Some of the most important questions in climatic science concern the warming of the oceans, both the surface and the sub-surface. When did it warm? How does its warming relate to greenhouse gas forcing of the climate? Is the sub-surface of the oceans the place where most of the energy of global warming is going?
It is clear in land and sea surface temperature datasets that the surface of the Earth has increased its temperature since the start of the instrumental era of temperature measurements (post-1860). However there has been a problem with ocean temperature measurements because they are harder to make and quantify and do not go back in time as far as land measurements. To try to resolve this problem Viktor Gouretski of the University of Hamburg and colleagues compare historical global temperature data time series for the upper 20 metres of the ocean based on the latest update of an historical hydrographic profile data set.
As Fig 1 shows (click on images to enlarge) the sea surface data shows an overall warming since 1900, which is to be expected. There is a rise in temperature between about 1910 and 1940 – as steep as anywhere in the data – followed by a long period of little change until the mid-1980s, then a rise and the familiar post-2000 standstill.
The 1940 – 1980 period of little change in sea surface temperatures is very interesting. When this feature is considered in the land only or combined land-ocean data it is often attributed to aerosol pollution reflecting solar radiation out into space and cooling the planet (though exactly how such a scenario maintains a roughly constant temperature in the face of increasing greenhouse gas forcing is problematical). Since aerosol pollution is often a regional effect it seems strange to explain the standstill in global ocean temperatures this way.
The researchers also extend records of temperature change in the upper 400 m of the ocean back to 1900 – some fifty years earlier than most previous analyses. Overall they conclude that the temperature change in the 0–400 m layer is characterised by two periods of temperature increase between 1900 and 1940–45 and between 1970 and 2003, separated by a period of little change. This is remarkably similar to the surface data.
Looking at their 0 – 400 m data it is obvious that most of the warming in this region too place prior to 1940 with a gradient of temperature increase that is the greatest in the dataset. Between 1940 and 1990 there is actually little evidence of any change, then an increase and a post 2000 standstill. Only in about ten of the past sixty years has the temperature of the 0 – 400 m region shown an increase!
Interestingly, the authors conclude; “These maps demonstrate that the first decade of the 21st century (2001–2010) was not uniformly warmer than previous decades. Before about 1920, the global ocean was almost everywhere colder than the reference decade of 2001– 2010. After 1920, several regions of the global ocean were warmer than in the reference decade.”
“Decadal mean SST (Sea Surface Temperatures) 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.”
Most of the sub-surface warming occurring prior to 1940. The first decade of the 21st century not uniformly warmer than previous decades. These are interesting facts that should be part of the debate about how our oceans are warming.