By Skip Rigney

This world is full of surprises. You don’t have to live very long to learn that things don’t always turn out the way you expected they would.

Two recent research studies illustrate that, despite an incredible increase over the past century in scientists’ understanding of weather and climate, the complex interactions between the atmosphere, ocean, land, and human activities can still surprise even the most knowledgeable experts.

The vast majority of climate scientists agree that the temperature of the air near the earth’s surface averaged across the entire globe is one to two degrees Fahrenheit warmer than it was back in the 1800s. Most of the warming has occurred since the middle of the 20th century.

Given that hurricanes and typhoons draw their heat energy from the warm waters near the surface of tropical and subtropical oceans, it would seem a reasonable expectation that as the climate has warmed, there would be an increase in the number of tropical cyclones.

Guess again. Not only has the annual number of tropical cyclones forming globally decreased by approximately 13% during the 20th century, the decrease may actually be due to the warmer climate. That’s according to a study published last month in the scientific journal Nature Climate Change.

The study was conducted by a group of 13 scientists, mostly from Australia and the United States, led by Savin Chand of Federation University in Australia. Chand and his colleagues hypothesize that, even though warmer ocean waters favor tropical cyclones, the warming climate has caused changes in broad and deep atmospheric circulation patterns that have been detrimental to tropical cyclone formation.

Even though there has been an increase in the number of tropical storms and hurricanes in recent decades in the North Atlantic basin, which includes the Caribbean Sea and Gulf of Mexico, Chand and his colleagues found “a statistically significant downward trend in North Atlantic tropical cyclone numbers” when they looked at the much longer period since the mid-1800s.

The uptick in hurricane numbers in the North Atlantic basin since the mid-1990s is probably due at least in part to natural long-term cycles in the climate. But, another surprising reason for the increase was uncovered by NOAA scientist Hiroyuki Murakami in a paper published in May in the journal Science Advances.

Murakami, who is an expert in tropical cyclone climate modeling and predictions, noted that efforts in Europe and North America over the last 40 years to reduce air pollution have resulted in a big drop in pollutant concentrations over those continents and the North Atlantic basin. Undoubtedly, this has been a good thing for our lungs. The pollution over the North Pacific has actually increased because of increased air pollution from South and East Asia.

With less pollutant particles to reflect sunlight over the tropical Atlantic Ocean, those surface waters absorb more heat and warm faster. According to Murakami’s study, those warmer Atlantic waters have contributed to a 33-percent increase in the number of hurricanes and tropical storms from 1980-2020.

Even if we forget the details of these two studies, two lessons worth remembering are that nature is full of surprises, and our actions always have unintended consequences.