VICE News is closely tracking global environmental change. Check out the Tipping Point blog here.
When talk turns to global warming, most of the attention is on rising temperatures on land and the impacts already being seen like historic droughts and melting ice sheets.
But to truly understand the remarkable ways in which human emissions are altering Earth's climate system, scientists need to find out what is going in the world's oceans. That was difficult until the past decade, when better technology like that of Argo, a network of 3,200 robotic floats, allowed them to get temperature and salinity data across the globe at depths of 6,500 feet below the sea surface.
The results have been a revelation.
Now, researchers have published one of the most detailed pictures yet of how much heat is going into the oceans.
Scientists from Lawrence Livermore National Laboratory, the National Oceanic and Atmospheric Administration, Princeton University, and Penn State University found that half the global ocean heat content since 1865 has occurred in the past two decades. And contrary to an early study that found the deep oceans were barely being touched by global warming, the researchers found as much as 35 percent of that was being taken up below 2,300 feet.
"The takeaway is that the rate, at which the global ocean is absorbing excess heat, has rapidly increased — so that in more recent times since 1997, it has absorbed as much heat as it took over 100 yeas to absorb," Peter Gleckler, a Lawrence Livermore scientists and the lead author on the paper, said. "That is alarming."
Increases in ocean temperatures near the surface mostly due to global warming have been documented since the 1970s. But there had been much less data collected from the deep ocean, especially in the Southern Hemisphere.
The paper helps fill in many of those holes by including data at greater depths to offer a more complete picture of the ocean warming — a step that many consider critical to understanding how the world is changing due to global warming and helping get an accurate picture of the pace of that warming.
"Given the importance of the ocean warming signal for understanding our changing climate, it is high time to measure the global ocean systematically from the surface to the ocean floor," NOAA oceanographer Gregory Johnson, a co-author on the paper, said in a statement.
The researchers combined some very old scientific data from the late-19th century voyage of the HMS Challenger with the most comprehensive data currently available of the world's oceans in order to arrive at their results, published Monday in the journal Nature Climate Change.
For climate scientists not involved in the study, the results confirm the ocean's role as a storehouse for much of the warming on the planet and echo climate model projections. The uptake of this heat is contributing to higher temperatures in the oceans, possibly impacting ocean circulation patterns as well as contributing to rising sea levels, which have increased 3.2 millimeters a year since 1993.
"This is an important study, largely confirming existing knowledge about human-induced climate change," Matt Palmer, climate scientist at the Met Office Hadley Centre who did not take part in the study, said in a statement.
"It highlights the need for more deep-ocean observations to better monitor current and future climate change," he said. "It also confirms that ocean heat uptake has been proceeding at the expected rate — the 'hiatus' is a surface phenomenon."
While surface temperatures have continued to rise over the past two decades, the rate of increase has diminished — a phenomenon that some have called a global warming hiatus. The study shows, however, that the warming has simply been absorbed by the oceans, rather than remain in the atmosphere.
Kevin Trenberth, a senior scientist at the National Center for Atmospheric Research, said he was "unimpressed with the paper," adding that he plans to publish a paper demonstrating that the rate of ocean warming in the past 40 years was about 25 percent more than found in current Nature Climate Change study and that more of it is being stored in the deeper ocean than estimated.
"The general story is that this paper has got is reasonable, except it's not the final answer," he said.
Gleckler countered that Trenberth missed the point of the paper, which wasn't aimed at providing new estimates of ocean warming. Rather, the goal was to provide "a broader picture, which hasn't been done previously, and that is to include the deeper ocean and look back in time to the 19th century."
But Glecker acknowledged that their paper wouldn't be the final word.
"Now that we are finding the deep ocean is becoming more and more important, we need more observations," he said.
Follow Michael Casey on Twitter: @Mcasey1