Global warming: The temperature of our planet has increased by 1.5°C according to 300-year-old sponges

A new measuring technique that examines the past indicates that the planet has already surpassed 1.5 °C of warming, which is the guardrail for climate change set at the 2015 Paris Climate Accords. The accord established that nations must not exceed 1.5 °C in order to prevent climate change from progressing beyond a certain point.

“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 of pre-industrial global mean temperatures that refers to the earliest international instrumental temperature records. This period of reference is around 1850-1900, when ship-based records of sea-surface temperatures became available.
According to McCulloch, however, long-lived marine sponges can provide indications of temperature dating back as far as the 18th century. McCulloch and his colleagues analyzed the ratio of the elements strontium to calcium in the calcium carbonate skeletons of a coral-like species of sponge called Ceratoporella Nicholson. These sponges grow off the coasts of Puerto Rico, and their ratio changes only with changes in water temperature, making them a reliable proxy thermometer. The study, which was published in Nature Climate Change today, provides evidence for this claim.

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 Caribbean arm where sponges grow is relatively protected from major ocean currents and climate cycles, such as the Atlantic Meridional Overturning Circulation and El Niño Southern Oscillation. As a result, this region experiences less variation in water temperatures compared to other ocean regions. The sponge skeletons indicate that the planet began to warm up in the mid-1860s, which is currently considered the pre-industrial baseline period.

“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.

Between 1700 and 1860, the global sea-surface temperatures remained relatively stable, varying by not more than 0.2 °C, except for brief periods of cooler temperatures due to volcanic eruptions. Researchers led by McCulloch used this period as a pre-industrial baseline to calculate that the global temperatures have increased by 0.5 °C more than what was predicted by the IPCC. This is a significant difference in the amount of warming that has occurred. The planet surpassed 1.5 °C of warming by 2010-2012, and it is expected to exceed 2°C in the next few years. Other proxies, such as ice cores and tree-ring samples, also indicate that temperatures have been rising since the 1860s.

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.

Hendry raises concerns that the sponge thermometry may not perform consistently throughout its entire lifespan as it did during the validation period. It is uncertain if the response is linear or if the biology changes over time. To build a comprehensive understanding of global warming, data from various sources around the world is required. Hendry suggests that every temperature proxy has its limitations and caveats, and the key is to combine as many proxies as possible. The more diverse pieces of information we have, the more robust our temperature reconstruction will be.

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