4,000 years prior to the Industrial Revolution, there is also data to suggest a warming trend over that same time period. The chart displayed in the video only shows ice surface temperatures at one location and does not reflect global temperatures.

Earth has warmed significantly due to human activity

The claim is off base about both of the more recent time periods.

Global land and ocean surface temperatures increased roughly 1.4 degrees in the 38 years between 1984 and 2022, according to NOAA data. The same data shows global warming of roughly 1.9 degrees in the 150 years between 1872 and 2022.

Multiple independent climate agencies have reported similar trends.

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

Temperature trends over last 4,000 years still under investigation

In the video, Plimer unequivocally asserts that global temperatures have cooled over the last 4,000 years. But there is not a clear scientific consensus on this point.

"Whether global average surface temperature warmed or cooled during the millennia prior to industrialization is still a matter of debate," Darrell Kaufman, a paleoclimatologist at Northern Arizona University, told USA TODAY in an email.

Modeling studies tend to show Earth was warming prior to industrialization, but analyses based on other types of physical data tend to suggest it was cooling, Kaufman said.

Ellie Broadman, a paleoclimatologist at the University of Arizona, who, with Kaufman, recently published a review of paleoclimate temperature trend research, says its most likely that a mild cooling trend driven by "slight variations in the Earth’s orbit around the sun" began about 6,500 years ago.

"It’s important to note, however, that this cooling trend sharply reversed starting during the Industrial Revolution – around 1850 – when humans dramatically increased our greenhouse gas emissions by burning fossil fuels," she told USA TODAY in an email.

Fact check: Graph shows dated data from one area in Greenland, not global temperature change

Yair Rosenthal, a Rutgers paleoclimatologist, recently co-authored a paper supporting the warming hypothesis. It argues the appearance of cooling in the climate record is related to seasonal temperature trends rather than annual trends. The paper describes a warming trend that began 12,000 years ago and was driven by receding ice sheets followed by increasing greenhouse gases, which may or may not have been related to pre-industrial human behavior.

However, even accounting for that warming, the paper reports a sharp and rapid increase in warming beginning with the Industrial Revolution.

"The (contemporary) rate of warming exceeds anything we have known from the past thousands of years," Rosenthal told USA TODAY in an email.

Chart does not show global temperature data

While the chart in the video appears to be offered as evidence supporting Plimer's claims about global temperature trends, it does not actually show global temperatures.

Instead, it shows ice surface temperature data derived from one ice core collected in Greenland in the 1990s, USA TODAY previously reported when fact-checking a different version of the graph. Further, the data derived from the core sample only goes up to the mid-1800s, prior to the warming caused by industrialization.

"To infer that this data reflects global temperature changes over the last 10,000 years is really misleading," Gary Clow, a geophysicist at the University of Colorado Boulder who published a 1997 paper that included an analysis of the core, previously told USA TODAY.

Fact check: April 1895 v. April 2023 temperature comparison does not disprove climate change

Despite these limitations, variations of the graph have been repeatedly used to minimize the significance of modern global warming.

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

The video was also debunked by Climate Feedback.

Our fact-check sources:

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

Our fact-check work is supported in part by a grant from Facebook.

This article originally appeared on USA TODAY: Video misstates warmings trends in recent decades | Fact check