But using ice cores and other ancient data points allows scientists to peer much deeper into the past, and establish that the climate today is likely warmer than it has been in 120,000 years.

How do they do it?

- Climate archive -

"When we want to know the climate of the past, we look for an archive that has recorded these temperature variations," said paleoclimatologist Etienne Legrain when reached by AFP during a research mission in Antarctica.

Marine sediment cores are one such trove, he says.

Composed of material that collects at the very bottom of the ocean, these are "full of critters -- foraminifera -- that record in their shells the temperatures of the time when they lived".

Ice cores extracted by drilling inside glaciers, sometimes several kilometres down, are literally frozen in time, offering scientists a glimpse into an ancient past.

"The deeper we go, the further back in time we can read," said Legrain, who had returned to base after a field expedition to the glaciers of the South Pole.

These icy cylinders allow scientists to reach back hundreds of thousands of years.

Another method is to weigh an ice molecule to derive the temperature at the time it was deposited, he said.

Legrain said these different data sources present "the same temperature curves. That's why we have a fairly strong degree of certainty" even if they do not achieve precision to a tenth of a degree.

- Off track -

The UN's expert panel on climate change says global temperatures in the ten years to 2020 were about 1.1 degree Celsius hotter than an 1850-1900 baseline.

These readings "exceed those of the most recent multi-century warm period, around 6,500 years ago", wrote the scientific panel known as the IPCC.

"Prior to that, the next most recent warm period was about 125,000 years ago, when the multi-century temperature overlaps the observations of the most recent decade."

Margins of error do not allow scientists to say today's climate is definitely hotter than a warm period known as the Eemian that began about 130,000 years ago, said Legrain.

"But what is certain is on the current trajectory of greenhouse gas emissions, the climate at the end of this century will be the warmest in at least 800,000 years", said the scientist from VUB university in Brussels.

- Rapid warming -

The UN says the world is on track for nearly 3C of warming compared to the 19th century.

Temperatures have not been that high in two to three million years, Legrain said, when the climate "was probably about 2C to 3C warmer than the pre-industrial period".

Comparisons between past warmer periods and today's climate can be fraught.

Warming then occurred gradually, with Earth tilted more strongly toward the Sun, exposing the poles to higher levels of solar energy.

During the Eemian, global temperatures rose roughly 1C every 2,000 years, said Legrain, allowing time for living beings to adapt.

"We have gained almost a degree in 50 years", he said, a rate 40 times faster.

- Take caution -

Maria Sanchez Goni, a paleoclimatologist from France's EPHE institute, warned against taking shortcuts when reading clues from the deep past.

She told AFP it was difficult to establish average temperatures beyond a certain region using these sources of data, let alone try and break them down year by year.

One study in 2020 sought to study pollens, insects and geochemical markers to reconstruct the climate of the past 12,000 years.

Goni said in its conclusions, the study was limited to offering just estimates of global temperatures beyond more than 150 years.

Given these uncertainties "it is not impossible that a single year was as warm as 2024" when considering the distant past this way, she said.