Humans may have already caused 1.5°C of global warming if measured from a time actually before the industrial revolution and the beginning of the carbon emissions on a large scale.
This is the conclusion of a new study published in Nature Geoscience by Dr Andrew Jarvis of Lancaster University and Professor Piers Forster of the University of Leeds.
The 2016 Paris Climate Agreement set a long-term temperature goal of “limiting the rise in global temperature to well below 2°C, while pursuing efforts to limit the increase to 1.5°C.” . The figure of 1.5°C of warming has since become the yardstick for judging progress, or lack thereof, on climate change.
The human-induced contribution to global warming is currently estimated at 1.31°C, but with an uncertainty range of 1.10 to 1.60°C, according to the preferred methods of the Intergovernmental Panel on Climate Change. (IPCC). This means that it is not clear from the estimates adopted by the IPCC whether the 1.5°C limit has been crossed or not.
Crucially, the IPCC’s preferred methods use temperature records from 1850 to 1900 as their “pre-industrial” baseline for their calculations. They do this because this is when the first temperature records were taken, although the exact way to measure global temperature increases has never been defined within the climate negotiations.
Using this same 1850-1900 baseline, Dr. Jarvis and Professor Forster’s method more than halves the uncertainty in the current estimate of human-caused warming, thus showing that human-caused global warming it currently remains below 1.5°C if measured this way. By this measure, crossing Paris’ 1.5°C barrier is less than 10 years away at current warming rates.
Estimate until before 1700
However, Dr Jarvis and Professor Forster go further. Their method allows for a more precise estimate of the true human contribution to long-term global warming, as it pushes back the reference period from which global temperature change is measured to before 1700.
The authors conclude that, when measured by this earlier, more precise definition of the pre-industrial period, the human contribution to long-term warming was 1.49°C * 0.11°C in 2023 and is now greater than 1 .5°C. This reveals that there is almost 0.2°C of warming within the 1850-1900 reference period currently used to define warming.
Dr. Jarvis, lead author of the study, said in a statement: “Measuring human-caused global warming is a difficult task because it requires us to compare the current temperature with what it was in pre-industrial times, which we call the preindustrial baseline. The closest we get to pre-industrial global temperature measurements are from the mid-19th century, and as you might expect, this data is somewhat spotty and the Industrial Revolution was already underway at that time.
“Therefore, using these early temperature data as a baseline, like previous methods, not only ignores warming that was already underway, but also introduces significant uncertainty into warming estimates.”
This new study, instead, uses CO2 records from air bubbles trapped in ice cores to establish a temperature baseline before 1700. These records go back thousands of years, long before the Industrial Revolution and the effects of carbon emissions of human origin. Scientists can use the CO2 record to inform estimates of global warming because of what they say is an overlooked relationship between the two.
“If you plot global temperatures against the concentration of CO2 in the atmosphere, they both fall on a remarkably straight line, much straighter than current theory would predict,” Dr. Jarvis said. “That line not only tells us how much the Earth has warmed since pre-industrial times, but also how much of that warming can be attributed to human activity.
The scientists believe their new method is a strong candidate for measuring progress against the Paris criteria of 1.5° and 2.0°.
Dr Jarvis said: “Our method has a number of strengths. First, it directly addresses the problem of how to establish a solid pre-industrial baseline, although it works just as well with the 1850-1900 baseline. Second, it produces estimates of human-caused warming that are at least 30% safer than current methods.
“Finally, it is easy and quick to apply, meaning we can produce warming estimates as soon as CO2 and temperature data are available without having to rerun complex climate models. This also means that the results are transparent, making it much easier to communicate them to non-specialists.”