Humans may have caused 1.5 deg of warming, study finds

Humans may have already caused 1.5 degrees of global warming when measured from a time before the industrial revolution, a new study has found, suggesting that the planet could be closer to the Paris Agreement's warming limits than previously thought.

Countries involved in signing the Paris Agreement are expected to pursue efforts to limit increase in global warming beyond 1.5 degrees Celsius above pre-industrial levels.

Researchers, including those from Lancaster University, UK, analysed data from ice columns, or 'ice cores', of Antarctica to analysed how human-caused climate change has impacted the planet in the last two millennia.

"Our study shows that human societies have caused more than 1.5 degrees of long-term global warming. However, this does not necessarily mean that the Paris Agreement's 1.5 temperature guardrail is breached, as we find that 0.18 degrees Celsius of warming happened before global temperature records began, and this baked-in warming would not have been factored into the Paris Agreement," said co-author Piers Forster, from the University of Leeds.

The Intergovernmental Panel on Climate Change (IPCC) uses data of global temperature anomalies during 1850-1900 as the pre-industrial era, or the 'baseline', as this is when temperature records were first taken, according to the researchers. IPCC is the United Nations body involved in providing scientific guidance for tackling climate change.

However, emissions and carbon dioxide levels in the air have been known to rise well before this period, the researchers said.

Using the data from Antarctic ice cores, the team re-evaluated the relationship between global surface temperatures and carbon dioxide trends, and extended their analysis back 2,000 years. The results are published in the journal Nature Geoscience.

The authors found that when measured from this earlier "more accurate" definition of pre-industrial time, the long-term human contribution to warming was 1.49 degrees Celsius with an error margin of 0.11 degrees Celsius in 2023 and is now above 1.5 degrees Celsius.

This suggested that there is almost 0.2 degrees Celsius of warming already present within the 1850-1900 baseline, which is currently being used to define the warming, the authors said.

Urgent actions can slow warming rates and push back the time of breaching the Paris 1.5 degree limit. Although breaching the limit is now inevitable, delivering action commensurate with the noble Paris goal is more important than ever, Forster said.