Will 2023 force the world to scrap one of the key commitments of the 2015 climate agreements? Global temperatures could exceed for the first time the famous threshold of 1.5 ° C above the temperatures of the pre-industrial era, according to the latest forecasts of the American NGO Berkeley Earth, reported by the journal Nature, Friday, September 22.

"Berkeley Earth is one of a handful of international institutes whose climate modelling is taken seriously by the scientific community," said Chris Smith, a climate scientist at the University of Leeds.

The fault in August

The organisation, which has been publishing climate forecasts since the early 2010s, concluded that there was "a 55% chance that global average temperatures in 2023 will be more than 1.5°C higher than in the pre-industrial era".

Berkeley Earth has become increasingly pessimistic as the months go by. Before the start of 2023, the experts of this NGO believed that there was only a little less than 1% risk of crossing the alarm threshold that had been set in 2015 by world leaders at COP21. In July, this overshoot was still only about 20% likely to occur.

But August was a fundamental game-changer for forecasters. "The month of July had already been very abnormal, August was even more so," confirms Robert Vautard, climatologist and research director at the Pierre-Simon-Laplace Institute. To be precise: global average temperatures have blowed the old heat record set in August 2016 by more than 0.3 ° C, which "represents a difference of a more than surprising magnitude," say Berkeley Earth experts.

The most likely cause of this sweltering end of summer can be summed up in two words: El Niño. The arrival of this climatic phenomenon linked to warm ocean currents having an impact on global temperatures did not take scientists by surprise, but "it seems to be more powerful than what had been anticipated by specialists," says Joeri Rogelj, a climatologist at Imperial College London.

" READ ALSO Climate: 2023, year of an El Niño or a Super El Niño?

There are therefore two reasons for the panic of the thermometer in 2023: "On the one hand, a fundamental trend related to global warming, and on the other exceptional phenomena such as El Niño that accentuate the rise in temperatures," says Stefan Hagemann, meteorologist at the Helmholtz-Zentrum Hereon Institute for Coastal Systems, a German multidisciplinary research center.

If the Berkeley Earth teams now think that there is more than one in two chance that 2023 will be the first year with an average temperature above 1.5 ° C, it is also because "the full development of El Niño will be felt more at the end of the year," explains Robert Vautard.

It's (still) only one year

Experts interviewed by France 24 consider Berkeley Earth's forecasts "plausible", although several point out that this institute generally makes the warmest assumptions in terms of temperature forecasting.

And even if 2023 set a new record above 1.5 ° C above the temperatures of the pre-industrial era, "this does not mean that the Paris agreement is to be thrown in the trash," reassures Chris Smith.

Indeed, it is only one year. "The experts of the IPCC [Intergovernmental Panel on Climate Change, editor's note] estimate that it takes an average global temperature above 1.5 ° C over a period of about 20 years to be able to conclude that global warming has passed this threshold set in 2015," notes Stefan Hagemann.

For now, the trajectory of global warming is still below a 1.2°C rise. The Berkeley Earth forecasts "do not yet call into question the projections of IPCC experts who estimate that the 1.5 ° C bar will be crossed between 2030 and 2052," says Robert Vautard.

"It is to be expected that once El Niño has passed [the phenomenon usually lasts at least a year, editor's note], temperatures will drop," says Joeri Rogelj.

In other words, "we should not panic yet if global temperatures exceed this threshold of 1.5 ° C, but it is nevertheless a worrying indicator of the general trend," summarizes Chris Smith.

Especially since it is possible that 2024 will be even worse. "Generally, in the second year, the effects of El Niño are still strong on temperatures," says Robert Vautard.

This push of the thermometer into uncharted territory may also raise the question of tipping points again. From certain temperature increases, the climate changes era and "we can not say today with certainty when it will happen," admits Robert Vautard.

In other words, temperature rises can lead to snowball effects on global warming. Thus, a trend that particularly worries scientists at the moment is "the melting of Arctic sea ice, which seems particularly strong this year," notes Stefan Hagemann.

If this sea ice does not reform sufficiently once the summer temperature period in the Arctic has passed, it "can become one of those tipping points," Rogelj said.

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