Bobby Corrigan is the rat master. Some call him the rat czar. To others, he is simply a rodentologist, or as NBC recently described him, “one of the nation’s leading experts on rats.” Call him what you want; he is mostly alarmed. “I travel all over the world with this animal, and the amount of complaints and feedback and questions I hear right now are all, ‘We’ve never seen rats in the city like this before,” he said. “They’re all expressing the same concern: Our rat problem is worse than ever.”

Most cities know rat woes well. Washington, D.C., for instance, has burned through countless plans to stymie its longstanding “rat problem” or “rodent crisis,” in which disease-ridden critters are not only growing in number but ballooning to the size of human infants.

What they don’t know is how this all will end. Houston, Texas, is seeing a rat spike this year, and so is New York City. In Chicago, rodent complaints for the early part of the summer have increased about 9 percent from last year, forcing city officials to start sprinkling the streets with rat birth control. Philadelphia and Boston were recently ranked the two cities with the most rat sightings in the country. And it’s not just this year; as USA Today reported last year, major cities saw spikes in rodent-related business from 2013 to 2015. Calls to Orkin, the pest control service, were reportedly “up 61 percent in Chicago; 67 percent in Boston; 174 percent in San Francisco; 129 percent in New York City; and 57 percent in Washington, D.C.”

It’s no surprise that rats thrive in cities, where humans provide an abundance of food and shelter. But experts now agree that the weather is playing a role in these recent increases. Extreme summer heat and this past winter’s mild temperatures have created urban rat utopias.

“The reason the rats are so bad now, we believe, is because of the warm winters,” said Gerard Brown, program manager of the Rodent and Vector Control Division of the D.C. Department of Health, at a 2016 rat summit.