New evidence indicates that the world’s seawater has been absorbing far more heat than expected over the last 45 years, scientists at the Lawrence Livermore National Laboratory are saying in a paper published Sunday in the journal Nature Climate Change.
In fact, they say, the oceans may be warming twice as fast as has been generally assumed.
Meanwhile, a second paper in the same journal suggests that most of the warming is not going into deep-ocean storage but rather is remaining in the upper levels, at depths above 6,000 feet.
Because the oceans are thought to store more than 90 percent of the heat associated with the global warming driven by greenhouse-gas emissions, it seems to me these findings are worrisome for at least four main reasons:
- While the oceans’ ability to bank a certain amount of warming means that global air temperatures rise more slowly, at least for a while, this capacity is neither limitless nor indefinite. At some point, stored heat must be returned to the atmosphere.
- Warmer waters in the upper oceans have larger long-term surface-level consequences than deep-ocean warming — like the melting of ice now stored on land, in glaciers and ice sheets in places like Greenland and Antarctica, which causes sea level to rise.
- A warmer ocean surface can also add immense energy to short-term storm events like hurricanes and typhoons; diminishing sea ice in regions like arctic Alaska, meanwhile, promote and storm-driven coastline erosion.
- Finally: At this relatively late date, our understanding of oceanic climate-change mechanisms — and thus our ability to craft reasonable strategies of adaptation — is plagued by gaps that make our foundation for decisionmaking disconcertingly unstable.
How can we know so little?
Perhaps you are wondering how it is possible for such a critical factor as the rate of ocean warming to be measured so poorly for so long, in this age of technological sophistication.
One answer is that while the means may be there, the will has lagged behind.
Despite decades of concern over global warming, and the ocean’s complicated role in all scenarios, we haven’t been measuring things like ocean temperature all that rigorously — unless the work was in support of better forecasting for, say, the needs of commercial shipping.
This means that we know a lot more about ocean temperature in the northern hemisphere than in the southern; even though the seas of the southern hemisphere account for nearly two-thirds of the ocean worldwide, the shipping lanes are disproportionately northern.
And while scientists have long agreed that our grasp of ocean warming is a series of tenuous underestimates, this new work is claimed to be the first effort to quantify the magnitude of error.
And even here, the researchers had to rely heavily on satellite altimetry measurements of sea level, which are plentiful, rather than direct temperature measurements, which are not.
As the announcement from Livermore explained the work, in terms slightly less technical than the paper itself:
The team found that climate models that simulate the relative increase in sea surface height — a leading indicator of climate change — between Northern and Southern hemispheres is consistent with highly accurate altimeter observations. However, separating the simulated upper-ocean warming in the Northern and Southern hemispheres is inconsistent with observed estimates of ocean heat content change. These sea level and ocean heat content changes should be consistent, and suggest that until recent improvements occurred in the observational system in the early 21st century, Southern Hemisphere ocean heat content changes were likely underestimated.
Since 2004, automated profiling floats (named Argo) have been used to measure global ocean temperatures from the surface down to 2,000 meters. The 3,600 Argo floats currently observing the global ocean provide systematic coverage of the Southern Hemisphere for the first time. Argo float measurements over the last decade, as well as data from earlier measurements, show that the ocean has been gradually warming, according to [chief author Paul] Durack.
“Prior to 2004, research has been very limited by the poor measurement coverage,” he said. “By using satellite data, along with a large suite of climate model simulations, our results suggest that global ocean warming has been underestimated by 24 to 58 percent. The conclusion that warming has been underestimated agrees with previous studies, however it’s the first time that scientists have tried to estimate how much heat we’ve missed.”
Let’s focus on that latter statistic for a moment: The real rate of warming in the ocean may well actually be more than twice as large as is generally assumed.
Historical (and personal) context
At this point I ask your leave for a small digression, in which I recall a 10-year-old interview with a naval commander that helped me to grasp for the first time, really, how little we knew about ocean and climate apart from commercial forecasting.
His name is Conrad Lautenbacher, and at that point he was in charge of the National Oceanic and Atmospheric Administration (he’s now retired). He had been dispatched to make a tour of American newspapers’ editorial boards, including the one at the Star Tribune where I was employed, in the runup to the elections of November 2004, to demonstrate to opinion leaders that the George W. Bush administration was engaged in some good things, too, like weather forecasting.
On meeting “the admiral,” as his NOAA escort described him, you might note his slight build and scholarly demeanor and have no difficulty believing he held a Ph.D. in applied mathematics, which he did, or was an expert on prediction of currents and wave action in Pacific atolls, which he was.
You might be less likely to guess he’d been a vice admiral and the U.S. Navy’s deputy chief of naval operations for surface warfare (in which role a knowledge of how atolls influence ocean currents had been rather valuable).
He was kind of candid about the political agenda for his tour of newspapers, and the relaxed conversation that followed became memorably intriguing. He liked to turn the tables on a questioner, too, and at one point he asked me to imagine a hemisphere of the earth centered on the Pacific Ocean, and then to guess how many sea-level climate-monitoring buoys were deployed in the whole of that half-planet.
By the sheerest luck, I had only recently read the answer somewhere and knew that it was: One. As in, (1).
But I didn’t know why that was so, and thus began a free-ranging discussion of how little we really invest in understanding the oceans and their role in potentially the greatest planetary problem of our time.
We also agreed that the admiral had a pretty cool idea for converting cruise missiles left over from the Gulf wars — and he had more than a passing familiarity with these, having commanded the U.S. Naval forces at Riyadh during Desert Storm — into robotic monitors carrying weather instruments rather than warheads.
There were lots of them around, he said, gathering dust and carrying a production cost of about a million bucks apiece, which he said was almost certainly going to be cheaper than any custom-built solution for widespread measurement and monitoring.
He was still waiting to hear back from the White House on that one, but was beginning to think the idea wasn’t really going to go anywhere, and that’s pretty much what happened as far as I can tell.
But the Argo project of robotic floats, mentioned above, is also pretty cool — and is beginning to prove its value in research like the two pieces published on Sunday.
If you’ve seen Errol Morris’s documentary “Fast, Cheap and Out of Control,” you have the basic concept in your grasp: A bunch of (relatively) low-cost and low-tech instrument stations that are cast into the void, where they operate independently and without much central control after deployment, gathering data and sending it home until the batteries die and they go dark.
Except this time the target is not the surface of Mars, but the surface and upper depths of our own ocean, about which we know much less.
And about which, it would seem, we have a lot to learn and maybe not so much time in which to learn it.
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Both of the papers published in Nature Climate Change can be read online, but you’ll have to pay for access if you don’t already subscribe. The Durack paper on underestimation of ocean warming globally is here; the paper on upper-ocean trends is here.