Humanity has warmed the planet by 1.5°C since 1700

Humans have already caused approximately 1.5°C of warming since the very start of the Industrial Revolution, according to new estimates based on temperature data gleaned from bubbles of air trapped in ice.

Measurements of human-caused global warming generally use the period from 1850 to 1900 as the pre-industrial baseline, since this is when temperature records began. 2024 is almost certain to be the first year where average temperatures rose more than 1.5°C above this baseline. This data for a single year is influenced by naturally occurring factors such as a strong El Niño event, which pushed up global temperatures.

Once this natural variability is removed, scientists think humanity alone has caused 1.31°C of warming since the Industrial Revolution. But by 1850, the Industrial Revolution was already well under way, with fossil fuel-powered engines in use around the world.

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

Using this new pre-1700 baseline, humanity had caused 1.49°C of warming by 2023, meaning the 1.5°C level “has now in effect been reached”, the team write in a paper reporting the findings. “We have provided a new scientifically defensible way of coming up with a pre-industrial baseline against which we are measuring the warming,” Jarvis told reporters in a press briefing.

Jarvis says the new method can also be used to reduce uncertainty around temperature estimates based on the current 1850-1900 baseline, which is used by the Intergovernmental Panel on Climate Change. Using ice core data to establish the 1850-1900 baseline, the team says humans have caused 1.31°C of warming. That is in line with existing central estimates, but with a vastly reduced uncertainty range, the team point out.

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

Jarvis and Forster hope their new method will be adopted by scientists and policymakers as the main way of judging humanity’s progress against global climate goals. “I do think there is still scope for the policy community and the science community to rethink the pre-industrial baseline,” Jarvis said. “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.”

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

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

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

He says the method will be useful to provide a more up-to-date picture of the current level of warming for policymakers, but warned changing the baseline used in assessments could be seen as “moving the goalposts” for climate action. “Even without changing the baseline, it’s clear that current warming is much closer to 1.5°C than expected from using an out-of-date 10-year average,” he points out.

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