The oceans have been absorbing the lion's share of global warming in recent years, sparing land dwellers the worst of the heat. That's about to change.
Forget the so-called 'pause' in global warming—new research says we might be in for an era of deeply accelerated heating.
While the rate of atmospheric warming in recent years has, indeed, slowed due to various natural weather cycles—hence the skeptics' droning on about "pauses"—global warming, as a whole, has not stopped. Far from it. It's actually sped up, dramatically, as excess heat has absorbed into the oceans. We've only begun to realize the extent of this phenomenon in recent years, after scientists developed new technologies capable of measuring ocean temperatures with a depth and precision that was previously lacking.
In 2011, a paper in Geophysical Research Letters tallied up the total warming data from land, air, ice, and the oceans. In 2012, the lead author of that study, oceanographer John Church, updated his research. What Church found was shocking: in recent decades, climate change has been adding on average around 125 trillion Joules of heat energy to the oceans per second.
How to convey this extraordinary fact? His team came up with an analogy: it was roughly the same amount of energy that would be released by the detonation of two atomic bombs the size dropped on Hiroshima. In other words, these scientists found that anthropogenic climate is warming the oceans at a rate equivalent to around two Hiroshima bombs per second. But as new data came in, the situation has looked worse: over the last 17 years, the rate of warming has doubled to about four bombs per second. In 2013, the rate of warming tripled to become equivalent to 12 Hiroshima bombs every second.
So not only is warming intensifying, it is also accelerating. By burning fossil fuels, humans are effectively detonating 378 million atomic bombs in the oceans each year—this, along with the ocean's over-absorption of carbon dioxide, has fuelled ocean acidification, and now threatens the entire marine food chain as well as animals who feed on marine species. Like, er, many humans.
According to a new paper from a crack team of climate scientists, a key reason that the oceans are absorbing all this heat in recent decades so well (thus masking the extent of global warming by allowing atmospheric average temperatures to heat more slowly), is due to the Pacific Decadal Oscillation (PDO), an El Nino-like weather pattern that can last anywhere between 15-30 years.
In its previous positive phase, which ran from around 1977 to 1998, the PDO meant the oceans would absorb less heat, thus operating as an accelerator on atmospheric temperatures. Since 1998, the PDO has been in a largely negative phase, during which the oceans absorb more heat from the atmosphere.
Such decadal ocean cycles have broken down recently, and become more sporadic. The last, mostly negative phase, was punctuated by a brief positive phase that lasted 3 years between 2002 and 2005. The authors of the new study, Penn State climatologist Michael Mann, University of Minnesota geologist Byron Steinman, and Penn State meteorologist Sonya Miller, point out that the PDO, as well as the Atlantic Multidecadal Oscillation (AMO), have thus played a major role in temporarily dampening atmospheric warming.
"In other words, the 'slowdown' is fleeting and will likely soon disappear."
So what has happened? During this period, Mann and his team show, there has been increased "heat burial" in the Pacific ocean, that is, a greater absorption of all that heat equivalent to hundreds of millions of Hiroshimas. For some, this has created the false impression, solely from looking at global average surface air temperatures, of a 'pause' in warming. But as Mann said, the combination of the AMO and PDO "likely offset anthropogenic warming over the past decade."
Therefore, the "pause" doesn't really exist, and instead is an artifact of the limitations of our different measuring instruments.
"The 'false pause' is explained in part by cooling in the Pacific ocean over the past one-to-two decades," Mann told me, "but that is likely to reverse soon: in other words, the 'slowdown' is fleeting and will likely soon disappear."
The disappearance of the 'slowdown' will, in tangible terms, mean that the oceans will absorb less atmospheric heat. While all the accumulated ocean heat "is certainly not going to pop back out," NASA's chief climate scientist Dr. Gavin Schmidt told me, it is likely to mean that less atmospheric heat will end up being absorbed. "Ocean cycles can modulate the uptake of anthropogenic heat, as some have speculated for the last decade or so, but… net flux is still going to be going into the ocean."
According to Mann and his team, at some point, this will manifest as an acceleration in the rise of global average surface air temperatures. In their Science study, they observe: "Given the pattern of past historical variation, this trend will likely reverse with internal variability, instead adding to anthropogenic warming in the coming decades."
So at some point in the near future, the PDO will switch from its current negative phase back to positive, reducing the capacity of the oceans to accumulate heat from the atmosphere. That positive phase of the PDO will therefore see a rapid rise in global surface air temperatures, as the oceans' capacity to absorb all those Hiroshima bomb equivalents declines—and leaves it to accumulate in our skies. In other words, after years of slower-than-expected warming, we may suddenly feel the heat.
So when will that happen? No one knows for sure, but at the end of last year, signs emerged that the phase shift to a positive PDO could be happening right now.
In the five months before November 2014, measures of surface temperature differences in the Pacific shifted to positive, according to the National Oceanic and Atmospheric Administration. This is the longest such positive shift detected in about 12 years. Although too soon to determine for sure whether this is, indeed, the beginning of the PDO's switch to a new positive phase, this interpretation is consistent with current temperature variations, which during a positive PDO phase should be relatively warm in the tropical Pacific and relatively cool in regions north of about 20 degrees latitude.
In January 2015, further signs emerged that the PDO is right now in transition to a new warm phase. "Global warming is about the get a boost," ventured meteorologist Eric Holthaus. Recent data including California's intensifying drought and sightings of tropical fish off the Alaskan coast "are further evidence of unusual ocean warming," suggesting that a PDO transition "may already be underway a new warm phase."
While it's still not clear whether the PDO is really shifting into a new phase just yet, when it does, it won't be good. Scientists from the UK Met Office's Hadley Center led by Dr. Chris Roberts of the Oceans and Cryosphere Group estimate in a new paper in Nature that there is an 85 percent chance the faux 'pause' will end in the next five years, followed by a burst of warming likely to consist of a decade or so of warm ocean oscillations.
Roberts and his team found that a "slow down" period is usually (60 percent of the time) followed by rapid warming at twice the background rate for at least five years, and potentially longer. And mostly, this warming would be concentrated in the Arctic, a region where temperatures are already higher than the global average, and which is widely recognized to be a barometer of the health of the global climate due to how Arctic changes dramatically alter trends elsewhere. Recent extreme weather events around the world have been attributed to the melting Arctic ice sheets and the impact on ocean circulations and jet streams.
What this means, if the UK Met Office is right, is that we probably have five years (likely less) before we witness a supercharged surge of rapid global warming that could last a decade, further destabilizing the climate system in deeply unpredictable ways.