The destruction and death of lives can be caused by tropical cyclones such as Hurricane Ida. Over the past few decades, hurricanes and tropical cyclones along the US East Coast have become more severe than other weather types. While there are some questions about the magnitude of this increase in intensity, there is evidence to suggest that these storms are moving slower than they did in the past. Storms that move slower produce more rain and last longer. It is not clear how uncommon these slow-moving Cyclones are in comparison to other weather patterns, as conventional weather records only date back to 1948.
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Recent research addresses this question using tree rings to reconstruct hundreds years of seasonal cyclone precipitation levels. Over 300-year-old trees were studied and found that precipitation extremes have increased by 2 to 4 meters per decade. This has resulted in an increase in precipitation of up to 128mm (5 inches) over the early 1700s. The most dramatic increases have been seen in the last 60 years. Recent extremes are unparalleled by previous events.
Researchers are using these data sets to make forecasts about the region's future.
Great for Tree Growth
Justin Maxwell and collaborators discovered that longleaf pine trees along the East Coast of the US could be used as indicators for tropical cyclone precipitation. This was based on their late-season (June to September) growth bands. These studies were smaller and more localized, and showed that recent precipitation levels were much higher than any they had previously experienced in their lives.
This is a surprising finding as tree-ring records often show evidence of extreme weather throughout their history. However, the frequency might vary. This discovery led to a new study that examined whether the pattern was consistent across a larger area.
Maxwell explained to Ars Technica that tree-ring reconstructions often show us that extreme climates we have observed with instruments (weather stations), over the past 120 years are not comparable. Our past research revealed that extremes in recent times were not comparable. The highest values are generally since the 1990s. This was a huge surprise and motivated us to explore a wider area to determine if the increase was localized or spread over a larger area.
The researchers combined existing data with two new locations to include trees from seven different sites in North and South Carolina. This region is the one that receives the most rainfall from tropical cyclones in North America. It also has the best record of this type precipitation.
New data sets contained a selection from 13 to 36 old growth trees per site. These were taken in a way that did not cause any damage to the trees and stumps. Next, the researchers had to calibrate their model using tree ring patterns and known rainfall data from 1948 through the present.
To Predict the Future, Rebuild the Past
Tree rings, as might be expected are more indicative of seasonal rain than individual storms' extremes or frequency. However, the patterns of growth clearly indicated that there was less cyclone season precipitation over centuries.
A year with lots of rain does not necessarily indicate a huge storm. Maxwell wrote that it could be rainfall from one hurricane or multiple hurricanes. We found that this area received more tropical cyclone rains over the course of the season.