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.

(Global change in ocean heat content through 2015. Image source: Skeptical Science and CMIP5.)

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.

(Ocean heat content change since 1958. Illustration: Cheng and Zhu (2018), Advances in Atmospheric Sciences.)

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, 61,500,000,000,000 Joules were produced by the Hiroshima bomb. The world ocean is now taking in a similar amount of heat every 3-5 seconds.

In the atmosphere, we tend to focus on El Nino years as the hot ones in an ongoing upward trend. This is because warm surface waters spreading across the Equatorial Pacific belch a bit of that huge volume of stored ocean heat back into the atmosphere. But during La Nina years, cooler surface waters across wide regions of the Equator swallow up more of the atmospheric heat. It is during these years that oceans tend to warm the most swiftly even as atmospheric warming tends to take a break. 2017 saw a weak La Nina and a comparatively strong rate of related ocean heat gain. And though atmospheric temperatures were ‘only’ the second hottest ever recorded according to NASA, ocean temperatures tracked further into uncharted territory.

(During El Nino years [left], the global oceans transfer a portion of their vast store of warmth to the atmosphere. During La Nina years [right] the oceans draw in more of the atmosphere’s heat. Image source: Climate.gov.)

It’s worth noting that ocean heat gain is presently both quite rapid and rather steady. All of the past five years were each one of the five hottest ocean years ever recorded. Global temperature gain thus hasn’t slowed. And though atmospheric temperature gain has accelerated during recent years, the ocean measure hints that overall heat gain per year has been pretty steady since the mid 1990s. At least for the top 6,000 feet of the world’s surface waters (though other measures provide some hints at acceleration [see image at top of this post]). An observation that would seem to reinforce the present decadal rate of temperature increase in the range of 0.15 to 0.20 C every ten years or about 30 to 50 times faster than the warming that ended the last ice age.

To be clear, the primary driver of what is a very rapid warming in the geological context is human fossil fuel burning and related carbon emissions in the range of 11 billion tons per year. Halting fossil fuel burning is therefore critical to slowing down and ultimately stopping the present rate of warming and dangerous related atmospheric and ocean carbon addition.