Global warming | The limit of 1.5 degrees already crossed

Global warming has started earlier than expected, say American and Australian biologists. From sponges collected near Puerto Rico, they claim Monday in Nature Climate Change that the threshold of 1.5 degrees Celsius of warming has already been crossed and that the border of 2 degrees will be crossed by the end of the decade.

“The planet’s temperature began to rise shortly after the mid-19th century.e century,” Malcolm McCulloch of the University of Western Australia, one of the authors of the study published Monday, explained in a press conference last Thursday. “So, we have a few more decades of increase compared to current assessments. »

The temperature increase is 1.7 degrees compared to the pre-industrial average. “We believe that the threshold of 2 degrees (Editor’s note for temperature increase compared to the pre-industrial era) will be crossed at the end of the 2020s, unless there is a significant reduction in emissions. »

The new study is based on a type of Caribbean sponge, Ceratoporella nicholsoni, which grows very slowly and lives at depths of almost 100 meters. Over the past thirty years, these sponges, which can live for several centuries, have increasingly been proposed as ideal organisms for assessing the planet’s temperature in the past.

The new study shows that the climate emergency is very real, believes Patrick Bonin, head of Greenpeace Canada’s climate-energy campaign. “But it’s a study on an organism in a specific location on the planet,” says Mr. Bonin.

The authors of the study Nature Climate Change think that the sponge they studied represents very well the evolution of the average temperature of the planet, because at this depth the annual variations of the climate have less impact. Also, the Caribbean is less affected than the Pacific by El Niño, a meteorological phenomenon that warms the atmosphere and the ocean.


The study of these sponges makes it possible to evaluate the temperature passed with an accuracy of one tenth of a degree, Mr. McCulloch said at a press conference. We see in the data a brief cooling linked to certain large volcanic eruptions, notably that of Tambora in 1815, sometimes nicknamed “the year without a summer” because the soot from this Indonesian volcano cooled the atmosphere everywhere on the planet for several month.

Partly because of the distortion induced by the Tambora, the study authors believe that the current temperature should be compared to the 18th century average.e century rather than the 1850-1900 average, as does the Intergovernmental Panel on Climate Change (IPCC). The IPCC uses the 1850-1900 average because before the 20e century, thermometer measurements of temperature were difficult to compare from one year to the next.

During the press conference, journalists questioned the relevance of mentioning the 1.5 degree threshold while changing the “pre-industrial” reference period. Currently, the IPCC announces negative effects linked to exceeding the 1.5 degree threshold, but with a reference period ranging from 1850 to 1900. Will the same negative effects of exceeding the 1.5 degree threshold materialize? if we compare the current temperature in the 18th centurye century ?

“I would say that the serious impacts of global warming will occur sooner than expected,” Mr. McCulloch said at a press conference. I think we have moved ahead of things (editor’s note the negative effects linked to the 1.5 degree threshold) by a decade. »


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