- The saying goes that Kansas City in August is hotter than two rats in a wool sock, but it turns out the rising temperatures from climate change are fueling explosive growth in the global rat population. The warmer weather potentially expands the time they have to breed, putting cities on the back foot in their fight against the vermin.
The latest chapter of New York’s centuries-long war on rats has the city throwing everything at the problem—from enforcing new garbage laws, to enlisting bands of vigilante rat hunters, to putting the rodents on birth control. But while the U.S.’s largest city may be gaining ground in the anti-rat battle, the global fight is getting much harder, thanks to problems of humans’ own making, new research suggests.
Rising temperatures and more people living in dense environments are set to increase rat presence in some of the world’s most important cities, suggests a study published last week in Science Advances. That’s bad news for a future where ongoing climate change is projected to push already elevated temperatures even higher.
“The warmer cities are getting, the faster their rat populations are increasing,” lead author Jonathan Richardson, assistant professor of biology at the University of Richmond, told Fortune.
Richardson and his coauthors found that 11 of 16 global cities for which they obtained data saw their rat populations grow over a decade-long period, with the biggest growth happening in Washington, D.C., San Francisco, Toronto, New York, and Amsterdam. Cities where temperatures were rising fastest showed the biggest rodent gains. High rates of urbanization (defined as low rates of green space) as well as larger populations were also objectively rattier, the researchers found.
Only three cities—New Orleans, Louisville, and famously clean Tokyo—saw rat populations drop over this period, Richardson said.
As perhaps the most successful mammal in colonizing the globe after Homo sapiens, rats are humans’ longtime foils and foes. A constant companion to people ever since the first urban settlements formed thousands of years ago, the rodents have metastasized into a $27 billion-a-year problem in the U.S. alone, damaging crops, chewing up wires, spreading at least 50 distinct diseases, and ripping up lawns, to name just a few issues.
Warmer weather, especially in the winter, gives rats more opportunities to secure food and a potentially longer breeding window, the researchers noted.
“A rat doesn’t hibernate, so if it has just a day or two to come above ground and replenish its food cache in its burrow, that can bolster its survivorship and lead to more baby rats come spring,” Richardson said. “A rat that is well-fed in the winter can reproduce in the winter.”
Rats’ remarkable multiplying properties are one reason that pest-control efforts have barely made a dent in urban rat populations: In good conditions, a female rat can produce a litter of a dozen or more pups every month. “This species is exquisitely adaptive to reproducing and pumping out new rats,” Richardson said. The other is their status as a commensal mammal, meaning one that lives alongside people, “feeding on our resources and exploiting our own mess that we create for ourselves.” In the Science Advances study, more urbanized environments, defined as those with less green space, were the second-biggest factor behind rat increases after warming temperatures.
All that means that to effectively deal with the problem, cities have to take a more proactive approach, Richardson said: Rather than trying to poison the rodents into oblivion, cities should make living environments more hostile to rats by eliminating easy food sources and comfortable breeding quarters. In this regard, he praised New York’s efforts to put more trash in secured containers and limit how much time garbage spends on the street before being collected. An early effort in upper Manhattan has shown positive results and is expanding to more neighborhoods, he noted. Cities could also encourage homeowners to clean up their properties to make them inhospitable to rats, even fining those who don’t comply.
“Sanitation is the number-one strategy that we can use effectively to limit rats,” he added. “Any city rodent management plan that doesn’t start with sanitation and trash removal is bound to fail.”
