Rising Temperatures Are Turbocharging City Rat Populations, Scientists Warn

If climate change is here to stay, then the rodents will come out to play. A study released today suggests increasing temperatures are helping to drive a surge in city-dwelling rats.

Researchers at the University of Richmond led the study, published Friday in the journal Science Advances. Based on an analysis of cities around the globe, they found a significant association between a warming climate and increased urban rat populations. The researchers say that many cities will have to prepare for larger rat infestations in the future, given projected trends in urbanization and the climate.

Rats have been a disease-carrying pest of humanity since essentially the moment we began to grow crops and live in permanent settlements. But as cities today have gotten increasingly larger, some evidence has suggested that rats have become an even bigger problem of late. According to the study authors, though, there’s surprisingly little research looking at how rats have actually fared in cities over the long term, let alone the factors that might be fueling or dampening their numbers.

To bridge this knowledge gap, the researchers analyzed public rat sighting and inspection data from 16 cities. Most of the cities they studied were in the U.S., including New York, Washington D.C., and San Francisco, but they also studied rat populations in Tokyo, Japan, and the capital city of the Netherlands, Amsterdam. The researchers had between seven and 17 years worth of data from these cities, the average being around 12.

All in all, 11 of the 16 cities (69%) experienced a clear rising trend in rat numbers over time, the researchers found, including Washington D.C., New York, and Amsterdam. And the cities that experienced the greatest increases in temperature during the study period also tended to experience larger increases in rat populations. Denser populations of people and more urbanization within these cities (defined as fewer greenspaces, including parks) were also associated with growing rat numbers.

“Most cities are seeing increases in rat numbers, which are linked to climate warming, urbanization, and human population densities,” the researchers wrote in their paper.

The findings certainly make intuitive sense. Like many small mammals, rats are less able to thrive in colder weather. Warmer temperatures, particularly during the chillier seasons, might be conferring rats a greater chance to not only survive but also continue reproducing year-round. The researchers cite other studies suggesting that climate change may also be expanding the range of environments where rats can safely call home.

The researchers note that their sample didn’t include any cities from tropical parts of the world (and only one in a subtropical climate, New Orleans). It’s possible that climate change has had different effects on the rat populations within these areas. Their own work also shows that climate change alone isn’t to blame for rising rat numbers in the cities they did study, but it’s likely that the world’s cities will only continue to get bigger and denser over time. And right now, the climate is on track to keep warming as well. In other words, the rat problem in cities is only expected to  worsen in the decades ahead—unless we take effective steps to keep their numbers in check.

“Given the projections of continued warming for the foreseeable future, cities need to be prepared for the potential for this warming to exacerbate current rodent pest infestation levels,” the researchers wrote. “More financial and personnel resources will need to be dedicated to municipal rodent control efforts to limit this expected increase in rat populations and activity.”