In predicting how storms might change in the future, it is helpful to know something about their past. According to historical records from the 1850s, hurricanes in the North Atlantic have become more frequent over the past 150 years.
However, scientists have questioned whether this upward trend reflects reality or is simply an artifact of one-sided records. Had 19th century storm trackers had access to 21st century technology, would they have recorded more storms? This inherent uncertainty has deterred scientists from relying on storm records and the patterns they contain for clues about how climate affects storms.
A new MIT study was published today in Nature communication has used climate modeling instead of storm records to reconstruct the history of hurricanes and tropical cyclones around the world. The study concludes that hurricanes in the North Atlantic have actually increased in frequency over the past 150 years, similar to what historical records have shown.
Large hurricanes in particular, and hurricanes in general, are more common today than in the past. And those who hit the land seem to have become more powerful and harbor more destructive potential.
Strangely enough, while the North Atlantic as a whole saw an increase in storm activity, this trend was not observed in the rest of the world. The study found that the frequency of tropical cyclones around the world has not changed significantly over the past 150 years.
“The evidence, like the original historical records, suggests a long-term increase in North Atlantic hurricane activity, but no significant changes in global hurricane activity,” says study author Kerry Emanuel, Cecil and Ida Green Professor of Atmospheric Science, Earth, Atmospheres – and planetary science from MIT. “It will certainly change the interpretation of the effects of climate on hurricanes – that it really is the regionality of the climate and that something has happened to the North Atlantic that is different from the rest of the world. It could have been caused by global warming, which is not necessarily globally uniform. “
The most comprehensive record of tropical cyclones is compiled in a database known as the International Best Track Archive for Climate Stewardship (IBTrACS). This historical record includes modern measurements from satellites and aircraft that date back to the 1940s. The 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 are from 1851, and overall the database shows an increase in North Atlantic storm activity over the past 150 years.
“Nobody contradicts the fact that this is shown in the historical records,” says Emanuel. “On the other hand, most sane people don’t really trust historical records that go so far back.”
Recently, scientists have used a statistical approach to identify storms that historical records may have missed. To do this, they consulted all digitally reconstructed shipping routes in the Atlantic for the past 150 years and mapped these routes using today’s hurricane tracks. Then they assessed the likelihood that a ship would encounter the presence of a hurricane or miss it altogether. This analysis indicated that a significant number of early storms were likely overlooked in the historical record. Taking these missed storms into account, they concluded that storm activity may not have changed in the last 150 years.
However, Emanuel points out that hurricane trails may have looked different in the 19th century than they are today. In addition, the scientists could have overlooked important shipping routes in their analysis, as older routes have not yet been digitized.
“All we know is that if there had been a change (in storm activity), it would not have been detectable with digitized ship records,” says Emanuel in general. “
Instead, he estimated past hurricane activity using dynamic downscaling – a technique his group developed and used over the past 15 years to study the effects of climate on hurricanes. The technique begins with a rough global climate simulation and embeds a model with a finer resolution in this model that simulates features as small as hurricanes. The combined models are then fed real measurements of the atmospheric and oceanic conditions. Emanuel then sprinkles the realistic simulation with hurricane “seeds” and runs the simulation forward in time to see which seeds bloom into full-blown storms.
For the new study, Emanuel embedded a hurricane model in a climate “reanalysis” – a kind of climate model that combines observations from the past with climate simulations in order to create precise reconstructions of past weather patterns and climatic conditions. He used a certain subset of climate analysis that only takes into account observations from the surface – for example, from ships that have consistently recorded weather conditions and sea surface temperatures since the 1850s, as opposed to satellites that didn’t start systematically monitoring until the 1970s .
“We decided on this approach in order to avoid artificial trends that arise from the introduction of increasingly different observations,” explains Emanuel.
He performed an embedded hurricane model for three different climate analyzes and simulated tropical cyclones around the world over the past 150 years. In all three models he observed “clear increases” in hurricane activity in the North Atlantic.
“There has been a pretty sharp increase in activity in the Atlantic since the mid-19th century that I hadn’t expected,” says Emanuel.
As part of this general surge in storm activity, he also observed a “hurricane drought” – a period in the 1970s and 80s when the number of annual hurricanes temporarily decreased. This lull in storm activity can also be seen in historical records, and Emanuel’s group suggests a cause: sulfate aerosols, which were byproducts of fossil fuel burning, likely triggered a cascade of climate effects that cooled the North Atlantic and temporarily suppressed hurricane formation.
“The general trend over the past 150 years has been increased storm activity, interrupted by this hurricane drought,” notes Emanuel. “And at this point in time we are more confident about why there was a hurricane drought than why activity has been increasing steadily and over the long term since the 19th century. This is still a mystery and how global warming could affect future Atlantic hurricanes. “
This research was supported in part by the National Science Foundation.