This map of the Earth shows the spacial pattern of temperature variance by percentage. The most variance is seen in the tropics with less at the poles. IMAGE: DANIEL J. BROUILLETTE. PENN STATE

UNIVERSITY PARK, Pa. — Volcanic eruptions, not natural variability, were the cause of an apparent “Atlantic Multidecadal Oscillation,” a purported cycle of warming thought to have occurred on a timescale of 40 to 60 years during the pre-industrial era, according to a team of climate scientists who looked at a large array of climate modeling experiments.

The result complements the team’s previous finding that what had looked like an “AMO” occurring during the period since industrialization is instead the result of a competition between steady human-caused warming from greenhouse gases and cooling from more time-variable industrial sulphur pollution.

“It is somewhat ironic, I suppose,” said Michael E. Mann, distinguished professor of atmospheric science and director, Earth System Science Center, Penn State. “Two decades ago, we brought the AMO into the conversation, arguing that there was a long-term natural, internal climate oscillation centered in the North Atlantic based on the limited observations and simulations that were available then, and coining the term ‘AMO.’ Many other scientists ran with the concept, but now we’ve come full circle. My co-authors and I have shown that the AMO is very likely an artifact of climate change driven by human forcing in the modern era and natural forcing in pre-industrial times.”

The researchers previously showed that the apparent AMO cycle in the modern era was an artifact of industrialization-driven climate change, specifically the competition between warming over the past century from carbon pollution and an offsetting cooling factor, industrial sulphur pollution, that was strongest from the 1950s through the passage of the Clean Air Acts in the 1970s and 1980s. But they then asked, why do we still see it in pre-industrial records?

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