Today’s global ocean temperatures hover around 15°C on average. About 400 to 500 million years ago, Earth’s ocean temperatures averaged 35-40°C and atmospheric CO2 concentrations were “5-10x higher than present day values” (Henke et al., 2018). During this period, the marine biosphere thrived (Voosen et al., 2019).
Image Source: Henke et al., 2018
Image Source: Voosen et al., 2019 and press release
During the last 5.4 million years, CO2 has varied between 180 and 500 ppm (Fletcher et al., 2019). During the Pliocene (5.4 to 2.5 million years ago), Arctic temperatures were said to have been 22°C warmer than today (Fletcher et al., 2017) and tropical ocean temperatures were 3-6°C warmer than present (Wycech et al., 2020).
Image Source: Fletcher et al., 2019
Image Source: Fletcher et al., 2017
Image Source: Wycech et al., 2020
During the peak of the last glacial (the Last Glacial Maximum) 19,000 years ago, the Southern Ocean was 2°C warmer than it is today even though CO2 levels were a dangerously low 180 ppm at this time. This was also the same period when the Earth’s bottom water temperatures warmed up by about 2°C 1,000 years before the oceans’ surface began warming and CO2 began to rise (Stott et al., 2007).
Image Source: Stott et al., 2007
As recently as 10,000 years ago, the Arctic Ocean’s bottom water temperatures were 6-10°C warmer than they are today (Beierlein et al., 2015).
Image Source: Beierlein et al., 2015
Image Source: Rosenthal et al., 2017
Image Source: Gebbie and Huybers, 2019
In sum, paleoclimate research strongly suggests there is nothing unprecedented or even unusual about ocean temperature changes in the modern era.