The icky problem facing cities because of global warming

Rising seas, intensifying droughts, worsening floods – these are well-known effects of climate change, the consequences of pumping too much heat-trapping pollution into the air.

Now we may be able to add another unexpected outcome of human-induced warming to that list: rats. Lots and lots more rats.

A rat on the run at Docklands in Melbourne.Credit: Justin McManus

Urban rat populations are booming around the world, due in large part to rising average temperatures, according to an analysis of 16 cities published on Friday. The trend is most pronounced in Washington, DC, followed by San Francisco, Toronto and New York.

The findings have hairy implications for warming cities already struggling to deal with the destructive pests, which spread disease, contaminate food with urine and faeces, bite people and pets, chew through wiring in homes and cars, and cause an estimated $US27 billion ($43 billion) in damage a year in the United States alone.

“It gives cities a better picture of the uphill task that they face,” said University of Richmond urban ecologist Jonathan Richardson, who led the work published in the journal Science Advances, said.

It’s hard out there for a rat. During the height of winter, the rodents flee underground or indoors, spending less time foraging for food to feed their young and grow their numbers.

Zookeepers at Taronga train a rat used as part of the bird shows.Credit: James Brickwood

“For a small mammal, it’s pretty tough to make your living in the very cold winter months,” Richardson said.

To assess whether milder temperatures alleviate that winter burden and allow rats to reproduce more, Richardson and his colleagues collected data on rat sightings over a dozen years in cities including Tokyo and Amsterdam.

Sightings rose significantly in 11 of the 16 cities analysed, a result unsurprising to those working in rodent management.

Over the past decade, rat sightings in Washington DC jumped by more than 300 per cent, while in New York they went up by 162 per cent, according to Richardson.

Rats are thriving in urban environments.Credit: New York Times

“We live in an infinite sea of rats,” University of California human-wildlife interactions adviser Niamh Quinn said. She was not involved in the study.

About 40 per cent of the overall increase in rat sightings is linked to rising temperatures in cities, according to the analysis. Researchers also found that the more densely populated a city is, and the less green space it has, the more urban rats thrived.

The research is the first to find a link between climate change and rat populations, Canada’s Simon Fraser University assistant professor and urban rat expert Kaylee Byers said.

“It’s a question we’ve had for a while,” Byers, who was not involved in the research, said. The study is “the first one of its kind,” she said, adding that she would like to see more research in labs on the physiology of rodents to see how they respond to temperature changes.

Uncollected garbage lines a Parisian street during a sanitation workers’ strike in 2023.Credit: Bloomberg

Of course, it’s more than just rising temperatures allowing rats to proliferate. Urban rodents thrive in areas with poor rubbish management, where broken garbage bags and open skip bins offer rats a daily feast.

Some cities are stepping up efforts to end the all-you-can-eat buffet. Washington, DC has a “rat academy” to train property managers and private exterminators to spot and attack infestations, while rogue groups of dog owners root out rats with terriers.

New York hired a “rat tsar” to pilot a plan for securing rubbish in bins, where rodents can’t get it.

But rat experts say none of this is enough. In the “war on rats,” the rodents are winning. It is hard to even know the scale of the problem because doing a census on an animal that is nocturnal and hides in sewers and alleys is difficult.

“Hardly any cities are collecting data on rat numbers,” Quinn said. “If you were to look at the cities in the United States, a huge proportion of them don’t have any municipal planning for rats.”

That lack of data forces researchers such as Richardson to use the number of complaints to public health officials as a stand-in for rat populations in cities. But rat complaints, he said, are still “a pretty faithful proxy” for population size.

Byers agreed. “It’s kind of the best data we have.”

With temperatures poised to continue to rise and make rat infestations worse, the findings underscore how municipal governments need to move away from simply trying to poison rats and instead remove the food waste and debris that give them sustenance and shelter, Richardson said.

“Understanding that climate warming may lead to a general increase in rats isn’t good news,” he said, “but it’s really important to know the challenges you’re facing ahead of time so that you can dedicate more resources to try to slow that trend.”

The Washington Post

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