Rat numbers are soaring in cities as global temperatures warm, according to recent research. The study, published in the journal Science Advances, analyzed data from 16 cities worldwide, revealing that eleven of them exhibited significant increasing trends in rat populations. The cities with the greatest increases included Washington, DC, San Francisco, Toronto, New York City, and Amsterdam.
Over the past decade, rats have increased by 390% in Washington, DC, 300% in San Francisco, 186% in Toronto, and 162% in New York. Researchers analyzed public sightings and infestation reports to determine these trends. Some major cities, such as London and Paris, were not included in the study due to a lack of data, but researchers believe the findings are applicable to many similar cities worldwide. “There’d be no reason to expect it to be different in other places,” said lead researcher Jonathan Richardson from the University of Richmond in Virginia.
Toronto, one of the worst-affected locations, is experiencing a “perfect rat storm” as residents grapple with a surging rat population. “When you walk the streets of Toronto, under your feet, deep in the sewage system, is a place teeming with rats,” said Alice Sinia, lead entomologist for Orkin, Canada’s largest pest control company. The city’s helpline fielded 1,600 rat-related calls in 2023, up from 940 in 2019, with Orkin also experiencing a surge in calls.
The exact size of Toronto’s rat population remains unknown as it has never been formally studied. Two city councillors, Alejandra Bravo and Amber Morley, called for a formal management plan to address the crisis. “It’s a really critical quality-of-life problem when people have all of a sudden been confronted with rats coming into their home or into their business or their place of work,” Bravo said, describing the situation as a “kind of perfect rat storm.”
The study found that rising temperatures correlated with rising rat numbers. As small mammals, rats struggle during winter, but higher temperatures allow them to breed for more of the year and forage for longer. In Toronto, cold winters had long acted as “mother nature’s pest control,” killing off significant portions of the population. However, milder temperatures have allowed rodents to continue breeding.
The fact that rat numbers increased fastest in cities that were warming fastest was “the gloomiest outcome of the study,” said Richardson. Last year was the hottest on record, with average temperatures 1.6C above preindustrial levels.
Rats cause billions of dollars in damage each year by infiltrating buildings and can transmit at least 60 diseases to humans. They also affect the ecology of other species living in cities and cause significant damage to biodiversity in regions where they are invasive. Research suggests that people who frequently encounter rats have poorer mental health. Globally, humanity’s “war on rats” costs an estimated $500 million each year.
Not all cities are experiencing an increase in rat populations. Tokyo, Louisville, and New Orleans have reported declining rat numbers. In Tokyo, cultural norms and expectations of cleanliness likely contribute to the swift reporting of rodent sightings. New Orleans has implemented educational outreach on how to prevent infestations. “There are important lessons probably to be gleaned from those cities,” said Richardson.
Researchers emphasize that the best pest management strategies involve making the urban environment less rat-friendly, such as putting rubbish in containers rather than in bags on the street. Despite thousands of studies on lab rats, little is known about wild urban rats. Richardson stressed the importance of understanding the battle being fought, stating, “Pretty much every city announces that they have a war on rats.”
Rats can form colonies of up to 100 individuals, usually occupying less than one block. “I do not like rats,” Richardson admitted, “but there is something fascinating about an organism that has been able to spread over the world and live in such proximity to us so successfully.”
As cities around the world continue to grapple with rising rat populations, the need for effective and sustainable pest management strategies becomes ever more critical.