Climate sceptics have latched onto a scientific study in an attempt to undermine human contributions to global warming.
They claim the study, which looks at the acceleration of established warming trends since 2000, proves temperatures are increasing due to natural variability, not greenhouse gas emissions.
But the study doesn’t make that claim, and its authors told AAP FactCheck the assertion is “totally misleading”.
The claim is made in a Facebook post (archived here) titled, “New Study: Greenhouse Gases May Not Be Causing 21st-Century Warming”.
The post links to this page, which reproduces a December 16, 2022 article published by Net Zero Watch, the campaign arm of the Global Warming Policy Foundation, a UK-based think tank.
That article features the heading: “21st century warming may not be due to greenhouse gasses, leading climate scientists say”.
Other Facebook posts feature the same article (see here).
The focus of the article is a November 2022 study headed “Is Anthropogenic Global Warming Accelerating?”. It examines the human and natural causes of an acceleration in the trend of global warming since 2000.
The study shows one of the major contributors to the post-2000 acceleration in the warming trend is changes in the aerosol effective radiative forcing (ERF) trend, resulting from aerosol emissions reductions.
This article explains how the reduction of some aerosols that have a cooling effect can contribute to global warming.
The Net Zero Watch article claims: “When considering estimates of the amount of warming due to aerosol reduction along with natural climate variability, they (the researchers) find a solution with all the post-2000 temperature trends being due to natural variability alone.”
But that’s not what the study says.
On page 4283 of the PDF copy of the study, it says: “Figure 5 suggests around 50% of the post-2000 GMST (global mean surface temperature) acceleration is attributable to the anthropogenic ERF trend change … The remaining 50% of the GMST acceleration is attributed to unforced internal variability.”
Citing this section of the study in its final paragraph, the article concludes “it’s a credible hypothesis that global temperature trend changes since 2000 could be ‘arising largely from internal variability’.”
But in reaching this conclusion, the article overlooks much of the study’s complexity.
In reality, the study concludes: “GMST observations do appear to have accelerated since 2000, with around 80% of this change attributed to aerosol ERF trend changes if the attribution uses the bulk anthropogenic ERF shape from Fig. 2.”
It then qualifies this with: “However, this anthropogenic ERF shape may not be the best description of the energy imbalance contributing to historical GMSTs.”
In short, the study concludes that human activity and natural variables have both contributed to the trend acceleration over the past two decades.
Study co-author Stuart Jenkins said the post and article had misrepresented their work by stating it was looking at the warming trend rather than the increase in the trend.
“The study we completed looked at the trend in global surface temperatures in the last two decades, and asked what was the likely cause of the increasing trend in global surface temperature since 2000,” he told AAP FactCheck in an email.
“We determined that the acceleration in global surface temperature … is probably caused by a mixture of declining aerosol emissions and natural climate variability.
“This is different to what is suggested in the post … which claims that the overall trend in global surface temperatures could be considered internal variability of the climate system – this is incorrect, and in fact we specifically rule out this possibility in the study.”
Mr Jenkins said the study found the trend in global surface temperatures (+0.2C/decade) was very likely caused by human emissions of carbon dioxide (CO2) and other greenhouse gases.
However, he said the increase in the global surface temperature trend from +0.2C/decade to +0.3C/decade between 2000 and 2020 “is potentially, in part, caused by internal variability”.
“Essentially, we couldn’t rule out the possibility of the +0.1C/decade increase in warming trend being natural variability (over a short period this is understandable), but still the best estimate is a 50-50 contribution to the increased surface temperature trends from human-caused aerosol emissions reductions and internal variability, respectively.”
He added: “The underlying long-term trend in global warming is very very likely human-caused, as we demonstrate (and explicitly state!) at several points through the study.”
Fellow co-author Myles Allen told AAP FactCheck the article had confused the rate of change with evidence for acceleration.
“It would be funny if it wasn’t so sad,” Professor Allen said via email.
Another co-author, Andrew Gettelman, said the article was “totally misleading and is climate misinformation”.
“Our study was discussing the recent past temperature record, and the statement only refers to the past 20 years. It does NOT refer to the future and this article mis-uses it,” Dr Gettelman said in an email.
The Verdict
The claim that scientists have discovered 21st century warming may not be due to greenhouse gases is false.
The study instead looked at the increase in the established warming trend since 2000. Authors of the study said their work found the trend acceleration was probably caused by a mixture of declining aerosol emissions and natural climate variability.
The authors said the post amounted to “climate misinformation”.
False – The claim is inaccurate.
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