The resurfaced December 2025 clip features former University of Ottawa professor Denis Rancourt claiming that doubling atmospheric CO2 would not noticeably change Earth’s mean surface temperature — a contention climate scientists reject. Experts point to more than a century of physics, starting with Svante Arrhenius, and modern observations showing CO2’s warming effect. Atmospheric CO2 has risen from ~278 ppm pre-industrially to about ~430 ppm in 2025, and a CO2 doubling is expected to raise global temperatures by roughly 1.5°C–4.5°C, with significant ecological consequences.
Resurfaced Video: Ex-Professor Claims Doubling CO2 Won't Warm Earth — Scientists Contest Decades Of Research
Screenshot from Facebook taken December 16, 2025
A short video that reappeared online in December 2025 shows former University of Ottawa physics professor Denis Rancourt asserting that doubling atmospheric CO2 "will not have a noticeable effect on the Earth's mean surface temperature, period." Climate scientists who spoke to AFP say this claim is incorrect and contradicts more than a century of atmospheric physics and climate research.
What the Video Shows
The clip, posted to Facebook on December 6 and shared widely on X, was recorded during a talk Rancourt gave at a 2023 business event in Ottawa. In the footage he also invokes his scientific credentials while denying the existence of climate change and making other contested claims, including a false statement that "there was no pandemic."
"You can double the amount of CO2, and it will not have a noticeable effect on the Earth's mean surface temperature, period. That's a hard calculation." — Denis Rancourt (video excerpt)
Experts Say The Claim Is Wrong
Climate scientists told AFP Rancourt’s statement misrepresents well-established physics. Katharine Hayhoe pointed to calculations dating to Svante Arrhenius in the 1890s showing that increases in carbon dioxide affect global temperature. Zeke Hausfather noted that the atmospheric physics underpinning CO2’s greenhouse effect have been refined across the 20th century — including engineering-related research in the mid-1900s — and have been repeatedly confirmed by observation and experiment.
Researchers estimate that a doubling of CO2 from pre-industrial levels is likely to increase global average surface temperatures by roughly 1.5°C to 4.5°C, depending on climate sensitivity and feedbacks. These figures are broadly consistent with IPCC assessments and decades of peer-reviewed literature.
Charts showing the build-up of atmospheric carbon dioxideSTAFFAFP(STAFF / AFP)
Context And Consequences
Burning fossil fuels has raised atmospheric CO2 from about 278 parts per million (ppm) in pre-industrial times to roughly 430 ppm in 2025, according to NASA estimates. Rising CO2 and associated warming are already linked to more frequent heat extremes, increased wildfire risk, biodiversity loss, coral decline, ocean warming and acidification, and sea level rise.
Oceans have absorbed most excess heat to date, but studies warn there is no guarantee they will continue to do so at the same rate as marine ecosystems become further stressed, with cascading ecological and societal impacts.
Credibility And Response
Rancourt was dismissed from his tenured position at the University of Ottawa in 2009. AFP has previously investigated and debunked other false or misleading claims attributed to him. AFP contacted the University of Ottawa for comment on Rancourt’s scientific credentials but received no response.
Bottom line: The statement that doubling CO2 would not noticeably change Earth's mean surface temperature contradicts established climate science. Cutting CO2 emissions remains essential to limit further warming and its wide-ranging impacts.
