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Climate modeling supports historical data indicating an increase in storm activity

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Climate modeling, rather than storm data, is increasingly being used by scientists to recreate the history of hurricanes and tropical storms throughout the globe. The analysis concludes that the frequency of North Atlantic storms has grown during the previous 150 years, as historical records have demonstrated.

When predicting how storms will alter in the future, it is useful to know something about their history. Hurricanes in the North Atlantic have been increasingly common in the previous 150 years, according to historical data going back to the 1850s.

However, experts have questioned whether this increased tendency reflects reality or is merely a result of unbalanced record-keeping. Would 19th-century storm trackers have documented 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 insights about how climate effects storms.

Climate modeling, rather than storm data, was utilized in a new MIT research published today in Nature Communications to recreate the history of hurricanes and tropical cyclones throughout the globe. The analysis concludes that the frequency of North Atlantic storms has grown during the previous 150 years, as historical records have demonstrated.

Major storms, in particular, and hurricanes in general, are more often now than in the past. Those that do reach ashore have gotten more stronger, with greater catastrophic potential.

Surprisingly, although storm activity in the North Atlantic has increased overall, the same trend has not been seen in the rest of the globe. The research discovered that the worldwide frequency of tropical cyclones has not altered appreciably over the previous 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 very probably modify the understanding of climate’s influence on storms — that it’s actually the regionality of the climate, and that something occurred in the North Atlantic that wasn’t seen elsewhere. It might have been induced by global warming, which is not always uniform.”

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Unexpected encounters

The International Best Track Archive for Climate Stewardship is a database that has the most complete record of tropical cyclones (IBTrACS). This historical record includes observations taken by satellites and airplanes as recently as the 1940s. The oldest entries in the database are based on reports from ships and islands that happened to be in the path of a storm. These previous records extend back to 1851, and the database generally reveals a rise in North Atlantic storm activity over the past 150 years.

“No one can deny that the historical record reveals that,” Emanuel argues. “On the other hand, most rational people don’t have much faith in the historical record so far back in time.”

Scientists have utilized a statistical method to detect storms that the historical record may have missed. They did this by consulting all digitally rebuilt maritime routes in the Atlantic during the previous 150 years and mapping these routes onto modern-day storm paths. They then calculated the likelihood that a ship would meet or completely miss the presence of a storm. According to our approach, a considerable number of early storms were most likely missing in the historical record. After accounting for these missing storms, scientists determined that storm activity had not altered during the previous 150 years.

However, Emanuel points out that storm routes in the nineteenth century may have resembled today’s tracks. Furthermore, since earlier routes have not yet been digitized, the scientists may have overlooked important cargo routes in their investigation.

“All we know is that if there was a shift (in storm activity), it would not have been identifiable using digitized ship records,” Emanuel explains. “So I thought, there’s a chance to do better, by not utilizing historical data at all.”

Storms are being seeded

Instead, he approximated previous hurricane activity using dynamical downscaling, a method developed and used by his lab over the last 15 years to explore the influence of climate on hurricanes. The method begins with a large global climate simulation and embeds a finer-resolution model that simulates characteristics as tiny as hurricanes inside it. Real-world data of atmospheric and ocean conditions are then incorporated into the integrated models. Emanuel then spreads hurricane “seeds” around the realistic simulation and runs the simulation forward in time to observe which seeds grow into full-fledged storms.

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Emanuel inserted a hurricane model into a climate “reanalysis” for the new research, which is a form of climate model that integrates historical data with climate simulations to build accurate reconstructions of previous weather patterns and climatic conditions. He employed a subset of climate reanalyses that solely account for data obtained from the surface, such as from ships, which have regularly documented weather conditions and sea surface temperatures since the 1850s, as opposed to satellites, which started systematic monitoring only in the 1970s.

“We adopted this technique to prevent any false trends caused by the insertion of increasingly varied observations,” Emanuel says.

He used an integrated hurricane model on three separate climate reanalyses to simulate tropical cyclones throughout the globe during the last 150 years. He reported “unequivocal increases” in North Atlantic storm activity across all three models.

“I didn’t anticipate to observe such a big rise in activity in the Atlantic since the mid-nineteenth century,” Emanuel explains.

Within this general increase in storm activity, he also saw a “hurricane drought” – a time in the 1970s and 1980s when the number of annual hurricanes temporarily decreased. This pause in storm activity can also be seen in historical records, and Emanuel’s group proposes a reason: 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 previous 150 years, the overall trend has been growing storm activity, which has been stopped by current hurricane drought,” Emanuel observes. “And, at this time, we’re more certain about why there was a hurricane drought than we are about why there is a long-term rise in activity that started in the nineteenth century. That remains a mystery, and it has implications for how future Atlantic storms may be affected by global warming.”

The National Science Foundation helped fund some of this study.

Source of the story:

The materials have been given by the Massachusetts Institute of Technology. Jennifer Chu wrote the original. Please keep in mind that content may be altered for style and length.

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