Global temperatures may have passed 1.5°C of warming a decade ago

Activists at the COP28 climate conference in Dubai in December 2023

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The world is already 1.8°C warmer than in pre-industrial times, having exceeded the limit of 1.5°C in 2010 or 2011, say researchers who used sea sponges to understand how temperatures have changed in the Caribbean over the past 300 years. .

“The increase in global average surface temperature has been half a degree higher than currently accepted estimates,” says Malcolm McCulloch at the University of Western Australia. “What our work says is that we are ten years ahead, if not further ahead, in the global warming scenario.”

However, other climate scientists say data from a single region is not a reliable way to know past global temperatures.

The 2015 Paris Agreement called on countries to limit warming to 1.5°C above pre-industrial levels, but did not define exactly what that meant. Climatologists writing reports for the Intergovernmental Panel on Climate Change (IPCC) therefore defined it as the average temperature between 1850 and 1900.

At that time, the planet had already started to warm due to emissions from burning fossil fuels. But because there were very few temperature measurements before 1850, there is enormous uncertainty about the extent of warming caused by fossil fuels in the early industrial era. The choice of 1850 to 1900 as a reference was therefore pragmatic.

McCulloch and his colleagues, however, believe they discovered the exact extent of fossil fuel warming early on, after analyzing samples of a very long-lived sponge (Ceratoporella nicholsoni) which forms a calcium carbonate skeleton.

10-centimeter-wide sponge could be around 400 years old, team member says Amos Winter at Indiana State University. “These sponges are extremely slow growing.”

The sponges were collected off the coast of Puerto Rico by divers at depths between 33 and 91 meters. The original goal was to examine past ocean pH, but the team also measured the strontium-to-calcium ratio, which varies depending on the water temperature when calcium carbonate formed.

What the researchers realized was that there was a close correlation between the temperatures “recorded” by the sponges and the average global surface temperature measured by the instruments, particularly after 1960, when measurements became more reliable.

“It’s sort of an accidental discovery, but the connection is very strong,” says McCulloch. “They evolve in proportion to the world average. The main differences occur when the instrumental recordings are poorest.

The team therefore calculated what the pre-industrial global average temperature was up to 1700, based on the assumption that sponges accurately reflect it.

The researchers believe that the IPCC should take their work into account when assessing whether the 1.5°C limit has been exceeded. “Ultimately, yes, this should be taken seriously by the IPCC,” says McCulloch.

He also thinks climate modelers need to take the results into account. While carbon dioxide emissions have so far caused more warming than expected, the impact of new emissions may be underestimated, he says.

But other climate scientists are far from convinced. “In my opinion, it is credulous to claim that instrumental records are wrong based on paleo-sponges from one region of the world,” says Michael Mann at the University of Pennsylvania. “That doesn’t make any sense to me. That said, our own previous work supports the idea that there was at least 0.2°C of additional warming before the end of the 19th century. »

In fact, human-caused warming may have started several thousand years earlier. According to the hypothesis of the beginning of the Anthropocene proposed by William Ruddiman At the University of Virginia, deforestation by early farmers and the creation of rice fields generated enough CO2 and methane to keep the planet from cooling and entering a new ice age.

Recent studies by other researchers have provided increasing evidence to support this hypothesis, although it is still far from being widely accepted.

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