New study says the world blew past 1.5˚C four years ago

Limiting average global warming to 1.5 degrees Celsius, or 2.7 degrees Fahrenheit, above preindustrial levels has been the gold standard for climate action since at least the 2015 Paris Agreement. A new scientific study published in the peer-reviewed journal Nature Climate Change, however, suggests that the world unknowingly passed this benchmark back in 2020. This would mean that the pace of warming is a full two decades ahead of projections by the Intergovernmental Panel on Climate Change, or IPCC, and that we’ll cross the 2˚C-threshold in the next few years.

Even more surprising than the findings, perhaps, is the fact that they were derived from the study of sea sponges. A research team led by Professor Malcolm McCulloch of the University Western Australia Oceans Institute analyzed sclerosponges, a primitive orange sponge species found clinging to cave roofs deep in the ocean. Sclerosponges grow extremely slowly — just a fraction of a millimeter a year — and can live for hundreds of years. This longevity is part of why they can be particularly valuable sources of climate data, given that our understanding of ocean temperatures before 1900 is very hazy.

By taking samples from these sponges, McCulloch’s team was able to calculate strontium to calcium ratios, which can be used to derive water temperature back into the 1700s. These ratios were then mapped onto existing global average water temperature data so that the team could fill the holes we have at the beginning of the industrial period, when humans began releasing large amounts of carbon dioxide into the atmosphere.

Given how well the information gleaned from the sponges matches ocean temperature records from recent decades, the researchers were able to support extrapolating far into the past to show that the average ocean temperature was lower than the IPCC supposes.

While the new paper was able to persuade skeptics of its findings during the peer-review stage, on its own, it’s unlikely to dislodge current consensus estimates about how much global warming has already occurred — roughly 1.2˚C, according to many current estimates, compared to the 1.7˚C posited by the new study, which is the first instrumental record of the preindustrial ocean temperature.

“I would want to include more records before claiming a global temperature reconstruction,” Dr. Hali Kilbourne, a geological oceanographer at the University of Maryland Center for Environmental Science, told the New York Times. With more research being undertaken — a team in Japan is looking into Okinawan sponges — we may have those records soon.