Where’s the heat? Greenhouse gases, particularly carbon dioxide, continue to be pumped into the atmosphere, but sometime around 1998, the rise in Earth’s average temperatures slowed, deviating from the rates predicted by models. Scientists have proposed that what some call “the pause” could be the result of a number of factors, including heat storage in deep ocean waters to unexpectedly high amounts of aerosols in the stratosphere helping deflect solar rays back into space. Now, a new study suggests that natural cycles in the Pacific Ocean are the culprit.
Since the end of last El Niño warming event of 1997 to 1998, the tropical Pacific Ocean has been in a relatively cool phase—strong enough to offset the warming created by greenhouse gas emissions. But, this is just a temporary balm: When the switch flips and the waters turn warm again, the researchers say, Earth will likely continue warming.
“What this study addresses is what’s better described as a false pause, or slowdown,” rather than a hiatus in warming, says climate scientist Michael Mann of Pennsylvania State University, University Park. Some climate change deniers have taken encouragement from the pause, saying they show warming predictions are flawed, but Mann, a co-author on the study, notes that “there have been various explanations for why [the slowdown is happening], none of which involve climate models being fundamentally wrong.”
One of the biggest lingering issues in the global warming slowdown is the full impact of the natural temperature cycles of Earth’s oceans. The waters of the Pacific flip back and forth between warm and cool as part of a 16- to 20-year-long cycle known as the Pacific Decadal Oscillation. That oscillation includes the 3- to 7-year-long warm El Niño/cold La Niña cycle. Overprinted on that is a longer term oscillation of sea surface temperatures in the Pacific, a cycle that lasts perhaps 50 to 70 years. Similarly, in the Atlantic, sea surface temperatures go through a long-term natural cycle (the Atlantic Multidecadal Oscillation) that lasts about 50 to 70 years.
In order to understand Earth’s recent temperature record, it’s essential to understand the impacts from these natural cycles, says Byron Steinman, a paleoclimatologist at the University of Minnesota’s Large Lakes Observatory in Duluth and lead author of the new study. Previous methods haven’t perfectly isolated the impact of these cycles, Steinman says. So, along with Mann and analyst Sonya Miller, also of Pennsylvania State University, he devised a new method that mined data from the full suite of state-of-the-art simulations now available. The 45 or so models used in the Intergovernmental Panel on Climate Change (IPCC) are run by giant supercomputers around the world to simulate ocean-atmosphere interactions, and they have created about 170 “realizations” of Earth’s temperature history, which don’t precisely match one another due to the complicated climate physics involved.
“Earth’s internal climate variability is inherently random,” Steinman says. “If you went back to 1850 and repeated history”—meaning the same volcanic eruptions, the same solar variability, the same greenhouse gas emissions—“the overall temperature increase would be about the same, but you would end up with somewhat different temperature records due to the inherent randomness in the climate.”
To cancel out that randomness, Steinman and his co-authors first took the realizations created by these different supercomputers and averaged them all together. Then, they subtracted the well-known inputs attributed to human and natural drivers—greenhouse gas emissions, volcanic aerosols, solar output—from the observational record. “What’s left over should be the internal component,” Mann says, including “El Niño and La Niña, the things that come and go without any driver to make them happen.”
What Steinman and colleagues found was that both the Atlantic Multidecadal Oscillation and the Pacific Decadal Oscillation had a big impact on the Northern Hemisphere’s temperatures over the last century or so—but in the last couple of decades, it’s the Pacific Ocean that’s been driving the slowdown, they report online today in Science. The team identified a cooling trend in the Pacific Ocean and a very slight warming trend in the Atlantic Ocean since the late 1990s. That’s in contrast to some recent work that has suggested the Atlantic Ocean is driving the slowdown by burying the missing heat in its deep waters. The research does show natural variability in the Atlantic playing a more significant role in modulating the planet’s temperature record earlier in the last century, however.
Steinman and his team’s approach is “novel for a couple of reasons,” says Ben Booth, a climate scientist at the Met Office Hadley Centre in Exeter, U.K. Although it’s already widely accepted in the community that the Pacific Ocean plays a large role, this paper gives a much longer time context, he says, highlighting the role of both oceans over many decades. “That’s a very new picture.”
The study also highlights the lingering question of the planet’s sensitivity to other drivers, particularly aerosols, Booth adds. “One reason that we haven’t appreciated the role of aerosols in the climate system is that many—most—models don’t include aerosol-cloud interactions,” including only a handful of those used in IPCC’s fifth assessment report, released in 2014. But, by including all such models that now exist, this study marks “the first time anyone has estimated the ocean’s role while accounting for these processes,” he says.
Because the Pacific cooling is just one piece of an ongoing cycle, the slowdown in warming isn’t going to last, Steinman says. “It’s fair to say that over the next couple of decades, we would expect to see the trend reverse, and internal variability accelerating the warming.” Mann words it more strongly: “It’s a double-edged sword, and we’re about to see the other edge of that sword.”