(Global change in ocean heat content through 2015. Image source: Skeptical Science and CMIP5.)
Where does most of the heat trapped by human fossil fuel and other greenhouse gas emissions ultimately end up? Given our fixation on global surface temperatures, many people would say ‘the atmosphere.’ But this answer is incorrect. The vast majority ends up in the world ocean.
The world ocean system is the largest heat sink on our planet’s surface. This is due to the fact that liquid water contained in the oceans both has a far greater mass and overall heat capacity than the atmosphere. Just a fraction — less than 1/30th of the heat trapped by human-emitted greenhouse gasses ends up in the atmosphere. Similar portions end up getting soaked in by the land and by melting glaciers. The rest, about 90 percent, finds its way into the oceans.
The ocean is thus the best, most reliable global thermometer available. For good reason, most scientists wait for readings from this big, wet thermostat to get an idea where global temperatures are headed and how fast. And what some of the world’s top ocean researchers found this week was that during 2017 the top 6,000 feet of the world’s oceans experienced their hottest year ever recorded.
Not only was 2017 the hottest ocean year on record, the heat gain from the previous hottest ocean year (2015) was quite considerable. In all 15,100,000,000,000,000,000,000 Joules of heat energy were added by the world ocean from 2015 to 2017. By comparison, 4,184,000,000 Joules were produced by the Hiroshima bomb. The world ocean is now taking in a similar amount of heat every 3-5 seconds.