There's been good evidence that the oceans were heating up faster than thought. Now, scientists have fitted the puzzle pieces together.
The trend in the amount of heat in the oceans is shown for the period 1993 to 2015. Yellow, orange and red tones show locations where ocean heat has increased. (Source: Lijing Cheng & NCAR. Retrieved from https://climatedataguide.ucar.edu/climate-data/ocean-temperature-analysis-and-heat-content-estimate-institute-atmospheric-physics.) So this morning, as I'm drinking my coffee and perusing news headlines, I see this in the New York Times: "Ocean Warming Is Accelerating Faster Than Thought, New Research Finds." The story was about a new paper published Thursday in the journal Science titled, "How fast are the oceans warming?" This is a big deal, because human-caused global warming doesn't affect just the land surface. In fact, more than 90 percent of global warming's heat is absorbed in the oceans. That has helped prevent much steeper increases in temperature on land. But all that heat going into the oceans isn't really a benign phenomenon. By causing ocean waters to expand, it contributes to sea level rise. The heat also can make storms more destructive, and it's putting enormous stress on ocean ecosystems — which we depend on heavily for food. And in the long run, what goes into the oceans doesn't all stay in the oceans. Heat eventually comes out of the water to contribute to warming atmospheric temperatures around the globe. So knowing exactly how much heat is going in is very important. With that in mind, I checked out other stories about the new paper in Science, and I saw that many featured similar headlines as the N.Y. Times. More about the scientific paper in a minute. But first, I have to say that I realized I had seen very similar headlines before. Just this past October, for example, I saw this in Scientific American: "The Oceans Are Heating Up Faster Than Expected." According to the story, a "new study published yesterday in the journal Nature concluded that the global oceans may be absorbing up to 60 percent more heat since the 1990s than older estimates had found." And nearly two years ago, the Washington Post ran this headline: "The world’s oceans are storing up staggering amounts of heat — and it’s even more than we thought." That was based on a study published in the journal Science Advances. In a press release about it, study co-author Keven Trenberth of the National Center for Atmospheric Research was quoted as saying that "the planet is warming quite a lot more than we thought." Hmmm. Two years ago we already knew that the planet was warming quite a lot more than we thought. So what's up with today's headlines, which seem to suggest that we didn't know this? For quite awhile now, scientists have actually had good reasons to believe the oceans have been taking up more global warming heat than was estimated in a major report in 2014 from the U.N.'s Intergovernmental Panel on Climate Change. And more recently, research has been confirming those suspicions. If you read beyond the headline and down into that New York Times story — which actually is quite good — you'll see that the new paper isn't at all a research article presenting a major new advance. It's actually an assessment based on previous original research of the state of knowledge about rising ocean heat content, or "OHC," as scientists label it. And as the assessment concludes, "Multiple lines of evidence from four independent groups thus now suggest a stronger observed OHC warming." Based on a lot of the coverage I encountered today, you might easily conclude that the new assessment produced dramatic new findings. But the findings have actually been piling up for a few years — as have those headlines, some of them quite dramatic. Now, the authors of the new assessment have pulled multiple strands of previous research together to provide a clearer picture of what's currently known. That picture shows that the oceans are heating up 40 percent faster than what the U.N. report estimated. And things are getting increasingly worse. As Trenberth, one of the authors, put it in an email to me today, "There are clear signs of acceleration." The better estimates of how much global warming heat is going into the oceans is based in part on new ways of piecing together data from different sources. Since the early 2000s, accurate data have been provided by a modern network of floating ocean heat sensors, called the Argo network. But prior to that, information was collected by less accurate sensors called expendable bathythermographs. Because of inaccuracies, the data from the older sensors contained biases. Thanks to recent research, scientists have found ways to deal with this issue, providing a better picture of just how much more heat the oceans have been sopping up compared to the past. The picture has also been improved by new ways of dealing with another vexing issue: In the past, larger portions of the oceans went unmonitored than today. "The oceans are not well observed as we go back in time," notes Trenberth. In the past, scientists tried to deal with this using various strategies for filling the gaps. But these tended to produce overly conservative estimates. More recently, satellite observations and computer modeling have helped improve estimates of what has been going on in largely unmonitored areas of the oceans. And still other researchers have analyzed ocean factors that are influenced by ocean temperature to derive independent estimates of how the ocean's store of heat has changed over time. Overall, the estimates derived by these studies are in line with what climate models have been saying. The models have tended to indicate more ocean warming than what had been observed, and that discrepancy had given fodder for critics of climate change science. But now, Trenberth and his fellow authors say that discrepancy is largely gone. One of the sobering conclusions of the new assessment is the likely consequences of failing to get off the business-as-usual scenario of high emissions of heat-trapping greenhouse gases. Model projections — which we now know have been line with observations — show that the likely amount of ocean warming "would have major impacts on ocean ecosystems and sea level rise through thermal expansion," the scientists write. When you combine estimates of thermal expansion with projected sea level rise from melting glaciers and ice sheets, you come up with numbers that "portend very bad consequences for many coastal regions," Trenberth told me in an email message. My point in writing all of this today is to point out that if you pay too close attention to headlines, you might get the impression that science happens in discrete bursts of dramatic new research findings. In fact, most of the time, research progresses incrementally, with different groups of scientists probing at a particular issue independently and often in different ways. One study usually doesn't provide definitive insight into a phenomenon. It takes multiple findings — and sometimes a group of scientists fitting those puzzle pieces together — to produce a clearer, convincing picture.
