In assessing the global-scale trends in near-surface (0-20 m) ocean temperatures between 1900 and 2010, Gouretski et al. (2012) determined that the world’s oceans warmed by about 1.1°C between 1900 and 1945 (~0.24°C per decade), but then only warmed by an additional net 0.3°C between 1945 and 2010 (~0.046°C per decade), including a cooling trend between 1945 and 1975.
The early 20th century warming was therefore about 4 to 5 times greater both in magnitude and rapidity as the post-1945 warming.
“Both time series show a temperature increase from 1900 to about 1945, a slight decrease to the mid-1970s, and a temperature rise to the end of the record.”
Interestingly, Gouretski et al. (2012) also point out that large regions of the oceans have been cooling since the 1990s.
“Decadal mean SST and 0–20 m layer anomalies calculated relative to the reference decade 2001–2010 give evidence of the general warming of the global ocean since 1900. However, large regions of the oceans have experienced cooling since the 1990s. Whereas cooling in the tropical Eastern Pacific ocean is associated with frequent La Nina events in the past decade, the cause of the cooling within the Southern Ocean remains unknown.”
According to Riser and 26 co-authors (2016), the globe’s oceans have warmed in some places, cooled in others, and the overall net change has been a warming of a little less than 0.2°C (0-1000 m) since about 1950, or about 0.03°C per decade.
The achievement of a few tenths of a degree of added warmth over the course of the last 6 ½ decades has been realized largely because the regions of the world where the oceans have been warming have slightly exceeded the cooling regions in volume.
The net difference between the warming and cooling trends for the globe is oddly referred to as global warming even though the warming trends have not been global, but regional.
“Most regions of the world ocean are warmer in the near-surface [0-700 m] layer than in previous decades, by over 1° C in some places. A few areas, such as the eastern Pacific from Chile to Alaska, have cooled by as much as 1° C, yet overall the upper ocean has warmed by nearly 0.2° C globally since the mid-twentieth century.”
According to climate models and anthropogenic global warming theory, it has been expected that a long-term, gradually rising warming trend in the world-wide ocean would follow the trends associated with the rise of CO2 emissions.
The the oceans have not cooperated.
Instead, the world’s regional oceans have followed a decadal-scale variability, with pronounced warming and cooling episodes. The lack of consistency with climate models has thus led scientists to conclude that it is “very difficult to determine whether significant anthropogenic change in [regional 0-2000 m ocean temperatures] … have occurred” (Yashavaev and Loder, 2017).
Below are several examples of the wide swaths of the Earth where ocean cooling (or non-warming) has been ongoing for at least the last decade to last several decades, including the North Atlantic Ocean, Pacific Ocean, Southern Ocean, and Indian Ocean.