‘Perfect rat storm': urban rodent numbers soar as the climate heats, study finds

The Guardian

31-01-2025

Rat numbers are soaring in cities as global temperatures warm, research shows.
Washington DC, San Francisco, Toronto, New York City and Amsterdam had the greatest increase in these rodents, according to the study, which looked at data from 16 cities globally. Eleven of the cities showed 'significant increasing trends in rat numbers', said the paper published in the journal Science Advances, and these trends were likely to continue.
Over the past decade, rats increased by 390% in Washington DC, 300% in San Francisco, 186% in Toronto and 162% in New York according to researchers, who analysed public sightings and infestation reports.
Some big cities, such as London and Paris, were not included because they did not provide the necessary data – but researchers said the findings would apply to many similar cities around the world. '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.
In Toronto, one of the worst-affected locations, a 'perfect rat storm' has taken hold, with residents of Canada's biggest city staring down a surging 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, the country's largest pest control company. 'Increasingly we're flushing them out into open spaces – either through construction or floods – and they have to go somewhere.'
Toronto city's helpline fielded 1,600 rat-related calls in 2023, up from 940 in 2019 and Orkin has also experienced a surge in calls.
'But the reality is, we don't know how big the population is because no one has ever really studied it formally,' said Sinia.
Two Toronto city councillors, Alejandra Bravo and Amber Morley, last year called for a formal management plan as a way to blunt 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 told the Canadian press, adding that it had morphed into a 'kind of perfect rat storm'.
Other cities with increasing rat populations included Oakland, Buffalo, Chicago, Boston, Kansas City and Cincinnati. The research focused on US cities, as well as Amsterdam, Toronto and Tokyo, as all of them gathered similar data on rat sightings. The research did not quantify the overall rat population, just relative increases in reports over time.
Rising temperatures correlated with rising rat numbers, researchers wrote in the paper. As small mammals, rats struggle during winter, but in higher temperatures they can breed for more of the year and forage for longer.
In Toronto, cold winters had long acted as 'mother nature's pest control', said Sinia, killing off swaths of the population. But mild temperatures had helped rodents of all kinds in the city to keep 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 by infiltrating buildings each year, and can transmit at least 60 diseases to humans, as well as affecting the ecology of other species living in cities. In regions where they are an invasive species, they do huge damage to biodiversity. Research suggests people who encounter rats frequently have poorer mental health. Globally, humanity's 'war on rats' costs an estimated $500m each year, according to the study.
Tokyo, Louisville and New Orleans bucked the trend with declining rat numbers. In Tokyo, Richardson speculated that cultural norms and expectations of cleanliness meant people were quick to report rodent sightings. In New Orleans there has been educational outreach on how to prevent infestations. 'There are important lessons probably to be gleaned from those cities,' said Richardson.
Researchers say the best pest management strategies involve making the urban environment less rat-friendly – for example by putting rubbish in containers, and not in bags on the street – rather than removing rodents that are already there.
Despite thousands of studies on lab rats, little was known about wild urban rats, said Richardson. 'We need to know the battle that we're fighting. Pretty much every city announces that they have a war on rats.'
Sometimes there can be up to 100 rats in a single colony, which usually occupies less than one block. 'I do not like rats,' he said, '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.'