Rahul Somvanshi
Dr. Robert Corrigan has spent his entire adult life studying rats, and he has a simple message about what climate change is doing to urban rat populations: “Planet Earth is changing and the rats are benefiting.”
Now in his 70s, Corrigan is one of the world’s most respected rodentologists. He first encountered rats while working as an exterminator in New York City’s sewers, funding his graduate education. There, he studied rodent pest management and spent weekends camping in barns to observe rat behavior firsthand.
“The rodents have paid for every single bill I’ve ever had — every mortgage, every car payment, every pint of Guinness,” says Corrigan. “It’s been a great ride and I’m still at it.”
His research into the relationship between rats and humans has been extensive. Corrigan co-authored work with ten other scientists on the subject last year, exploring the practical and ethical dimensions of urban rat management. As he reflects: “Do I love that species and do I respect it as a scientist? Heck, yes. On the other hand I know that they cannot cohabitate with us because they can kill us.”
The connection between climate change and rat populations is straightforward. The two most common rat species—the black rat and the brown rat—do not hibernate. In regions with cold winters, their reproduction slows or stops during the cold months. But as global temperatures rise and winters become consistently warmer, this natural population control is breaking down.
“When you have repeating warm winters, one after the other, the rats benefit,” Corrigan explains. “For many families of rats, the warmer temperatures may only offer a marginal advantage, but the problem escalates when you start multiplying that little bit of extra rat production by hundreds of millions of rats.”
Rats are also vectors for serious diseases that can spill over into human populations, including hantavirus and leptospirosis, with Weil’s disease the severe form of leptospirosis. Traditional rodenticide treatments are becoming less effective as rats develop poison resistance. Moreover, these poisons cause slow, cruel deaths and can secondarily poison wildlife that eat poisoned rats. Similar arguments over chemical controls and non-target species reached the courts when environmental groups sued over a pesticide regulatory loophole, and the wider public health stakes are visible in research showing disease carriers multiplying as scavenger species decline.
Cities worldwide are struggling with growing infestations. Coastal towns are not exempt either, as residents found when a rodent infestation shocked beachgoers in Cork. A promising alternative is fertility control. These methods use contraceptives to slow rat reproduction rates without the ecological damage of poisons. Richard Parr, director of the Center for Wild Animal Welfare, notes: “Developing new forms of birth control is the key to managing the soaring number of poison-resistant rats compassionately.” However, he adds that outdated EU biocidal product regulation is “stifling innovation” in this space.
Not all experts are convinced fertility control alone is sufficient. Steven Belmain, professor of ecology at the University of Greenwich, remains skeptical: “People are sometimes quite naive thinking we can use fertility control. If you’ve got a rat in your house, you’re not going to feed it contraceptives and just let it live there with you.” Belmain emphasizes the broader role of poison management: “Our restaurants, our hotels, our hospitals, our food warehouses will all just collapse without having any sort of poison management.”
The most successful rat eradication program on Earth operates in Alberta, Canada. Since 1950, the province has maintained a near-zero rat population through a coordinated program involving citizen reporting requirements, public education campaigns, and systematic eradication along its eastern border with Saskatchewan. When the program began, Alberta dealt with 600 to 700 infestations per year. That figure has dropped dramatically. In 2025, the province recorded just five infestations across its entire territory.
However, even Alberta is becoming concerned. The fall of 2025 was exceptionally warm, undermining the province’s natural cold-weather protection. Urban expansion is also creating more sanitation challenges that rats exploit. Additionally, more residents are traveling out of province and returning home with rodent hitchhikers from warmer regions, introducing a new risk to the province’s eradication success. Elsewhere, shifting conditions have already reshaped local species balances, as seen where python predation on native wildlife fuelled a rat invasion, and in Britain, where wildlife populations have fallen sharply amid climate and drought pressure.
Back in New York, Corrigan argues that rat problems ultimately stem from human failures: “Cities are not strict enough on punishing people that ruin the block or the area or the province for everybody.”
