How do we know that the current climate is the warmest in at least 120,000 years ?

How do we know that the current climate is the warmest in at least 120,000 years? VANDERLEI ALMEIDA/AFP

2024 is undoubtedly the warmest year ever measured on Earth since 1850. But ice cores, sediments on the ocean floor and other “climate archives” allow scientists to establish that the current climate is unprecedented in at least 120,000 years. Explanations.

“When we want to know about past climates, we look for an 'archive' that has recorded temperature variations,” paleoclimatologist Etienne Legrain explained to AFP, contacted during his research mission in the Antarctic.

For example, “marine sediment cores, composed of sediment that falls to the bottom of the oceans” : these “are full of creatures — foraminifera — that record in their shells the temperatures of the time they lived,” he says.

“There are also ice cores, with several methods: a very simple one consists to put a thermometer in a hole that we are going to drill over 3 km and the deeper we go, the further back in time we go to read the temperature of the ice, which always remains the same as at the time of its formation”, describes this scientist from the Free University of Brussels, who has just returned from an expedition to the glaciers of the South Pole.

“Another consists of weighing the weight of an ice molecule, which gives the temperature at the moment when the ice was deposited”. Ice cores allow us to go back more than a million years in time.

“When we analyze all these very different archives”, the distinct research communities “fall on the same temperature curves, that's why we have a fairly high certainty”, he summarizes, even if they do not achieve precision to the tenth of a degree.

Conclusion, according to the IPCC: global temperatures in the last decade (2011-2020), approximately 1.1 °C warmer than the period 1850-1900, “exceeded those of the most recent multi-century warm period” of about 6,500 years ago, which was only 0.2 to 1 °C warmer than pre-industrial times.

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“Before that, the most recent warm period was about 125,000 years ago, when the multi-century temperature (0.5 °C to 1.5 °C relative to 1850-1900) overlaps with observations of the past decade”, write the UN-commissioned climatologists.

Cause natural

“The margins of error do not allow us to say that the current climate is warmer than the Eemian”, the name of this period of a few thousand years of warming, just before the last ice age, explains Etienne Legrain.

“But what is certain, if we stick to current greenhouse gas emissions trajectories, is that the climate expected at the end of the century will be the warmest in at least 800,000 years”, warns the researcher, since, according to the UN, humanity is putting the planet on the path to a warming of nearly 3°C compared to the 19th century.

To find a point of comparison, we will then have to go back “2-3 million years” back, when the climate “was probably about 2 to 3°C warmer than the pre-industrial period”, adds the researcher.

The Comparison between past and current warm periods has its limits: warming was natural, the result of the Earth's closer proximity to the Sun and a greater tilt, increasing the exposure of the poles to solar energy.

“The Eemian corresponds to an increase of 5-7°C in average temperature over 10,000 years, or about +1°C every 2,000 years. We have gained almost one degree in 50 years!”, underlines Etienne Legrain. The very slow and gradual warming of the past era allowed living beings to adapt.

But beware of shortcuts, warns paleoclimatologist Maria Sanchez Goni: with climate archives, “it is difficult to obtain average temperatures on a global scale” and not just region by region, “and even more difficult to have them year by year”, she tells AFP. When we talk about climate, we are talking about averages over at least a decade.

We must deduce an estimate of global temperatures from fragmentary indices, as in this reference study published in 2020 which reconstructed the climates of the last 12,000 years from pollen, insects and geochemical markers. In her conclusions, she limited herself to giving estimates of global temperatures over more than 150 years, the scientist emphasizes.

With these uncertainties, even in the less warm climate of 6,500 years ago, “it is not impossible that a single year was as warm as 2024”, warns this research director at the École Pratique des Hautes Études.