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Drought in Brazil (picture from October): boats running aground


Bruno Kelly / REUTERS

Extreme rainfall and droughts, enormous heat: time and again, scientists report new records in this area, and again and again they are in the context of the Paris climate target. Accordingly, global warming is to be limited to below 1.5 degrees Celsius in the long term compared to pre-industrial times. However, according to the currently applicable criteria, experts could only attest to the fact that this limit has been exceeded many years later. British researchers warn against this in a commentary in the journal Nature.

The group led by Richard Betts and his colleagues from the British weather service Met Office and the University of Exeter makes a new proposal: to calculate the state of global warming from observational data from the past ten years as well as model projections for the next ten years. This further ensures that the average value of a period of 20 years is looked at. However, if the 1.5-degree threshold is exceeded, it could be detected in time and a tightening of measures could be initiated, according to the researchers.

Using the proposed method, the scientists calculated that global warming at the end of 2022 was around 1.26 degrees Celsius compared to pre-industrial levels.

Compliance with the 1.5-degree mark is also on the agenda at the United Nations Climate Change Conference currently underway in Dubai. The 1.5°C target refers to longer-term values and not to individual days, months or years. But there are different calculation approaches to the question of when exactly it was missed. "It may come as a surprise that the Paris Declaration does not contain a formally agreed definition of the current state of global warming," Bett and his team explain.

"Without an agreement on what counts as exceeding 1.5 degrees Celsius, we risk distraction and confusion at the very moment when action to avert the worst effects of climate change becomes even more urgent," Betts said. Many experts now assume that the 1.5-degree target can no longer be achieved.

The United Nations Intergovernmental Panel on Climate Change (IPCC) has defined when a certain temperature mark is considered to have been exceeded. To do this, the experts look at the global average temperature averaged over a period of 20 years. If, for example, the average value is 1.5 degrees above the pre-industrial average, the experts determine the middle of the 20-year period as the moment at which this threshold was first crossed. "This means that we can't detect a threshold being exceeded until 10 years after that date," Chris Hewitt, director of climate services at the World Meteorological Organization (WMO), said when asked.

The problem is illustrated by the example of the 1-degree threshold: the period from 2002 to 2021 was the first in which the global average temperature was on average one degree above pre-industrial times. It was determined – and only at the end of this long period – that the 1-degree threshold was exceeded around the year 2012.

The WMO is considering hiring an international team of experts to examine alternative methods to allow for a more timely assessment, Hewitt said.