Story at a glance

  • Using preindustrial climate and pollution data, scientists are walking back the idea that the Atlantic Multidecadal Oscillation is a real meteorological entity.
  • Instead, human activity is responsible for fluctuations in hurricane seasons, they argue.

When analyzing and tracking hurricanes and storms that develop in the North Atlantic Ocean, meteorologists and scientists have long looked toward the Atlantic Multidecadal Oscillation (AMO) for justification.

Understood as a naturally occurring phenomenon, the AMO is supposed to cycle through warm and cool phases every 20-40 years, which accounts for seasonal hurricane activity. 

New research posits that there is no AMO at all, however, and that changes in hurricane activity within the Atlantic are directly related to human-caused climate change.

Published in the journal Science, a team of researchers argue that the AMO is not an entity in and of itself; rather, it is a manifestation of the effects of anthropogenic greenhouse gas emissions, or fossil fuels emitted into the atmosphere from human activity.

In short, the AMO is not responsible for varying hurricane activity — humans are.


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“We can more convincingly now conclude that the increases in Atlantic hurricane activity are tied to warming that is human-caused, not natural, in nature," said Michael Mann, a professor of atmospheric science at Penn State University who coined the term AMO. His latest research walks his initial theory about cyclical weather patterns back. 

“I was wrong about the existence of an internal AMO oscillation when I coined the term 20 years ago," he told CBS News.

Mann cites pollution data in reevaluating the existence of the AMO, specifically when modeling the countering effects emissions have on global temperatures.

When greenhouse gases are emitted into the atmosphere, they also include sulphate aerosols, which are fine solid particles that have the ability to block light. Greenhouse gases and carbon emissions may work to trap heat in the atmosphere, causing spikes in global temperatures, but the accompanying aerosol particles actually block light from entering the atmosphere, resulting in a cooling effect.

This fluctuation in sporadic cooling and warming runs parallel to the assumed effects of the AMO. Furthermore, the new research adds that preindustrial oscillations in Atlantic Ocean temperatures and weather can be attributed to explosive volcanic eruptions, which can release large volumes of carbon emissions into the atmosphere. 

Given these factors, researchers were not able to recreate the effects of the AMO in a controlled climate system with no external forces.

This study has big implications for both meteorology and climate change studies. Primarily, the frequency and severity in hurricanes is likely another adverse effect of human-driven climate change. 

Recent weather patterns fall into lockstep with this theory; 2020 had an above-average active hurricane season, with two Category 4 hurricanes touching down in Central America.

Still, not all experts are convinced. Kevin Trenberth of the National Center of Atmospheric Research told CBS News that computer models may not be accurate and Mann’s conclusions might be premature.

"There is now great URGENCY in acting on the climate crisis, but there is also AGENCY," Mann told reporters via email. "If we cease adding carbon pollution to the atmosphere, state of the art climate models tell us that the surface warming (which appears tied to more destructive hurricanes) stabilizes within a few years. We can prevent these impacts from getting worse if we act now."


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Published on Mar 09, 2021