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A Sclerosponge (Acanthochaetetes wellsi) from Palau, Western Pacific Ocean, collected in 1996.

A coral-like sponge, Ceratoporella nicholsoni, which grows in the Caribbean, can act as a proxy for sea temperatures going back 300 years.Credit: Kristen Grace/Florida Museum

The planet has already passed 1.5 °C of warming, according to a new measuring technique that goes back further in time than current methods. At the 2015 Paris Climate Accords, nations agreed not to exceed 1.5 °C, a guardrail of climate change.

“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 analysed 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 today1.

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 metres, 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.

“The baseline is where we measure our current temperatures from, so when we say 1.5 [degrees of warming], it’s to do with this reference point,” says McCulloch.

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.

Golden chalice

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Other proxies for global temperatures include ice cores and tree-ring samples. Some of these show temperatures rising from the 1860s as well.

The search for accurate temperature data from before instrumental data collection is “a golden chalice in terms of climate research”, says chemical oceanographer and marine biogeochemist Kate Hendry, at the British Antarctic Survey in Cambridge. “If we’re going to agree on climate targets, we need to know what we’re basing everything against,” she says.

Although there is a lot of interest in these geochemical proxies of temperature, Hendry says that this approach is still in its infancy, and that there needs to be greater understanding of these proxies “before we jump to any very strong conclusions”.

The research team verified the accuracy of the sponge-derived temperature data by comparing it with global-average temperature records from 1964 to 2012, and “they agree perfectly”, says McCulloch.

However, Hendry says that this assumes that the sponge thermometry would perform the same across its entire lifespan as it does during the validation time period. “Is it a linear response, which is essentially what they’re assuming, or does the biology change somehow?”

Hendry says that building a global picture of warming will need data from around the planet and from a variety of sources.

“Every single proxy for temperature that we have will have problems, will have caveats, will have limitations, so it’s a matter of putting as many of these together as possible,” she says. “The more different bits of the puzzle we can put together, the more robust we are going to be able to reconstruct these temperature differences.”

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