When British Capt. James Cook undertook his second voyage in the Southern Ocean in 1772, scientists on board measured the temperature 183 meters below the surface. It was colder than at the surface.
Scientists have since graduated to vastly improved technologies for measuring the ocean's temperatures. By 2004, they had launched Argo ("swift" in Greek), a network of 3,000 floating devices spread out throughout the world. The devices record the temperatures down to 6,500 feet, where only the deepest divers, like sperm whales and great white sharks, visit.
Scientists are decoding the oceans using these instruments. The oceans are major players in the climate system, absorbing about 90 percent of the heat of global warming. To understand global warming, scientists must first understand the oceans.
"When we think about global warming, what we should really thinking about, to be honest, is ocean warming," said Paul Durack, a climate modeler at Lawrence Livermore National Laboratory (LLNL).
Improved data about the oceans from the Argo floats caused a splash this week as two studies in Nature Climate Change challenged conventional thinking.
Durack and his colleagues at LLNL found that the Southern Hemisphere's oceans have warmed at a higher rate over the past 35 years than previously thought.
If that is true, the repercussions would be huge. It would mean that scientists have missed accounting for a portion of the heat resulting from human emissions. Scientists have calculated that a doubling of carbon dioxide concentrations would warm the planet by 1.5 to 4.5 degrees Celsius. Durack's results would place the planet's sensitivity to CO2 toward the higher end of this range.
A second study, also published in Nature Climate Change, found that the deepest parts of the ocean, beyond 6,500 feet, have not warmed by very much in the past decade. Much of global warming's impacts are playing out closest to the surface, said Joshua Willis, a scientist at NASA's Jet Propulsion Laboratory and co-author of the study.
The study set off a furious debate among scientists and oceanographers studying climate change. The world's surface temperatures have risen at a slower rate over the past 15 years than at any time since 1951, according to the Intergovernmental Panel on Climate Change. Some scientists have tied the phenomenon, called the global warming "pause," to the deep oceans' taking up more heat. But the NASA study suggests that may not be the case.
Resolving the reasons for the "pause" would require even better measurements. Scientists are on the case and the Argo devices are plunging to ever greater depths and surfacing with new information.
"We are getting better equipment, better instruments, more of them," Willis said. "Both of these studies are looking at how [ocean temperature] is changing over time. And the more we can learn about what happened in the past, the better we'll be able to predict what is going to happen in the future."
The puzzle of the missing heat
One way to think about global warming is as an imbalance of energy. Solar energy enters the planet and, due to the greenhouse effect, gets trapped within the atmosphere instead of being reflected back into space.
Scientists estimate that every square meter of the planet has received between 0.5 to 1 watt (an average light bulb emits 60 watts of heat) of excess energy in the last few decades. And more than 90 percent of that energy has entered the oceans and warmed them.
The oceans contain 252 billion billion gallons of water, and the energy imbalance caused by climate change is so huge that it affects this vast system. If water is warm, the oceans expand and rises. The energy has contributed to a global sea-level rise of 3.2 millimeters every year since 1993. Sea levels have also risen due to melting glaciers and ice sheets at the poles.
Scientists have been measuring the heat in the warming upper layers since the 1970s, but these measurements have not been very accurate. The Southern Hemisphere's oceans, especially, have been a dark spot.
So to cross-check the heating of the oceans, Durack of LLNL and his colleagues took a roundabout route. They first verified that climate models are accurate using real-world satellite data of sea-level rise. Then they used the climate models to simulate by how much ocean heat content has risen since the 1970s.
Their simulations did not agree with measurements of ocean heat made by scientists since the 1970s, particularly in the Southern Hemisphere. Prior to the Argo initiative, very few measurements were taken in the south, said John Abraham, a professor at the University of St. Thomas in St. Paul, Minn., who was not involved in the study.
"They find the warming of the ocean since 1970 is biased low," he said, "which means there really was more warming than we've thought."
Since the advent of the Argo project, the measurements have improved and the ocean's heat content has matched the predictions of climate models, Durack said.
An argument over the 'nitty-gritty'
The Durack paper suggests that the upper oceans have been warming much more rapidly over the past 35 years than previously thought.
A second paper, by Willis and his colleagues, suggests that the deeper oceans' warming has not contributed to global sea-level rise in the last 10 years. Sea-level rise occurs due to glacier melt and thermal expansion of warming water.
The scientists used data from the Argo floats to figure out by how much the upper oceans have warmed and expanded. They also knew from satellite data the amount of water added to the oceans from glacier melt.
The two measurements, plus warming of the deep ocean, would equal the global sea-level rise of 2.78 millimeters over the last decade. So, through the process of elimination, they figured out the contribution of deep-ocean warming to the observed sea-level rise.
It was negligible.
The study was called "deeply flawed" by Kevin Trenberth, a climatologist at the National Center for Atmospheric Research. He faulted the authors' choice of data and sampling methodology.
The challenge goes to a key problem in climate science today. Sea surface temperatures over the last decade have essentially been at a standstill, which is a problem, since the ocean warms from the top down. So, it would appear, global warming has "paused."
Trenbeth and others have used simulation-based studies to suggest that the ocean is continuing to warm, but the deeper layers have been warming up more in the last decade.
Willis' study suggests this is not the case. That's not to say Willis believes global warming has paused; he does not. He simply thinks other mechanisms are likely to account for it.
"Global warming is still happening; there is still sea-level rise; we are still sucking up more heat than spitting back out to space," Willis said. "What we are arguing about is the nitty-gritty."
The arguments will eventually be resolved by better data, some of which will come from the "Deep Argo" project, by which ocean temperatures will be sampled down to 19,700 feet. The first two floats were launched off the coast of New Zealand in June this year.
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