“We have an alternate record of global warming,” says coral-reef geochemist Malcolm McCulloch, at the University of West Australia Oceans Institute in Crawley, and lead author of the study. “It looks like temperatures were underestimated by about half a degree.”

The Intergovernmental Panel on Climate Change (IPCC) uses a baseline for pre-industrial global mean temperatures that reference the earliest global instrumental temperature records. This period is around 1850–1900, when the first ship-based records of sea-surface temperatures became available.

However, McCulloch says that long-lived marine sponges can provide indications of temperature as far back as the eighteenth century.

He and his colleagues analyzed the ratio of the elements strontium to calcium in the 300-year-old calcium carbonate skeletons of a coral-like species of sponge, Ceratoporella nicholsoni, that grows off the coasts of Puerto Rico. This ratio changes only with changes in water temperature, making it a proxy thermometer, according to the study published in Nature Climate Change today.

The sponges were sampled from one particular section in the Caribbean — the only place that they are found. They were collected at a depth of 33–91 meters, in what’s called the ocean mixed layer.

“Sea-surface temperature can be highly variable on top,” says McCulloch. “But this mixed layer represents the whole system down to a couple hundred metres, and it’s in equilibrium with the temperatures in the atmosphere.”

The arm of the Caribbean that the sponges grow in is also relatively sheltered from big ocean currents and climate cycles, such as the Atlantic Meridional Overturning Circulation and El Niño Southern Oscillation, which means that it experiences less variability in water temperatures than other ocean regions.

The sponge skeletons suggest that the planet started to warm up in the mid-1860s, during the period currently defined as the pre-industrial baseline.

During the relatively stable period of 1700–1860, global sea-surface temperatures varied by less than 0.2 °C — with the notable exception of brief cooler periods attributed to volcanic eruptions.

Using this earlier period as the pre-industrial baseline, McCulloch and colleagues calculated that global temperatures had in fact increased by 0.5 °C more than what was estimated by the IPCC. “That’s a huge difference relative to the total amount of warming,” says McCulloch. Furthermore, the planet exceeded 1.5 °C of warming by around 2010–2012, and is on track to surpass 2 °C in the next few years.

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