Hurricane Bob struck 30 years ago. Scientists unsure how climate change will alter Atlantic hurricanes?

Editor's note: This article is from August 2021, the 30th anniversary of when Hurricane Bob made landfall in the New England region

Tropical Storm Henri is projected to intensify into a hurricane and, for now, is following in the footsteps of Hurricane Bob, tracking towards the Cape and Islands.

"It’s inevitable we are going to get hit by a major hurricane in New England. I can’t say when, but from our history, you just have to expect it,” said Robert Thompson, who retired from the National Weather Service in 2018 after 45 years. During his career he served as the meteorologist in charge at the East Boston and Norton regional offices for 29 years.

Following a chilling report by the United Nation’s Intergovernmental Panel on Climate Change two weeks ago — which U.N. Secretary-General António Guterres called a “code red for humanity” — the question is how global warming affects hurricanes in the Atlantic.

When our time comes, will the Cape and Islands be looking at something a lot bigger and stronger than Bob, or Carol, or the Hurricane of ’38 or even the Great Colonial Hurricane of 1635?

That is something that is far harder to parse out from climate data than global trends, experts say, but most hurricane watchers do agree the Northeast and Cape Cod and the Islands are way overdue.

The National Weather Service estimates the average return time for a hurricane hitting the Cape is 16 years. By the time Bob, which at its peak was a Category 3 hurricane, hit the Cape and Islands in 1991 it wasn’t considered a major hurricane. A Category 3 or higher is considered a major hurricane with sustained winds exceeding 111 mph and causing damage to even well-built homes.

We are right on schedule for a major hurricane, according to the National Weather Service which estimates the Cape experiences an interval of 58 years between those bigger storms. It's been 67 years since our last direct hit from a major hurricane — Carol in 1954, which was a Category 3 storm.

The quiet before Hurricane Bob hit the Cape and Islands

Thirty years ago, Thompson was one of the few on weekend duty in East Boston as Hurricane Bob intensified into a hurricane off the Bahamas on a Friday, then made the turn north. By noon on Sunday, a hurricane watch was declared for the Cape and Islands.

The office was eerily quiet all weekend, Thompson recalled, with no phones ringing.

On a glorious late summer weekend, he felt many weren’t aware of the danger on their doorstep, and he started urgently calling emergency management personnel to let them know.

Overnight, Bob strengthened into a Category 3 hurricane, picked up forward speed and barreled towards Southern New England and the Cape and Islands.

Many on the Cape woke up Monday morning with beach plans that quickly turned to evacuation planning. Bob struck Rhode Island as a Category 2 storm, according to National Weather Service accounts, lashing the Cape and Islands with sustained winds of 75 to 100 mph with gusts up to125 and a storm surge of six to nine feet.

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It caused $1.5 billion in damage along the East Coast, with $680 million in Southern New England, and killed 17, including at least four on the Cape.

There were hundreds of thousands of downed trees across the Cape, entire neighborhoods carried away or damaged by the storm surge, power outages that took weeks to restore for some, and hundreds of vessels thrown up onshore and damaged.

Will Atlantic ocean hurricanes be more intense due to climate change?

The question researchers are pondering now is how much human-induced global warming will affect the strength of the hurricanes to come.

“One of the issues with the Atlantic basin is that there is strong multi-decadal variability. That makes it hard to determine trends that may be caused by increasing greenhouse gases,” said Thomas Knutson, a senior scientist at NOAA’s Geophysical Fluid Dynamics Laboratory in Princeton, N.J.

The problem in detecting the fingerprint of greenhouse warming on Atlantic hurricanes is that multi-decadal variability caused by either natural cycles in the ocean or changes in human-caused aerosols (particulates and gases emitted from cars, manufacturing, power plants and other human activities) over the Atlantic can mask greenhouse gas-related trends caused by man.

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Warm water temperatures are the fuel that drive hurricanes. Even though scientists agree that ocean temperatures are rising and that anthropogenic (human activity) global warming plays a role in that, large oceanic variations in climate such as the Atlantic Multi-decadal Variability — in which Atlantic sea surface temperatures fluctuate between roughly 30-year periods play a major role in modulating hurricane activity.

“It’s hard for us to say what part of observed Atlantic hurricane changes is human-caused. There are a lot of open questions still, that become more pointed at regional scales,” Knutson said.

Still, there are at least trends discernible at the global level that over 190 scientists worldwide agreed to in consensus statements in the latest Intergovernmental Panel on Climate Change (IPCC) report.

For tropical cyclones, the consensus opinion is:

• Human activity was very likely the main driver of sea-level rise that would allow storm surge and the destructive power of waves from hurricanes to reach further inland.

• The scientists had medium to high confidence that under a 2-degree C (3.6 F) global warming scenario there would be 10%-15% increased rainfall from hurricanes and cyclones within about 60 miles of the storm due to human-caused warming of the atmosphere allowing it to hold more moisture.

• The scientists had medium to high confidence that elevated sea surface temperatures and other factors would cause an increase of hurricane intensity of between 1% and 10% depending on the model used.

• The scientists had a medium to high confidence that the global proportion of hurricanes reaching the highest categories (4 and 5) with the most destructive potential would also increase, although the question of whether there would be more hurricanes and extreme storms remained debatable.

“The whole climate system is very complex, (and) you have to be careful reading it,” Knutson said.

When the IPCC says an increase in global tropical cyclone intensity is expected, they mean the global average intensity, he said. That doesn't mean it will happen that way in every region.

"For a relatively small region, such as New England, there could be changes in hurricane track or frequency that make the problem worse or not as bad, but those potential changes are the ones that scientists currently have less confidence in,"  Knutson said.

Rapidly heating North Atlantic waters worries scientists

One recent development that worries physical oceanographer Glen Gawarkiewicz of the Woods Hole Oceanographic Institution is the warm-core rings spinning off the Gulf Stream and warming coastal waters particularly at the edges of the continental shelf.

Gawarkiewicz is concerned that the waters off Cape Cod and in the Northwest Atlantic are warming rapidly, faster than just about any other ocean. Warm-core rings are enormous swirling cylinders of warm water 30 miles across and up to 2,600 feet deep, spun off from the Gulf Stream that may be contributing to that warming, he said.

Gawarkiewicz was co-author with Jacob Forsyth and Magdalena Andres of a paper last year that looked at the role of these eddies in slowing, even reversing the flow of a coastal cold water current known as the Shelfbreak Jet.

Hurricanes feed off warm water with temperatures in excess of 80 degrees F. In water temperatures below that threshold, they begin to weaken. Traditionally, that helped to blunt the force of storms heading our way, but research by Gawarkiewicz and his colleagues found an increase in the number of warm water eddies and he worried they might allow storms to retain more power.

While Gawarkiewicz agrees that there are reasons to be cautious about predicting trends in Atlantic hurricanes, he also noted that we have little time to get it right.

"Things are happening quickly now and we don't understand the dynamics of these changes, which is a worry," he said.

In other areas of the world, scientists are already documenting some of the projections noted in the IPCC report. Gawarkiewicz said fellow researcher Avijit Gangopadhyay of UMass Dartmouth is working with Indian scientists looking at the Bay of Bengal where the ocean is also warming rapidly. Recently, two major storms, Category 4 and 5, occurred before the onset of the monsoon season.

"That didn't used to be the case," said Gawarkiewicz.

Research review points to more intense hurricanes in the future

Other mechanisms could also come into play. Some studies show that wind shear, a change in wind speed or direction in the upper atmosphere that tears tropical storms and hurricanes apart, could increase as the tropical Atlantic experiences substantial warming, meaning fewer hurricanes and tropical storms will survive to reach land.

In an Aug. 9 review of existing studies and computer models to assess global warming’s effects on hurricanes, Knutson found that while “it is likely that greenhouse gas warming will cause hurricanes in the coming century to be more intense globally and have higher rainfall rates than present-day hurricanes … there is little evidence from current dynamical models that 21st-century climate warming will lead to large … increases in tropical storm numbers, hurricane numbers or PDI (an index combining frequency, intensity and duration) in the Atlantic.”

Models do show that the lifetime maximum intensity of hurricanes could increase by 5% over this century, Knutson wrote in his analysis and there does seem to be growing evidence that a small fraction of tropical cyclones that reach higher Category 4 or 5 intensity levels has increased globally and will continue to increase.

“From a purely tropical cyclone standpoint, a number of factors go into how bad a season we have,” Thompson said. “However … warmer air does tend to hold more moisture and that can result in more intense rainfall.”

Sea level rise expected to continue even if CO2 emissions are reduced

Other factors also influence the extent of hurricane damage. In the 30 years since Bob, there's been a lot of development, larger more valuable homes, along our shoreline. And seemingly implacable sea level rise will extend the punishing reach of waves and tidal surge farther inland.

The consensus in the IPCC report was that scientists were very confident about sea-level rise continuing despite emissions reduction as higher atmospheric temperatures already present will continue to melt glaciers and ice sheets and heat and expand ocean water volume.

“We are starting from a higher (sea level) base if there is a storm and that is a concern,” Thompson said. “Storm surge from hurricanes and nor’easters will now be on top of that higher base sea level.”

While forecasting storm tracks, predicting storm intensity, and the emergency notification, preparedness, and evacuation protocols have all been improved, some substantially, emergency planners worry that people may be complacent.

“A big concern, aside from global warming, is that it has been so long (since a hurricane hit our region),” said Thompson.

The biggest damage potential is close to the eye and a relatively subtle shift in the hurricane track makes a big difference. If the 1938 hurricane had tracked just 40 miles to the east, Thompson said, Buzzards Bay would have seen that plus-20-foot storm surge.

People in high-risk areas like Buzzards Bay have never seen that kind of storm in their, or their parents' lifetimes, and might rely on past experience in making their decision on whether to evacuate.

“Despite our preparedness efforts, will people move out of harm’s way?” Thompson said.

Tips from NOAA for hurricane evacuation 

Contact Doug Fraser at dfraser@capecodonline.com. Follow him on Twitter: @dougfrasercct.

This article originally appeared on Cape Cod Times: Hurricane Bob and Tropical Storm Henri: Are Cape Cod storms changing