It helps to have some knowledge of how hurricanes have changed in the past when predicting how they may change in the future. Hurricanes in the North Atlantic have been increasingly common in the previous 150 years, according to historical records dating back to the 1850s.
Scientists, on the other hand, have questioned whether the growing tendency is a reflection of reality or simply an artifact of skewed data collection. Would 19th-century storm trackers have recorded more storms if they had access to 21st-century technology?
Because of this inherent uncertainty, scientists have been hesitant to rely on storm records and the patterns found within them for insights into how climate affects storms.
Climate modeling, rather than storm records, was utilized in a new MIT study published today in Nature Communications to reconstruct the history of hurricanes and tropical cyclones around the world. The analysis reveals that the frequency of North Atlantic storms has grown over the last 150 years, which is consistent with historical records.
Major hurricanes, in particular, and hurricanes in general, are more often today than in the past. Those that do make ashore have become more powerful, with greater destructive potential.
Surprisingly, while storm activity in the North Atlantic has increased overall, the same trend has not been observed in the rest of the world. The frequency of tropical cyclones has not altered considerably in the last 150 years, according to the study.
The evidence does point, as the original historical record did, to long-term increases in North Atlantic hurricane activity, but no significant changes in global hurricane activity. It will almost probably affect the interpretation of climate’s influence on storms, implying that it’s truly the regionality of the climate and that something unique to the North Atlantic occurred. It could have been triggered by global warming, which isn’t always uniform around the world.
Kerry Emanuel
“The evidence does point, as the original historical record did, to long-term increases in North Atlantic hurricane activity, 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 probably affect the interpretation of climate’s influence on storms, implying that it’s truly the regionality of the climate and that something unique to the North Atlantic occurred. It could have been triggered by global warming, which isn’t always uniform around the world.”
Chance encounters
The International Best Track Archive for Climate Stewardship (IBTrACS) is a database that has the most comprehensive record of tropical cyclones. Modern measurements from satellites and aircraft dating back to the 1940s are included in this historical record.
The oldest records in the database are based on reports from ships and islands that happened to be in the path of a storm. These previous records go back to 1851, and the database demonstrates that North Atlantic storm activity has increased over the last 150 years.
“Nobody disagrees that that’s what the historical record shows,” Emanuel says. “On the other hand, most sensible people don’t really trust the historical record that far back in time.”
Scientists have recently utilized a statistical approach to discovering storms that may have been missed by the historical record. To do so, they looked at all of the digitally recreated shipping routes in the Atlantic during the last 150 years and mapped them over current storm paths. They then calculated the likelihood that a ship would encounter or completely miss a hurricane.
A considerable number of early storms were likely missing in the historical record, according to this analysis. They found that there was a potential that storm activity had not altered over the last 150 years after accounting for these missed storms.
However, Emanuel points out that storm routes in the nineteenth century may have differed from those seen now. Furthermore, because older routes have not yet been digitized, the scientists may have overlooked important cargo routes in their investigation.
“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 calculated past hurricane activity using dynamical downscaling, a technique created by his group and used to analyze climate’s impact on storms for the past 15 years. The method begins with a coarse global climate simulation and then embeds a finer-resolution model that simulates characteristics as small as hurricanes within it.
After then, real-world data of atmospheric and ocean conditions are incorporated into the integrated models. Emanuel then spreads hurricane “seeds” around the realistic simulation and runs it forward in time to see which seeds grow into full-blown hurricanes.
For the new research, Emanuel used a hurricane model into a climate “reanalysis,” which combines historical observations with climate simulations to provide accurate reconstructions of past weather patterns and climatic conditions.
He employed a subset of climate reanalyses that solely account for data made on the ground, such as from ships, which have been recording weather conditions and sea surface temperatures since the 1850s, as opposed to 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 used an embedded hurricane model to simulate tropical cyclones around the world for the last 150 years on three separate climate reanalyses. He saw “unambiguous rises” in North Atlantic storm activity across all three models.
“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 noticed a “hurricane drought” in the 1970s and 1980s, when the number of yearly hurricanes decreased for a brief period.
Sulfate aerosols, which were byproducts of fossil fuel combustion, likely kicked off a cascade of climate changes that cooled the North Atlantic and briefly reduced hurricane formation, according to Emanuel’s group.
“The general trend over the last 150 years was increasing storm activity, interrupted by this hurricane drought,” Emanuel notes. “And at this point, we’re more confident of why there was a hurricane drought than why there is an ongoing, long-term increase in activity that began in the 19th century. That remains a mystery, and it has implications for how future Atlantic hurricanes may be affected by global warming.”
This research was supported, in part, by the National Science Foundation.
