Sea sponges suggest we have surpassed 1.5°C of warming

Global mean surface temperatures may have already passed 1.5°C of warming and could exceed 2°C by the end of the decade, according to a new paper published in the journal Nature Climate Change. The paper’s projections are based on 300 years of ocean temperature records preserved in sclerosponge skeletons from the Caribbean. Previous estimates of warming are based on sea-surface temperature records, which only date back to the mid-1800s.

The 2015 Paris Agreement aimed to keep temperatures to below 2°C above pre-industrial warming and to pursue efforts to limit temperature rises to 1.5°C above pre-industrial levels. Historical observations and data are limited for ocean temperatures, as direct temperature measurements didn’t become widespread until well into the 1900s; however, proxy records allow for the examination of historical events. One of those proxies is the sclerosponge, a long-lived species that records chemical changes in its calcium carbonate skeleton, serving as a natural archive of ocean temperatures.

“The ratio in which the sea sponges store strontium versus calcium in their skeletons is directly related to the temperature of the water they grow in,” explained Dr Georgy Falster, postdoctoral fellow with the ARC Centre of Excellence for Climate Science, who was not involved in the study. “The sea sponges used in this research grew in a part of the ocean that closely tracks global temperature.”

An international team of researchers, led by Malcolm McCulloch from the UWA Oceans Institute, used samples of sclerosponges collected in the Eastern Caribbean, where the natural variability of temperatures is less than at other locations, to explore temperatures in the ocean mixed layer (the region in the water that interacts with the atmosphere) over the past 300 years. These data were then calibrated against observational records detailed in the HadSST4 dataset of sea surface temperature, and when the authors compared these datasets, the new temperature record showed a high correlation from 1961 to the present.

On the basis of these sponge records, the study authors show that the pre-industrial period can be defined by stable temperatures from 1700 to 1790 and from 1840 to 1860, with a gap defined by cooling related to volcanic activity. They suggest that warming related to human activity commenced from the mid-1860s, with clear emergence by the mid-1870s. This is around 80 years before instrumental sea surface records (taken from the Hadley Centre Sea Surface Temperature Dataset version 4) indicated, but is in line with previous palaeoclimate reconstructions.

These findings have implications for current projections of global warming, with the authors noting that for the reference period (1961 to 1990) used to calculate anomalies, the ocean mixed layer and land surface temperatures increased by around 0.9°C relative to the newly defined pre-industrial period. This is compared to current estimates from the Intergovernmental Panel on Climate Change (IPCC) 1850–1900 pre-industrial period of 0.4°C. Using their temperature record, the authors estimate that 1.5°C of warming may have been reached and that a mean surface warming of 1.7°C could have occurred between 2018 and 2022. On this basis, McCulloch and colleagues indicate that the opportunity to limit Earth’s warming below 1.5°C may have passed, and the goal to keep warming below 2°C could be exceeded by the end of the decade.

That said, experts have been quick to point out that the new data, if accurate, does not necessarily change the goals of the Paris Agreement. Yadvinder Malhi, Professor of Ecosystem Science at the University of Oxford’s Environmental Change Institute, stated, “This early industrial era warming, if real, is almost certainly not human-caused… Our models of climate warming impacts are based on warming relative to 1850–1900, and moving the baseline definition of pre-industrial does not make these expected impacts worse. Human-caused climate warming to date is still around 1.4°C.”

“The opportunity to limit global warming to no more than 1.5°C has in fact not necessarily passed,” added Dr Shaun Fitzgerald, Director of the Centre for Climate Repair at the University of Cambridge. “It looks like it has passed if the only lever being considered is emissions reduction, but there are other levers too such as greenhouse gas removal. Various approaches are being developed and if scaled up quickly could help keep the target alive.”

Dr Duo Chan, Lecturer in Climate Sciences at the University of Southampton, concluded, “The impacts of climate change, including intensified heatwaves, extreme rainfall and sea-level rise, are tangible and will only exacerbate as temperatures continue to rise. Setting specific goals is beneficial for political collaboration and global action, but failing to meet a target should not lead to despair.

“Just like in our daily lives, where a missed objective in our schedule doesn’t mean we abandon the rest of the day’s plans. In the face of climate challenges, we adapt and persevere; we recalibrate and re-energise, because every step counts and every fraction of a degree matters.”

Image caption: The sclerosponge (orange) was collected in the Bahamas. Image credit: I Macintyre/Smithsonian Institution.