How much cooler was the pre-industrial world? New study sets quantitative benchmark

In a groundbreaking study published in Nature, researchers established a new pre-industrial temperature baseline, revealing that human-driven global warming has reached 1.49°C above pre-industrial levels in 2023. The study argues that by re-evaluating the baseline to pre-1700 levels, the planet has effectively reached the critical 1.5°C warming threshold set by the Paris Agreement.

The research was conducted by Andrew Jarvis from Lancaster University and Piers Forster from the University of Leeds. They re-evaluated the relationship between global surface temperatures and atmospheric CO₂ trends by analyzing air bubbles trapped in Antarctic ice cores to establish CO₂ concentrations during the 1700s. By extending their analysis back 2,000 years, they reconstructed the average global temperature before 1700, establishing a pre-industrial baseline from AD 13 to 1700 when CO₂ levels were around 280 parts per million.

Using this new baseline, they found that atmospheric CO₂ had risen by 142 ± 7 ppm above its pre-1700 levels by 2023, leading to an increase of 1.49°C in global temperatures. This figure is higher than the globally accepted estimate of about 1.3°C of warming, which is based on the commonly used 1850-1900 time period as the pre-industrial baseline. The researchers argue that the standard 1850-1900 baseline doesn't fully capture pre-industrial warming that was already occurring before 1850, as carbon dioxide levels and temperatures were increasing long before this period.

"By 1850, the Industrial Revolution was already well underway, with fossil fuel-powered engines in use around the world," said Jarvis. "We know that there is warming baked into the 1850-1900 estimate, simply because that is not the beginning of the Industrial Revolution. We are offering a way out there, to a much more scientifically secure baseline to operate from," he added, according to New Scientist.

To address the underestimation of warming, the researchers used Antarctic ice-core data that record atmospheric CO₂ levels over the past 2,000 years, allowing them to push the base period back to before 1700. Using a linear relationship between CO₂ concentrations and global temperatures, they estimated that human-induced warming reached 1.49 degrees Celsius in 2023, meaning the 1.5 degrees Celsius warming threshold has nearly been reached. "The 1.5 °C level for human-induced warming has now effectively been reached if estimated using the CO₂-temperature linearity," noted the researchers.

This has significant implications for global climate policy, as the Paris Agreement aims to limit global warming to well below 2°C, preferably to 1.5°C, compared to pre-industrial levels. Andrew Jarvis emphasized the urgency of reassessing the pre-industrial baseline: "I do think there is still scope for the policy community and the science community to rethink the pre-industrial baseline," he said.

Piers Forster, co-author of the study and a climate physicist at the University of Leeds, added, "Policy makers set the Paris temperature goal to limit the devastating climate impacts many around the world are already experiencing. It was set to push countries to higher national ambition. It is clear we need to do more."

The study's findings suggest that current approaches for estimating global temperature changes, which rely on climate models and statistical methods, might underestimate the increase in human-driven warming. By adopting the new pre-1700 baseline, the researchers' method produces estimates of human-caused warming that are at least 30% more certain than current methods. "This line tells you not only how much the Earth has warmed since pre-industrial times, but also how much of that warming can be blamed on human activity," Jarvis said.

However, the researchers acknowledge that their approach does not directly quantify how much warming may have been influenced by factors other than atmospheric CO₂. They caution that while the observed relationship between CO₂ and temperature change appears to have been statistically linear to date, this cannot be guaranteed going forward. "Although atmospheric CO₂ is responsible for the bulk of human-induced warming so far, it is not solely responsible, and we know other factors such as methane could become increasingly important in the future, especially if we encounter climate tipping points," Jarvis noted.

Climate experts not involved in the study have commented on its significance. Richard Betts, a climate scientist at the UK Met Office, said the new method "provides a clear and simple way to give up-to-date estimates of the current level of human-induced global warming." Andrew King, a climate scientist at the University of Melbourne, said the research was interesting and useful, but that the Paris Agreement had not necessarily been breached yet. "This new study doesn't mean we have failed to keep to the Paris Agreement but rather that the baseline that has been used doesn't account for some pre-1850 warming," he stated, according to ABC News Australia.

King emphasized the need to reduce greenhouse gas emissions and decarbonize more quickly, regardless of the baseline used. "This would help us to avoid the most dangerous climate change impacts," he said. The study also highlights the need for immediate action to reduce greenhouse gas emissions and mitigate climate change.

The increase of 1.5°C in global temperature has serious consequences, including risks presented by climate tipping points and the survival of many species and ecosystems. "Urgent actions can slow warming rates and push back the time of breaching the Paris 1.5°C limit. Although breaching the limit is now inevitable, delivering action commensurate with the noble Paris goal is more important than ever," Professor Piers Forster said.

The researchers suggest that their method could provide early warning for the onset of nonlinearities, including tipping points, and could continue to supply robust global mean surface temperature change and human-induced warming estimates. "Providing the pre-industrial baselines for both CO₂ and temperature have been accurately specified, CO₂-temperature sensitivity can be estimated in near real time directly from the data," the researchers noted in their study.

Sources: Nature, Última Hora, El País Uruguay, Phys.org, New Scientist, ABC News Australia, Rosario3

This article was written in collaboration with generative AI company Alchemiq