A new study published today in Nature Geoscience by Dr. Andrew Jarvis at Lancaster University and Professor Piers Forster at the University of Leeds shows that humans may have already caused 1.5°C of global warming when measured from a time genuinely before the industrial revolution and the start of large-scale carbon emissions.
The human-induced contribution to global warming is currently put at 1.31°C, but with an uncertainty range of 1.10 to 1.60°C, according to the Intergovernmental Panel on Climate Change (IPCC’s) preferred methods. This means it is unclear, from the IPCC’s adopted estimates, whether the 1.5°C boundary has been breached.
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 human-caused warming estimate, thereby showing human-caused global warming currently remains below 1.5°C if measured this way. On this measure, crossing the 1.5°C Paris guardrail is under 10 years away at current warming rates.
However, Dr. Jarvis and Professor Forster go further. Their method makes a more accurate estimate of the true long-term human contribution to global warming by pushing the base period from which the global temperature change is measured back to before 1700.
The authors find that when measured from this earlier, more accurate definition of pre-industrial time, the long-term human contribution to warming was 1.49°C ± 0.11°C in 2023 and is now above 1.5°C. This reveals that there is almost 0.2°C of warming within the 1850–1900 baseline currently being used to define the warming.
This new study instead uses CO2 records from air bubbles trapped in ice-cores to establish a pre-1700 baseline for temperature. These records stretch back thousands of years, well before the Industrial Revolution and the effects of human-derived carbon emissions. The scientists are able to use the CO2 record to anchor global warming estimates because of what they say is an overlooked relationship between the two.