Global warming is helping rats thrive in major cities around the world, with Washington DC seeing the fastest growth in rats
By Michael Le Page
31 January 2025
Rat sightings are on the rise in New York City
Fatih Aktas/Anadolu/Getty Images
It has long been predicted that many pest species will thrive as the planet warms – and now a study of 16 major cities has found that rat populations are growing fastest in areas where average temperatures are rising quickest.
It is extremely difficult to estimate the number of rats in a city, so Jonathan Richardson at the University of Richmond in Virginia and his colleagues didn’t attempt this. Instead, they got a sense of how populations are changing by looking at the number of complaints about rats recorded by cities.
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In the US, this information is often publicly available and the team was also able to get data for a few places outside the US by contacting city officials. The researchers only included cities in their study if at least seven years of data was available and the methods for collecting it hadn’t changed. That left them with data for 13 US cities, as well as Tokyo, Amsterdam and Toronto.
Their analysis suggests rat numbers are declining in New Orleans, Louisville in Kentucky and Tokyo, are stable in Dallas and St Louis, and are rising in the other 11 cities, with the fastest growth in Washington DC, San Francisco, Toronto, New York and Amsterdam.
Richardson and his colleagues then looked at several factors that might explain the trends. They found the strongest link was with the average temperature increase over the past century. The next strongest link was with urbanisation, assessed from satellite photos, followed by human population density. The city’s GDP did not show a link with rat trends.