Ocean surface warming

Average Global Sea Surface Temperatures

Average Global Sea Surface Temperature: 1880 - 2015

(Average Global Sea Surface Temperature, 1880–2015: USA Environmental Protection Agency)

  • The thick orange line shows “the average sea surface temperature of the world’s oceans” movement between 1880 and 2015.  
  • These temperatures have been rising since 1970.
  • The grey band shows the range of uncertainty in the data.

This sea surface temperature increase is further evidence that the planet is warming.

The phytoplankton feedback loop



  • transfer the most carbon dioxide from the atmosphere to the ocean and then into the deep ocean, so they play a crucial part in the global carbon cycle,
  • produce up to half of the oxygen we breathe,
  • are the base of the ocean food chain, and
  • numbers have dropped by about 40% since 1950.

When sea surface temperatures rise, this activates the “phytoplankton feedback cycle”, another cyclic sequence of cause-and-effect that tends to raise global air temperatures.  (See Self-amplifying feedback loops on this website.)

Higher air temperaturesMore ocean surface heating
More carbon dioxide in the airLess nutrient upwelling
Less photosynthesis & less carbon in the deep ocean Reduced phytoplankton population

If this feedback dominated, we would see:

  • higher global air temperatures, causing
  • more ocean surface warming, causing
  • the surface water to be less dense and so more buoyant, causing
  • less upwelling of cold, heavy, nutrient-rich water, causing
  • a reduced population of phytoplankton, which causes
  • less photosynthesis, and less carbon accumulating in the deep oceans, causing
  • more carbon dioxide in the air, which closes the cycle by causing
  • higher air temperatures.

Ocean heat waves

Extreme temperatures are destroying kelp, seagrass, and corals – with alarming impacts on humanity. The number of heatwaves affecting the planet’s oceans has increased sharply, killing swathes of sea life like wildfires on land that take out colossal forest areas. The damage caused in these hotspots is also harmful to humans. Humans rely on the oceans for oxygen, food, storm protection and the removal of climate-warming carbon dioxide from the atmosphere.

(Heat-waves sweeping oceans like wildfires: The Guardian: 5 Mar 2019: Damian Carrington)

2021 was the hottest year for oceans

In 2021 our oceans were the hottest since recording began in 1955. Ocean temperatures have broken this record every year for the last six years, starting in 2017.

(Oceans hottest temperatures: The Guardian: 11 Jan 2022)

In 2018 the rate of ocean warming was equivalent to five Hiroshima-size atomic bombs exploding every second.

(Our Oceans broke heat records in 2018, and the consequences are catastrophic: The Guardian: 16 Jan 2019)

Updated 31 Mar 2022