Crunching the numbers on cyclone trends and global warming
22 November 2023
Two papers published in the prestigious Nature Climate Change have argued that using different datasets and analysis techniques may lead to conflicting results on tropical cyclone trends.
Tropical cyclones are extreme weather systems that often have devastating ramifications in tropical regions when they reach landfall.
There has long been a scientific debate about how tropical cyclone frequency has changed over the past century due to anthropogenic – or human-induced – global warming or how it will change in future climate under enhanced warming.
In a study published in Nature Climate Change in 2022, Federation climate scientist Associate Professor Savin Chand and his co-researchers showed that the annual number of tropical cyclones forming globally decreased by about 13 per cent during the 20th century compared to the 19th. These findings complement other studies that have predicted tropical cyclone frequency will decrease due to global warming.
The researchers used the Twentieth Century Reanalysis dataset to reconstruct cyclone numbers as far back as 1850. This reanalysis project uses detailed metrics to paint a picture of global atmospheric weather conditions before the use of satellites. The reconstructed record reveals a decrease in the annual number of tropical cyclones since 1850 at both a global and regional scale.
In a ‘matter arising’ article published this week in Nature Climate Change, Professor Emeritus of atmospheric science at Massachusetts Institute of Technology and leading climate researcher Kerry Emanuel wrote that the limitations of climate datasets for long-term tropical cyclone trend analysis were mostly an artificial consequence of the greatly increasing density of surface pressure observations during this time, particularly after the World War II when locations of weather stations increased substantially.
In their reply, published in the same journal on the same day, Assoc Prof Chand and his co-authors provided a counterargument on why climate datasets and analysis techniques, when used intelligently, can provide useful insights on long-term tropical cyclone trends.
“Our climate system has undoubtedly warmed by over 1°C over the past century. However, it is not uncommon to see scientific debates on how such observed changes in the climate system will affect day-to-day extreme weather events such as tropical cyclones,” Assoc Prof Chand said.
Detection and attribution of climate change involves assessing the causes of observed changes in the climate system through systematic comparison of climate models and observations using various statistical methods.
The challenges for climate scientists
Detecting and attributing climate change can be challenging for several reasons, including a lack of long-term, high-quality observation records to properly delineate natural variability from any anthropogenic signal.
Assoc Prof Chand says that while Professor Emanuel’s article may be correct in terms of data suitability and availability, his specific analysis to counteract the original work is not suitable for tropical cyclone trends as it detects too many circulations that are too weak, too small, too dry and often too short-lived to be classified as a tropical cyclone.
“It is feasible that the broader class of circulations detected have no strong trend, while real tropical cyclone numbers decline,” Assoc Prof Chand said.
Assoc Prof Chand and co-authors also provide comprehensive evidence that downward tropical cyclone trends published in their original article are robust through use of multiple data sources, including climate model experiments performed under different warming conditions.
“Both studies acknowledge that all climate datasets have limitations, so can have implications on detection and attribution studies. However, as improved data products and climate models become available, further insights can be drawn not only on tropical cyclone frequency trends but also on other tropical cyclone characteristics,” Assoc Prof Chand said.
“For climate science to progress, we need and encourage this healthy scientific debate.”
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