Researchers at the University of Richmond in Virginia have looked at rat populations from 16 cities around the world. In eleven of them, the number of rats has increased significantly over the past twelve years, which, according to the study, can be linked to a warmer climate. The trend is therefore expected to continue.
Cold winters are tough for rats, and the low temperatures act as a kind of natural pest control. But when winters become milder, the rats survive to a greater extent. This also increases their opportunity to reproduce for a larger part of the year.
Several cities follow the pattern
The study looks at data from, among other places, Washington DC, where the number of rats has increased by 390 percent over the past decade. For San Francisco, the increase was 300 percent during the same period.
Jonathan Richardson, the research leader for the study, tells The Guardian that several cities around the world are likely to follow the same pattern.
Record number of rats in Sweden
In Sweden, too, the number of rats is increasing, and 2024 was a record year. Anticimex caught over one million rats, which is an increase of five percent compared to the previous year.
Whether this has to do with a warmer climate is difficult to answer, says Magnus Rosenholm, pest control expert at Anticimex.
However, we have seen an increase in rats in sewage pipes, where the temperature is more even than outdoors, he says.
Magnus Rosenholm believes that the winters in Sweden are still so cold that it is primarily during the summer, spring, and autumn that the rats are most active above ground.
So far, we have not made a comparison between the rat population and the climate in Sweden.
The study is published in Science Advances.
