Climate modeling, rather than storm records, is now being used by scientists to reconstruct the history of hurricanes and tropical cyclones around the world. The study concludes that the frequency of North Atlantic hurricanes has increased over the last 150 years, as historical records have shown.
It helps to understand the history of storms when forecasting how they will change in the future. Hurricanes in the North Atlantic have become more common in the last 150 years, according to historical records dating back to the 1850s.
However, scientists have questioned whether this upward trend reflects reality or is simply a result of lopsided record-keeping. Would 19th-century storm trackers have recorded more storms if they had access to 21st-century technology? Because of this inherent uncertainty, scientists have avoided relying on storm records and the patterns within them for clues about how climate influences storms.
Climate modeling, rather than storm records, was used in a new MIT study published today in Nature Communications to reconstruct the history of hurricanes and tropical cyclones around the world. The study concludes that the frequency of North Atlantic hurricanes has increased over the last 150 years, as historical records have shown.
The evidence does point to long-term increases in North Atlantic hurricane activity, as the original historical record did, but no significant changes in global hurricane activity. It will almost certainly change the interpretation of climate’s effects on hurricanes — that it’s really the regionality of the climate, and that something happened in the North Atlantic that wasn’t seen elsewhere.
Kerry Emanuel
In particular, major hurricanes, and hurricanes in general, are more frequent today than in the past. And those that make landfall appear have grown more powerful, carrying more destructive potential.
Curiously, while the North Atlantic has seen an overall increase in storm activity, the same trend was not observed in the rest of the world. The study found that the frequency of tropical cyclones globally has not changed significantly in the last 150 years.
“The evidence does point to long-term increases in North Atlantic hurricane activity, as the original historical record did, but no significant changes in global hurricane activity,” says study author Kerry Emanuel, the Cecil and Ida Green Professor of Atmospheric Science in MIT’s Department of Earth, Atmospheric, and Planetary Sciences. “It will almost certainly change the interpretation of climate’s effects on hurricanes — that it’s really the regionality of the climate, and that something happened in the North Atlantic that wasn’t seen elsewhere. It could have been caused by global warming, which is not always uniform.”
Chance encounters
The most comprehensive record of tropical cyclones is kept in a database known as the International Best Track Archive for Climate Stewardship (IBTrACS). This historical record includes measurements from satellites and aircraft dating back to the 1940s. Older records in the database are based on reports from ships and islands that happened to be in the path of a storm. These earlier records date back to 1851, and the database overall shows an increase in North Atlantic storm activity over the last 150 years.
“No one disputes that that is what the historical record shows,” Emanuel says. “On the other hand, most reasonable people don’t really trust the historical record that far back in time.”
Scientists recently used a statistical method to identify storms that the historical record may have missed. They did this by consulting all of the digitally reconstructed shipping routes in the Atlantic over the last 150 years and mapping them over modern-day hurricane tracks. They then calculated the likelihood that a ship would encounter or completely miss the presence of a hurricane. According to this analysis, a significant number of early storms were most likely missed in the historical record. After accounting for these missed storms, they concluded that storm activity had not changed over the previous 150 years.
But Emanuel points out that hurricane paths in the 19th century may have looked different from today’s tracks. What’s more, the scientists may have missed key shipping routes in their analysis, as older routes have not yet been digitized.
“All we know is, if there had been a change (in storm activity), it would not have been detectable, using digitized ship records,” Emanuel says “So I thought, there’s an opportunity to do better, by not using historical data at all.”
Seeding storms
Instead, he estimated past hurricane activity using dynamical downscaling, a technique developed and used by his group over the last 15 years to study the effect of climate on hurricanes. The method begins with a coarse global climate simulation and embeds a finer-resolution model that simulates features as small as hurricanes within it. Real-world measurements of atmospheric and ocean conditions are then fed into the combined models. Emanuel then scatters hurricane “seeds” across the realistic simulation and runs the simulation forward in time to see which seeds bloom into full-fledged storms.
For the new study, Emanuel embedded a hurricane model into a climate “reanalysis” — a type of climate model that combines observations from the past with climate simulations to generate accurate reconstructions of past weather patterns and climate conditions. He used a particular subset of climate reanalyses that only accounts for observations collected from the surface — for instance from ships, which have recorded weather conditions and sea surface temperatures consistently since the 1850s, as opposed to from satellites, which only began systematic monitoring in the 1970s.
“We chose to use this approach to avoid any artificial trends brought about by the introduction of progressively different observations,” Emanuel explains.
He ran an embedded hurricane model on three different climate reanalyses, simulating tropical cyclones around the world over the past 150 years. Across all three models, he observed “unequivocal increases” in North Atlantic hurricane activity.
“There’s been this quite large increase in activity in the Atlantic since the mid-19th century, which I didn’t expect to see,” Emanuel says.
Within this overall increase in storm activity, he also noticed a “hurricane drought” – a period in the 1970s and 1980s when the number of yearly hurricanes temporarily decreased. This pause in storm activity can also be seen in historical records, and Emanuel’s group proposes a reason for it: sulfate aerosols, which are byproducts of fossil fuel combustion, likely set off a chain reaction of climate effects that cooled the North Atlantic and temporarily suppressed hurricane formation.
“Over the last 150 years, the general trend has been increasing storm activity, which has been interrupted by this hurricane drought,” Emanuel observes. “At this point, we’re more certain about why there was a hurricane drought than we are about why there is a long-term increase in activity that began in the nineteenth century. That is still a mystery, and it bears on the question of how global warming might affect future Atlantic hurricanes.”
