The 2020 hurricane season was one of the most extreme on record. Between May and November, there were 30 named storms (the most ever recorded), and 14 of those reached hurricane level (the second-most ever). Cumulatively, all these storms caused hundreds of deaths and more than an estimated $US40 ($56) billion of damage. Now, we can officially say climate change made them worse, according to a study published Tuesday in the journal Nature Communications.
Warmer ocean temperatures caused an average of 5% more total rainfall across all named storms in 2020 than would’ve happened without human-caused climate change, the research found. And that rain came down at a 10% faster rate. For hurricanes, the strongest of the named storms, those percentages were even higher: 8% and 11% respectively, according to the paper.
Those percentages might seem small, but they add up, explained Kevin Reed, an extreme weather researcher at Stony Brook University in New York and lead author of the new study. “When you take that 10% into context over the entire spatial scale of the storm, that can actually lead to flooding much larger than 10%,” he said by phone. All the water that falls over a landscape flows to the lowest-lying places first, compounding small differences in total precipitation into big differences in flood severity.
To calculate their results, the researchers used computer models of “hindcasts” (i.e. forecasts for the past). They produced 1,200 unique scenarios for all of 2020’s storms under our actual climate conditions and compared them with an equal number of scenarios modelled around a world without climate change.
Previous research has been able to link particularly bad individual storms to climate change, but Reed’s and his colleagues’ findings are the first to draw a direct connection between a whole season’s worth of storm precipitation and climate change. “It’s important to understand that the changes that we’re seeing aren’t just related to the most extreme events,” Reed said. “Our day-to-day weather has changed.”
Kerry Emanual, an atmospheric scientist at MIT who was not involved in the new study, told the Associated Press that “The expected increase in hurricane rainfall is probably the most robust prediction concerning the response of hurricanes to climate change.” However, he also noted that the research says nothing about the relationship between climate change and the overall intensity or number of hurricanes.
The relationship between climate change and hurricane frequency isn’t yet well established, but this study adds to the pile of evidence suggesting climate change juices up existing storms.
So far, humans have caused an approximate 1.1 degrees Celsius of warming across the entire Earth’s surface since 1850 through greenhouse gases released by the burning of fossil fuels like coal, oil, and methane gas. By 2020, the year the new research focuses on, the Atlantic Ocean’s hurricane basin had warmed by an average of about 0.6 degrees Celsius.
That rise in sea surface temperature is largely responsible for all the extra storm rain. Hurricanes draw their strength and moisture from the ocean. Just as muggy summertime air can hold more humidity the hotter it gets, a warmer ocean surface leads to more moisture evaporating upwards. So, warmer oceans mean wetter storms. And the phenomenon doesn’t start or end with 2020.
The next Atlantic hurricane season will soon be upon us. The upcoming season officially starts June 1, although it could begin even earlier. Some forecasts are already predicting it will be particularly active, with about 30% more storms than the average. Chances are, climate change will make those upcoming storms wetter, too. “The increase in rainfall due to hurricanes will only [accelerate] as the globe continues to warm,” Reed said, adding that “climate change is here now. It’s impacting our weather now.”
If we want to minimise the number of people swept away in future super-storms, our society needs to adapt fast and sharply reduce global greenhouse gas emissions.