More bad news for rat-infested cities: Climate change is making it worse

More rats: that's the latest indignity that climate change is dumping on major cities around the world, including in Canada's largest city, according to a new study from a group of global rodent and public health scientists.

Growing rat populations are correlated with rising temperatures driven by global warming in at least 11 major cities across the world, according to the study. The cities seeing rat increases include Toronto, famously rat-prone New York City, and many other major centres like Washington, San Francisco and Amsterdam.

The study's authors say it's a wake-up call for cities to move away from a whack-a-mole approach to dealing with rodent complaints.

"I think every large city should have a dedicated team that focuses on nothing but rodents and the issues with rodents," said Jonathan Richardson, lead author of the study and an urban ecologist at the University of Richmond in Virginia. 

The study used data on public complaints and inspections about rats from 16 cities around the world. The 11 cities that saw significant increases in rats also experienced greater temperature increases over time, though that correlation doesn't prove the temperature caused the increase. Cities with more dense human populations and more urbanization also saw larger increases in rats.

The research comes at a time when rats have become a big issue in Toronto, which is seeing a large increase in rodent-related complaints after a brief decline during the COVID-19 pandemic. City officials are currently working on a new strategy to proactively deal with the rats.

"Even when you leave city hall or walk around city hall, you can see the burrows in the tree wells. I've walked along one of the streets and a giant rat ran past me in the middle of the day," said Alejandra Bravo, a Toronto city councillor who proposed the successful motion to build a rat strategy. 

"I think people need to see action."

Why are rats thriving?

Rats are resilient and remarkably adaptable to different environments, says Alice Sinia, entomologist with pest control company Orkin Canada. It makes biological sense that warmer temperatures would be helping them out, for three big reasons.

The harsh winter acts as a sort of "nature's pest control," with the cold killing rats every year. Climate change has led to milder winters in Toronto, allowing greater numbers of rats to make it through the season.

Meanwhile, the warmer seasons have become longer, and that's when the rats breed and reproduce. Sinia says that could further boost their numbers.

And finally, climate disasters themselves could be bringing rats closer to people and buildings. Sinia used the example of the floods in Toronto in July last year, caused by a line of storms that dumped rain on the city in rapid succession in a matter of hours.

Studies suggest that Canada will see more extreme rain, with warmer air able to hold more moisture that it will dump in short, severe storms.

The flash floods last year caused nearly a billion dollars in insured damages, flooding basements, highways and transit stations across the city. But apart from the damage, floods also overwhelm the city's sewer system — which are teaming with rats that get displaced.

"Now instead of the underground sewers, they [the rats] will come to the surface and when that happens … they're going to start occupying other areas," Sinia said. 

"They're going into people's homes, structures… and they're going to have an opportunity to reproduce very fast."

But none of this means people will just have to resign themselves to a rat-filled future, the experts said. 

Keeping up the fight

Richardson, the study author, said that they key was a proactive rat strategy that addressed the root causes of infestations. That's similar to what the Toronto council is working on, according to Bravo. City staff will look at food disposal, managing construction sites, new approaches like birth control and other ways to manage the rat population on a larger scale.

Ultimately, Richardson says, cities need to put more money and resources into fighting rats, because simply tolerating them is not a good option.

"They carry at least 50 known zoonotic pathogens and parasites that can make residents sick in cities," he said. A zoonotic infection or disease is one that can transmit between animals and humans.

"They also have mental health impacts for residents where if you're living alongside rats regularly and see them in your daily life, you tend to have worse mental health outcomes and feel less secure in your environment.

"If we can reassess how much tolerance we have for rats in our everyday daily life as we're walking around, maybe that applies some pressure to the city governments and advocates for more resources to knock these populations back."