Ocean warming speeds vary with depth

The world’s oceans are a vast reservoir of heat, a slow register of natural climate change − and ocean warming speeds differ widely.

Climate scientists who have found a new way to chart temperature change in the world’s seas over time say ocean warming speeds are much slower in deep water than on the surface.

Planet Earth is mostly ocean. Human-linked changes have started to raise global temperatures to what could be alarming levels and, as the thermometer rises, so will sea levels. So detailed understanding of temperature and ocean is vital. But two separate studies confirm that the connection is far from simple.

One study of the Atlantic confirms that in the last 150 years, the oceans have taken up 90% of the excess energy released by the combustion of fossil fuels to drive human economic growth and power − and to fuel potentially-catastrophic global warming and runaway climate change.

But what the oceans will actually do with that colossal burst of heat has yet to be fully explored. And a separate examination of the deep history of the Pacific Ocean confirms that change may be inexorable, but it is also very slow: the deeper parts of the Pacific are still registering the onset of the so-called “Little Ice Age” several centuries ago.

“These waters are so old and haven’t been near the surface in so long, they still ‘remember’ what was going on hundreds of years ago”

Both studies are reminders that oceanography is still a relatively new science and researchers still have a lot to learn about the fine detail of the ways in which temperature, atmosphere and ocean interact to affect climate over the world’s continents.

But repeated research has confirmed that the oceans are warming in response to human-triggered changes on land, that this warming presents several different kinds of hazard  to marine life, and that there is a link between overall ocean temperatures and the behaviour of the ocean’s currents, a link that plays out in dramatic shifts in regional climates.

So the rewards for a more precise understanding are considerable. But understanding starts with accurate and comprehensive data, and systematic measurement of ocean temperatures began only with the voyage of the British research ship HMS Challenger in 1871.

So Laure Zanna, a physicist at the University of Oxford and her colleagues, report in the Proceedings of the National Academy of Sciences that they deployed sophisticated mathematical techniques to calculate the heat uptake of the oceans and the way the blue planet has responded since 1871.

Huge heat uptake

Altogether, in the last 150 years, the deep waters have absorbed 436 zettajoules: a joule is the unit of energy required to deliver one watt for one second and a zettajoule is a number followed by 21 zeroes. This is an enormous amount of heat, roughly 1,000 times the energy consumed by 7 billion humans in the course of a year.

The researchers’ results so far show that roughly half the observed warming of the last 60 years – and the associated sea level rise – is linked to changes in ocean circulation. They were able to reconstruct two considerable bouts of warming, over the years 1920 to 1945 and between 1990 and 2015. What they have yet to do is sort out what this means for the behaviour of the oceans over the decades to come.

“The technique is only applicable to tracers like man-made carbon that are passively transported by ocean circulation,” Professor Zanna said. “However, heat does not behave in this manner as it affects circulation by changing the density of seawater. We were pleasantly surprised by how well the approach works. It opens up an exciting new way to study ocean warming in addition to using direct measurements.”

What the research also underlines is that the oceans have a long memory: so extensive and so deep are the five oceans that the surface waters may respond to 20th century greenhouse gas emissions while the deepest trenches contain water that last warmed more than 1,000 years ago in the reign of Charlemagne, the first Holy Roman Emperor.

Still adjusting

US oceanographers report in the journal Science that they matched predictions from computer models and modern data and ancient evidence with readings from the Challenger expedition to show that two kilometres under the waves, the Pacific Ocean is still adjusting to cooling that began with the onset of the Little Ice Age centuries ago.

Such studies count as basic research: as a way of testing techniques and establishing ground rules from which more discovery could follow. They also offer new ways to understand oceans as registers of climate change over long intervals.

“These waters are so old and haven’t been near the surface in so long, they still ‘remember’ what was going on hundreds of years ago when Europe experienced some of its coldest winters in history,” said Jake Gebbie, of the Woods Hole Oceanographic Institution.

“The close correspondence between prediction and observed trends gave us confidence that this is a real phenomenon.”