Humans have already caused about 1.5°C of warming since the very beginning of the industrial revolution, according to new estimates based on temperature data collected from air bubbles trapped in ice.

Measurements of human-caused global warming typically use the period from 1850 to 1900 as a pre-industrial benchmark, since that is when temperature records began. 2024 will almost certainly be the first year that average temperatures rise more than 1.5°C above this benchmark. These single-year data are influenced by natural factors such as a strong El Niño, which caused global temperatures to rise.

Once this natural variability is removed, scientists believe that humanity alone has caused 1.31°C of warming since the Industrial Revolution. But by 1850, the industrial revolution was well underway, with fossil fuel-powered engines in use all over the world.

Andrew Jarvis at Lancaster University and Pierre Forster at the University of Leeds, UK, set out to establish a new pre-industrial benchmark using data from Antarctic ice core samples. The duo analyzed the composition of air bubbles trapped in ice cores to establish the concentration of CO2 in the atmosphere during the 1700s, before humans had a significant impact on atmospheric temperatures. They then used this CO2 data to establish global average temperatures over the same period, assuming a linear relationship between CO2 and temperature increase.

Using this new pre-1700 baseline, humanity had caused warming of 1.49°C by 2023, meaning the 1.5°C level “has now effectively been reached,” writes the team in an article reporting the results. “We have provided a new, scientifically defensible way of establishing a pre-industrial baseline against which we measure warming,” Jarvis told reporters at a press briefing.

Jarvis says the new method can also be used to reduce uncertainty around temperature estimates based on the current 1850 to 1900 baseline, used by the Intergovernmental Panel on Climate Change. Using ice core data to establish the baseline between 1850 and 1900, the team says humans caused 1.31°C of warming. This is consistent with existing central estimates, but with a significantly reduced range of uncertainty, the team points out.

“The problem with just looking at surface temperature observations is that the further back in time you go, the more uncertain they become,” says Forster. “We can be much more confident than before that we are currently at around 1.3°C.”

Jarvis and Forster hope their new method will be adopted by scientists and policymakers as the primary way to judge humanity’s progress against global climate goals. “I think it’s still possible for the policy community and the scientific community to rethink pre-industrial bases,” Jarvis said. “We know there is warming built into the estimate for 1850-1900, simply because that is not the start of the industrial revolution. We offer a path to a much more scientifically secure baseline from which to operate.

However, the new method may not be future-proof. The linear relationship between CO2 concentrations and global temperatures could weaken as climate change progresses, for example if we trigger tipping points in Earth’s systems that cause a cascade of warming events.

The new method also does not change the impacts of climate change felt on the ground, Forster emphasizes. “The impacts that we’re experiencing today, people being killed in Spain and by these hurricanes, the impacts are exactly the same whether you call it 1.3°C above pre-industrial levels, or whether you call it 1.5 °C above pre-industrial levels. levels. The impacts are the impacts.

Richard Betts of the UK Met Office, says the new method “provides a clear and simple way of giving up-to-date estimates of the current level of human-induced global warming”. This is partly because it is able to produce a “real-time” estimate of human-induced warming rather than relying on a 10-year rolling average like current estimates.

He says the method will be useful in providing policymakers with a more up-to-date picture of the current level of warming, but he warns that changing the baseline used in the assessments could be seen as “shifting the goalposts” of climate action. “Even without changing the reference, it is clear that current warming is much closer to 1.5°C than one would expect from an obsolete ten-year average,” he emphasizes.