Intensifying Hurricanes are Real, not Artifact

March 13, 2021


In the year 2000, esteemed climatologist and geophysicist Michael E. Mann of Pennsylvania State University coined a term he would come to loath. “Atlantic Multidecadal Oscillation”, or AMO for short, was used to describe an oscillation pattern between the North Atlantic ocean currents and the wind patterns above. Similar to the El Nino/Southern Oscillation (ENSO), however the AMO lasting over the course of decades rather than a year or two like ENSO.

In an article recently published by Mann, he explains “back in the 1980s and 1990s, a number of articles pointed to a pattern of North Atlantic warming during the 1930s-1950s, subsequent cooling in the 1960s and 1970s, and warming thereafter, which seemed to resemble a natural oscillation in the climate system”.

At the time, Mann and his colleagues had mountains of data collected from the analysis of a long-term ocean-atmosphere model from the Princeton Geophysical Fluid Dynamics Laboratory. This state-of-the-art (at the time) model processed data that expanded over a thousand years in the past and presented the conclusion that over time, the North Atlantic went through several internal warming and cooling periods. Basically, it was something that simply just happens. However, there was one major (unbeknownst) caveat to the model. It ran as a “control” model, meaning it did not factor in external “forcing” such as no greenhouse gas changes, no variations in solar output, no volcanic eruptions, etc.

In Mann’s article, he states “at times I feel like I created a monster when I gave a name to this putative climate oscillation in 2000. The concept of the AMO has since been misapplied and misrepresented to explain away just about every climate trend under the sun, often based on flawed statistical methods that don’t properly distinguish a true climate oscillation from a time-varying trend.”

Essentially, this was the fuel climate change deniers needed to fund a 20-year long campaign that involved a respected climatologist conducting a study that proves that climate change is not caused by humans (anthropologic forces), but rather a natural warming/cooling phenomenon that occurs every few decades.

CBS News:

The image below, from the National Oceanic and Atmospheric Administration (NOAA), shows how hurricane activity seems to flow in roughly 60-year waves — active for around 30 years when the Atlantic in its warm phase and inactive for around 30 years when in the cool phase.

But today, in a newly released paper in the journal Science, the Atlantic Multidecadal Oscillation may have been dealt a deadly blow — by the very man who named it. Mann now concludes the AMO is very likely an artifact of climate change, driven by “human forcing” from rising carbon emissions in the modern era and “natural forcing” due to massive volcanic eruptions in pre-industrial times.

The finding — which is bound to generate significant controversy and pushback from the weather and climate communities due to how broadly accepted the concept of the AMO has become — may very well shake the foundations of understanding of what has been driving historical hurricane cycles. 

Simply put, if true, this discovery means that during the 20th century and beyond, humans — not natural variability — have been the main driving force in the up-and-down cycles of hurricane activity in the Atlantic Ocean.   

“A scientist has to admit when they are wrong,” acknowledges Mann. Mann, who is best known for his now famous Hockey Stick Curve, which illustrates the abrupt spike in temperatures since 1900, says unfortunately he and his colleagues were not wrong about the recent unprecedented warming due to human-caused climate change. “But I was wrong about the existence of an internal AMO oscillation when I coined the term 20 years ago,” he said.

This also has important implications for the future, because as the oceans continue to warm due to climate change, it’s more clear than ever that hurricane activity in the Atlantic will continue to intensify.

“Unfortunately, it tells us that the more we allow the planet to warm up, the worst off we’ll be,” said Mann.

This is not the first paper that Mann has published which casts doubt on the concept of the Atlantic Multidecadal Oscillation. Last January, Mann’s team concluded that the apparent AMO cycle in the modern era was an artifact of industrialization-driven climate change. 

When climate scientists speak about climate change driven by humans, they are generally referring to the warming from the release of greenhouse gases like carbon dioxide and methane. But there is also a counter cooling effect, which was dominant during much of the 20th century. That’s because when fossil fuels are burned it also releases particulate matter pollution, which is partially responsible for the hazy sky over cities, especially on stagnant days. 

Also known as aerosols, the suspended pollution blocks some sunlight from reaching the Earth, and this results in the cooling of the air and oceans. This effect was especially significant in the eastern U.S. and over the Atlantic Ocean adjacent to the U.S. before the Clean Air Act was passed in the early 1970s. The cooling then slowly faded as pollution dropped off gradually, but substantially, over the decades to follow.

Since then, the decline in air pollution combined with warming from heat-trapping greenhouse gases has led to a rise in air and ocean temperatures. This tug-of-war in the Atlantic Ocean between the cooling influence of aerosols and the heating from greenhouse gases, Mann and colleagues claimed in last year’s paper, is the real culprit behind the apparent Atlantic Multidecadal Oscillation in the modern era.

For this new paper, the team took the research a step further and asked: Why does the AMO still seem apparent in pre-industrial records, before humans were an influential factor in shaping the climate?

Their conclusion is that the fluctuation was caused by the periodic cooling effect of large volcanic eruptions in past centuries. The signal observed had an average spacing that resembles an irregular, roughly 60-year AMO-like oscillation.

To determine this, the researchers used state-of-the-art climate models to test two scenarios for pre-industrial times to look back over the past thousand years. In one scenario, external forces such as variances in solar output and the impact of volcanic eruptions were factored in. In the other scenario, “control” simulations were run with no external drivers — since with no external forcing, any changes that happen must be internally generated naturally from within Earth’s climate system. 

When they looked at simulations for the short 3-to-7-year El Niño Southern Oscillation cycles, they found that these cycles occurred in the models without any added forcing by climate change, volcanic activity, or anything else. They appeared to occur as a natural part of the climate system.

However, when they looked for the AMO in the models, it did not occur in the scenarios that left out solar and volcanic forcing. In fact, it only appeared in modern times when climate change variables were included in the simulation, and in preindustrial times when volcanic eruptions were factored in. 

In other words, the Atlantic Multidecadal Oscillation could not be replicated in the models with only the internal natural climate system and no external drivers added in. Therefore, if these models are an accurate representation of the real world, then the apparent AMO is not a natural oscillation at all. 


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