There’s a new study out (and unfortunately gated) from European researchers in Nature Climate Change adding to the case that the oceans have absorbed much of the effect of global warming since 2000.
One of the more popular recent arguments among climate change deniers is that temperatures have not increased since roughly 2000, even as we’ve continued dumping carbon emissions into the atmosphere. The claim falls apart in several different ways. But one of the main ones is that it simply fails to account for the fact that the oceans are themselves part of the planetary ecological system being affected by global warming. And as Reuters reported, one of the findings of the study is that surface temperatures could begin accelerating again if that heat moves back out of the oceans:
Experts in France and Spain said on Sunday that the oceans took up more warmth from the air around 2000. That would help explain the slowdown in surface warming but would also suggest that the pause may be only temporary and brief.
“Most of this excess energy was absorbed in the top 700 meters (2,300 ft) of the ocean at the onset of the warming pause, 65 percent of it in the tropical Pacific and Atlantic oceans,” they wrote in the journal Nature Climate Change.
Lead author Virginie Guemas of the Catalan Institute of Climate Sciences in Barcelona said the hidden heat may return to the atmosphere in the next decade, stoking warming again.
“If it is only related to natural variability then the rate of warming will increase soon,” she told Reuters.
Several previous studies, including Balmaseda et al. (2013) and Nuccitelli et al. (2012) used a combination of improved data collection and new computer modeling to reach essentially the same conclusion. Furthermore, they concluded that anywhere from 30 to 40 percent of the warming over the last ten or so years has occurred in the ocean depths below 700 meters — depths that aren’t always accounted for.
On top of that, there was an unusually high level of La Niña events since 2000, which also covered up some of the warming. La Niña and El Niño are the two poles of the Southern Oscillation, a natural cyclical process that brings cold and warm water, respectively, upwelling to the surface of the equatorial Pacific Ocean. The cold waters accompanying the La Niña oscillation in 2008 were the strongest seen since 1999, and drove sea-surface temperatures as much as two degrees colder than they otherwise would be.
All told, the oceanic temperature increase accounts for much of the “missing heat” — warming that was anticipated based on the scale of carbon emissions, but that standard measurements couldn’t pick up — identified by Kevin Trenberth, one of the authors of the previous studies. It also means that, again contrary to some claims, the Earth’s climate sensitivity probably isn’t lower than previously thought. More likely, we’ll simply see less short-term warming than scientists have anticipated, but offset by greater long-term warming and more rapid sea level rise.
“Global warming is continuing but it’s being manifested in somewhat different ways,” Trenberth told Reuters, adding that the pause in surface warming could last 15 to 20 years. Meanwhile, “recent warming rates of the waters below 700 meters appear to be unprecedented.”