The world has warmed 1.5 °C, according to 300-year-old sponges

By the time that official temperature records began, global temperatures had already risen by half a degree

Too bad it wasn't published in 1998

300-year-old sponges show 1.5 °C warming

Evidence from long-lived marine sponges suggests that the planet has already passed 1.5 °C — a milestone of global warming that nations pledged to avoid in the 2015 Paris climate agreement. The ratio of two elements — strontium and calcium — in the skeletons of Ceratoporella nicholsoni reflects changes in water temperature, making the coral-like sponges a proxy thermometer. The data indicate that the planet had already started to warm in the 1860s, around the time when the first ship-based records of sea-surface temperatures began. The approach is still in its infancy, but could indicate that warming has been hugely underestimated, “by about half a degree”, says coral-reef geochemist and study co-author Malcolm McCulloch.

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

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.