A 20-Year-Old Climate Mystery Has Finally Been Explained

Scientists giveth and scientists taketh away. 

Michael Mann, distinguished professor of atmospheric science at Pennsylvania State University, came up with a novel term 20 years ago to describe something he and his colleagues had found in their research. About every half-century, wind and waves conspired to warm up or cool down part of the North Atlantic, with probable large-scale effects on weather. Drawing on the same Earth simulations on which most climate research was then based, they concluded that the back-and-forth swing must be a feature intrinsic to the natural system itself. Mann dubbed it the “Atlantic Multidecadal Oscillation,” or AMO.

Since then, the AMO has become a commonly recognized feature of meteorology, similar to the occasional temperature flips in the tropical Pacific that mark warm El Niño or cool La Niña episodes but with smaller significance, and thought to influence the strength of Atlantic hurricanes.

Only here’s the thing—the Atlantic Multidecadal Oscillation turns out not to actually exist. That’s the latest and definitive conclusion now from Mann and three colleagues, who write in the journal Science that the newest climate models can no longer find any evidence of a natural temperature flip in the Atlantic every few decades. 

Instead, the data has led them to another smoking gun—smoking volcanoes. 

Research of ancient climates suggests that before industrialization took off in the 19th century, volcanoes periodically belched out enough sulfate aerosols to explain the “oscillation” going back a thousand years. These aerosols have a cooling effect by reflecting about a quarter of sunlight back into space until they wash out of the atmosphere within months or a few years. In the 20th century, the steady concoction of manmade warming and cooling pollution likely influenced the temperature swings.

“This article is the final nail in the coffin,” said Mann, who last month chronicled his experiences bringing climate science into the public sphere in a book, The New Climate War.

The AMO’s illusory nature, if confirmed, might cause some dissonance for weather forecasters, who have built it into their meteorological models. The AMO is frequently mentioned in seasonal storm predictions, including those issued by the U.S. Climate Prediction Center and many commercial forecasters. Those who subscribe to the theory often base predictions of greater-than-normal storm activity, for example, on the Atlantic being in a warm phase.

Mann said that by focusing on the climate change-related warming of seawater instead of decadal oscillations, his team was able to beat the U.S. Climate Prediction Center and Colorado State University in forecasting what turned out to be last year’s epic 30-hurricane season. Mann’s group predicted 24; CPC said 13 to 19, and CSU 16. The previous record was 2005’s 28 storms. The long-term average is 12.

Retiring the AMO would also end the confusion around its closely named cousin, the Atlantic Meridional Overturning Circulation, or AMOC. Also called the Gulf Stream, this liquid conveyor belt carries heat from the tropics up to the North Atlantic before cooling, sinking and returning south. Climate models have long predicted that the Gulf Stream would slow down as the world warms—a forecast that’s now coming true, according to a study published last month in the journal Nature Geoscience. U.K. and German scientists found that human-driven changes to the climate system have reduced the circulation to its lowest level in a millennium. Further weakening could lead to more extreme weather events in Europe and faster sea-level increases along the U.S. East Coast.

Not everyone was persuaded that the AMO mattered that much to annual hurricane forecasts. Phil Klotzbach, an atmospheric research scientist at CSU who wasn’t involved in the paper, said that conditions that change only once every generation or two don’t play a role in his annual work predicting hurricane seasons. “If for some reason the AMO is completely just an artifact, it wouldn’t change the way that we do our seasonal forecasts,” he said. “At the end of the day, warm water and low shear”—a measure of wind speed or direction—“lead to active hurricane seasons, regardless of what's driving them.”

For Mann, the emergence of the AMO—and, if the community agrees with him, its de-emergence—is a perfect example of the scientific process working itself out. “A scientist has to admit when they are wrong,” he wrote in an essay posted today to the RealClimate.com.

Decades of research keep confirming that, when it comes to climate change, they’re not wrong—or if they are, it’s thinking that changes would occur more slowly than they have. “We’re seeing some climate change impacts play out faster than expected,” Mann said. “But the latest science also conveys agency.”

The planet should halt its warming within a few years after greenhouse-gas emissions end, he said. “That means there’s a direct and immediate impact of our efforts to reduce carbon emissions now.”

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