Humans, not nature, are the cause of changes in Atlantic hurricane cycles, new study finds

It's well known in science that for more than a century hurricane activity in the Atlantic Ocean has oscillated between active and inactive periods, each lasting a few decades. For the past couple of decades, meteorologists and climate scientists have believed that this ebb and flow was due to a natural warming and cooling cycle built into the climate system called the Atlantic Multidecadal Oscillation, or AMO.

The term was coined in the year 2000 by world-renowned climate scientist Dr. Michael Mann, distinguished professor of atmospheric science at Penn State University and author of the new book "The New Climate War." The concept of the AMO has become ubiquitous in explanations and forecasts of active or inactive hurricane seasons. 

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.

Mounting evidence against the AMO

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. 

What it means for hurricane seasons

"Some hurricane scientists have claimed that the increase in Atlantic hurricanes in recent decades is due to the uptick of an internal AMO cycle. Our latest study appears to be the final nail in the coffin of that theory," said Mann. 

If Mann and his colleagues are correct, this is a big deal because it means hurricane activity has been significantly influenced by human activity for many decades. 

"It means that 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," explains Mann.

And in recent decades, there is robust evidence that Atlantic hurricane seasons are getting worse. Just this year the baseline used to determine what's "normal" for a season changed from using a 30-year average centered on 1980-2010, to an average now centered on 1990-2020. When that happened, the typical number of named storms per year jumped markedly, from 12.1 to 14.4 — a 19% increase.

Hurricanes have also gotten stronger. According to research from NOAA climate scientist Dr. James Kossin, in the Atlantic, there's about twice the chance that a hurricane will be at major hurricane intensity (Category 3, 4 or 5), rather than a weaker Category 1 or 2, compared to the chances four decades ago. 

In fact, this past fall, two very late-season Category 4 hurricanes hit Central America back to back, displacing over 500,000 people and triggering a surge of climate-related migration.

Tropical systems also seem to be forming earlier in the season, with six preseason storms in the last five years. This has recently prompted the World Meteorological Organization to contemplate starting the Atlantic hurricane season earlier, on May 15th rather than June 1st. Also, the National Hurricane Center just announced it will start releasing its routine Atlantic tropical outlooks earlier, on May 15th rather than June 1st.

While many meteorologists feel technological advances, like better satellite detection, are a bigger part of the reason for the uptick in these weaker early-season systems, there is also evidence that warmer waters due to climate change play a role in early formations as well.  

Reaction from the scientific community

Given that the Atlantic Multidecadal Oscillation has long been accepted among most weather and climate scientists, CBS News reached out to other prominent scientists for their take on Mann's study.

Dr. Kerry Emanuel, a professor of Atmospheric Science at MIT who studies climate and hurricanes, agrees with Mann's latest conclusion. "My own work pretty much agrees with Mann's work. We also think that aerosols, including man-made aerosols, are to blame for the 1970s-1980s depression [dip in North Atlantic sea surface temperatures and hurricane activity]," explains Emanuel.

However, Dr. Kevin Trenberth, Distinguished Scholar at the National Center of Atmospheric Research, has a somewhat different interpretation. Trenberth said: "I am skeptical of the main result because it depends on climate models which may not all simulate Atlantic variability properly. In the past, a few models have exhibited large AMO-type variability arising from ocean dynamics. But many models may not be realistic in this regard and do not provide a basis for concluding the way this paper does."  

In other words, Trenberth feels many climate models are just not up to the task of simulating the Atlantic Multidecadal Oscillation and, in addition, it is important to validate the ability of the models used in the research. Therefore, although the role of factors external to the climate system, such as volcanic eruptions, may well have been underestimated, Trenberth feels using today's climate models to disprove the existence of the AMO is premature.

With that said, Trenberth does agree that variability from aerosols and greenhouse gases have played at least some role in sea surface temperature changes and therefore changes in hurricane activity in the North Atlantic.

So, while there may not be complete consensus across science about whether human activity explains some or all of the modern cycles of hurricane activity in the North Atlantic, Mann's research does lend significant evidence that the choices made by humankind have influenced our climate system in general, and hurricanes in particular.

And if it is clear that our choices have significantly shaped our hurricane past, that also means our choices can shape our hurricane future. 

"There is now great URGENCY in acting on the climate crisis, but there is also AGENCY," Mann said–– in an 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."

In:
• Atlantic Hurricane Season