Extreme heat waves, devastating storms, and prolonged droughts have yet to instill widespread fear of climate change's consequences. However, one additional phenomenon may finally raise alarm: as temperatures climb, rats will multiply at an alarming rate in cities and urban areas, challenging existing population control efforts.

$27 Billion in Annual Damage

According to West Hawaii Today, people globally—especially in the United States—will face this rat population boom as another disaster linked to climate change. Currently, Americans endure over $27 billion in property damage annually caused by urban rats. This figure excludes the costs associated with diseases spread by these rodents, such as hantavirus, murine typhus, and bubonic plague, as well as the mental health struggles of living alongside them.

Recent statistics published in Science Advances draw from rat sighting records in 16 cities worldwide. Unfortunately, 11 of these cities saw their rat populations grow during the study, while only two remained stable, and three experienced measurable declines.

This population spike shouldn’t come as a surprise: rats thrive in urban environments, making their homes in walls, basements, and subway stations, and feeding on garbage, sewage, pet waste, and leftover food.

A Rat-Free Continent

Antarctica is the only continent that rats have not invaded.

“These rats are highly adaptable to new temperatures and urban food sources, allowing them to reproduce effectively and introduce new rodents to neighborhoods,” explains Jonathan Richardson, a biologist at the University of Richmond who studies urban wildlife and its health impacts.

While cold weather typically helps curb rat populations, climate change is reducing that advantage. Global warming raises average temperatures and decreases the number of winter days. In cities, this issue is compounded by the urban heat effect, which causes built environments to retain more heat than rural areas.

To explore the connection between rising temperatures and rat populations, Richardson and his colleagues examined reliable data from the 200 most populous cities. While conducting a comprehensive rat count was impractical, they utilized municipal inspection records and reported rat sightings to government agencies.

13 Cities Analyzed

The researchers identified 13 cities with consistent rat infestation records spanning at least seven years. Because cities collected data from different sources over varied periods, the researchers standardized their measurements of changes in rat sightings.

Washington Leads

The study found the greatest increase in rat reports in Washington, D.C., followed by San Francisco, Toronto, New York City, Amsterdam, Oakland, Buffalo, Chicago, Boston, Kansas City, and Cincinnati.

Three cities—New Orleans, Louisville, and Tokyo—successfully reduced their rat populations during the study period, while Dallas and St. Louis showed no significant changes. Los Angeles was not included in the analysis due to the absence of systematic rat records, though it frequently ranks among the top three most rat-infested cities in America. According to Richardson, the high volume of rodent complaints there is more a result of the city’s sprawling size than a particularly rat-friendly environment.

The researchers employed statistical methods to determine which factors contributed to the varying outcomes of rat control in different cities. They considered five factors, including human population density and the amount of vegetation.

An Important Factor

The most significant factor affecting rat populations was the change in average temperature within a city—the higher the temperature, the more rats there were. Surprisingly, changes in the minimum temperature did not impact rat populations. Initially, the team expected that cold weather would prolong the time it takes for female rats to become fertile and reduce the number of newborns.

In favorable climates, a female rat can become pregnant at just two months of age, with a gestation period lasting only three to five weeks. The researchers noted that if higher temperatures lead to winter ending a week or two earlier, it could allow for an additional reproductive cycle each year.

According to Santtu Pentikainen, a researcher at the University of Helsinki who was not involved in the study, the authors made a compelling case that global warming benefits rat populations.