City Rat Numbers Are Rising With The Climate

Rats are everywhere. Two species, the brown rat and the black rat, have spread to nearly every continent on Earth, and they’ve done so by thriving alongside us. They damage buildings, eat crops, contaminate food, and cost an estimated US$27 billion in damage each year in the U.S. alone. Beyond the financial toll, rats carry more than 50 pathogens and parasites transmissible to people. Living near rats has even been linked to poorer mental health outcomes in affected communities.

Despite all this, cities have largely been fighting rats the same way for decades: poisons and traps. These lethal approaches are expensive, running about US$500 million globally each year. At the same time, it’s unclear how effective rat control efforts actually are because there’s been surprisingly little research tracking their populations over time. The assumption that rat numbers are increasing is widespread, but the data to back this up just isn’t there. As cities grow and the climate shifts, understanding what’s driving rat population trends is becoming increasingly urgent.

This study set out to do exactly that: quantify how rat populations have changed across cities around the world, and identify which environmental and social factors are most strongly linked to those changes.

The researchers gathered between seven and 17 years of public rat complaint and inspection data from 16 cities, including Amsterdam, Boston, Buffalo, Chicago, Cincinnati, Dallas, Kansas City, Louisville, New Orleans, New York City, Oakland, San Francisco, St. Louis, Tokyo, Toronto, and Washington, D.C. Because traditional rat surveying, like trapping, is rarely done at a city-wide scale, public complaint data served as a reliable data source. Prior studies have shown these reports track closely with rat abundance estimates from trapping. The team then compared rat trends against five variables for each city: long-term temperature increases, average minimum winter temperatures, human population density, vegetation cover (as a measure of urbanization), and per capita economic output.

The results were striking. Rat numbers increased significantly in 11 of the 16 cities (69%) studied. Washington, D.C. showed the steepest rise, followed by San Francisco, Toronto, New York City, and Amsterdam. Only three cities, Tokyo, Louisville, and New Orleans, saw declines. Dallas and St. Louis showed no significant trend either way.

Together, the five variables explained 66% of the variation in rat trends across cities. The single strongest predictor was long-term temperature increase, which accounted for about 41% of that variation. Cities that had warmed more over time saw larger increases in rat sightings. This likely comes down to rat biology: cold temperatures limit how long rats can forage and breed, so as winters grow milder, rats can stay active and reproduce for longer stretches of the year.

The second biggest factor was vegetation cover, explaining about 34% of the variation. Cities with less green space, meaning more pavement and built infrastructure, experienced faster rat growth. Human population density came in third at around 19%. More people means more food waste, which in turn supports larger rat populations. Notably, a city’s wealth had almost no connection to rat trends at just 3%, suggesting that even well-resourced cities are struggling equally. Average minimum winter temperature explained the least amount of variance at around 2%, indicating that how cold a city’s winters are in absolute terms may matter less for rat numbers than how much warmer that city has become over time.

One key limitation of the study is its reliance on public complaint data rather than direct population surveys. These reports can be influenced by factors like residents’ comfort level interacting with city agencies or awareness of reporting systems. Additionally, none of the 16 cities were in tropical climates, with only one subtropical location (New Orleans), so the findings may not fully apply to cities in tropical zones closer to the equator. The researchers also couldn’t determine exactly why warmer temperatures lead to higher rat numbers, whether through lower winter mortality, higher birth rates, or simply more time spent foraging.

For animal advocates, this study carries an important message about the limits of how cities currently manage urban rat problems. The heavy reliance on rodenticide poisons has repeatedly shown poor long-term results and causes serious harm to non-target animals, including birds of prey, coyotes, and others who feed on poisoned rats. The most promising path forward involves making cities less hospitable to rats in the first place: better food waste management, secure garbage containers, stricter building codes to prevent rats from entering, and preserving green spaces. Advocates can use these findings to push for rodent management policies that reduce reliance on rodenticides, protecting both urban wildlife and public health. The study also underscores that as climate change continues, inaction will only make urban rat problems and the harmful control methods that follow worse.