How much of global warming has happened already?


The 2015 Paris Agreement aimed to limit temperature rises to 1.5 °C above pre-industrial levels, but a recent study involving sea sponges suggests we may have already failed

Global mean surface temperatures may have already passed 1.5 °C of warming and could exceed 2 °C by the end of the decade, a paper published in Nature Climate Change suggests. The projections are based on 300 years of ocean temperature records preserved in sclerosponge skeletons from the Caribbean.

Historical observations and data are limited for ocean temperatures; 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.

Malcolm McCulloch and colleagues 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.

On the basis of these sponge records, the 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 indicated, but is in line with previous palaeoclimate reconstructions.

The authors suggest that these findings have implications for current projections of global warming. They indicate that their records show 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 the basis of their records, 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.