If we are, as some city officials have said, in a war with rats, we are clearly losing. We’ve been losing for years.

Although cities have ramped up their use of poisons and traps, the number of rats in places like New York City, San Francisco, and Toronto has increased in recent years, according to a new study published in the journal Science Advances. The researchers analyzed rat complaints and inspection reports for 16 cities that had consistent, long-term data available. More than two-thirds of those cities saw a significant increase in rat sightings.

Washington, DC, had the largest increase in sightings over roughly the last decade, according to the study, which is the most comprehensive assessment of city rats to date.

“We are on our heels and being pushed backward,” Jonathan Richardson, the study’s lead author and an ecologist at the University of Richmond, said about the fight against rat infestations.

There’s more bad news: The study found a strong link between an increase in rats and rising temperatures, a consequence of climate change. Cities that warmed more quickly had larger increases in rat sightings, the research found. This is in part because, with warmer winters, rats can spend more time eating and reproducing and less time hunkering down underground.

Scientists project that urban areas will warm by between 3.4 and nearly 7.9 degrees Fahrenheit by the end of the century, depending on how much oil and gas we burn. Cities tend to be hotter than rural areas — because concrete and other human infrastructure absorb and re-emit more heat than vegetation — and warm faster. That means that not only are current rat control methods failing, but the problem is likely to get much worse.

It’s a good thing, then, that there’s an obvious solution. And better yet, it’s simple.

The cities where rat sightings are growing the fastest

While rats are easily the most common urban mammal, cities don’t actually know how many of them there are. They don’t run a census for rats like they do for, say, squirrels. So to figure out how their populations are changing, researchers rely instead on proxies, such as 311 complaints — when disgruntled tenants or parkgoers or diners report an infestation to city officials. Those complaints have been shown to correlate with the abundance of rats, though they’re imperfect approximations. Plenty of factors, beyond the sheer number of rats, influence whether or not someone complains, including their relationship with their landlord and trust in the city government.

The new study relies on those public complaints, though it also uses inspection reports, which are created by city officials who inspect a property for rats,........

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