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It helps to know something about the past when forecasting how storms will change in the future. Over the last 150 years, hurricanes in the North Atlantic have become more frequent.
Scientists have questioned if the upward trend is a reflection of reality or just an artifact of record keeping. If 19th-century storm trackers had access to 21st-century technology, would they have recorded more storms? Scientists have not been able to rely on storm records for clues to how climate influences storms.
The MIT study used climate modeling to reconstruct the history of hurricanes and tropical storms around the world. Historical records show that the number of hurricanes in the North Atlantic has increased over the last 150 years.
Major hurricanes are more frequent today than in the past. Those that make landfall have grown more powerful.
While the North Atlantic has seen an increase in storm activity, the rest of the world has not. In the last 150 years, the study found no change in the number of tropical cyclones.
Kerry Emanuel, the Cecil and Ida Green Professor of Atmospheric Science in MIT's Department of Earth, Atmospheric, says that the evidence shows that there have been long-term increases in North Atlantic Hurricane activity. It will change the interpretation of climate's effects on hurricanes, that it's really the regionality of the climate, and that something happened to the North Atlantic that's different from the rest of the globe. It may have been caused by global warming.
There are chances of encounters.
The International Best Track Archive for Climate Stewardship is a database that contains the most comprehensive record of tropical cyclones. The records include modern measurements from satellites and aircraft from the 1940s. The database's older records are based on reports from ships that were in the path of a storm. Over the last 150 years, the database shows an increase in storm activity in the North Atlantic.
Emanuel says that the historical record shows that. Most sensible people don't really trust the historical record that far back in time.
Scientists have been using a statistical approach to identify storms that may have missed the historical record. They consulted all the reconstructed shipping routes in the Atlantic over the last 150 years and mapped them over the modern-day Hurricane tracks. They estimated the chance that a ship would miss a storm. The analysis found that a lot of early storms were missed. They found that there was a chance that storm activity had not changed over the last 150 years.
Emanuel believes that the 19th century's Hurricane paths may have looked different than they do today. The scientists may have missed some important shipping routes because they haven't yet uploaded them to the internet.
Emanuel says that if there had been a change in storm activity, it would not have been visible using the ship records.
The storms are being seeded.
He used a technique that his group has used over the last 15 years to study climate's effect on hurricanes. The technique starts with a coarse global climate simulation that is embedded in a model that is small and hurricanes. The models are fed with real-world ocean and atmospheric conditions. Emanuel runs a forward simulation in time to see which seeds bloom into full-blown storms.
Emanuel used a Hurricane model to create 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 subset of climate reanalyses that only account for observations from the surface, which have been recorded from ships since the 1850s, as opposed to satellites, which only began systematic monitoring in the 1970s.
Emanuel says that they chose to use this approach to avoid artificial trends brought about by the introduction of different observations.
He ran an embedded model on three different climate reanalyses to model tropical storms around the world over the past 150 years. He observed "unequivocal increases" in the activity of hurricanes in the North Atlantic.
Emanuel didn't expect to see a large increase in activity in the Atlantic since the mid-19th century.
He observed a "hurricane drought" in the 70s and 80s when the number of yearly hurricanes dropped. Emanuel's group suggests that sulfate aerosols, which were the result of fossil fuel combustion, may have caused the pause in storm activity.
The last 150 years have seen an increase in storm activity, interrupted by the hurricanes. At this point, we're more confident of why there was a hurricanes dry spell than of why there is an ongoing increase in activity that began in the 19th century. That is still a mystery, and it bears on the question of how global warming might affect future Atlantic hurricanes.
The activity of Atlantic tropical cyclones has increased over the past 150 years.
Nature Communications is a journal.
Climate modeling confirms historical records showing a rise in hurricanes.
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