http://www.newsweek.com/rat-explosion-climate-change-2-degrees-global-warming-moisture-heat-pests-nyc-1200780

Scientists are warning that rising temperatures caused by climate change will turn cities worldwide into ideal rat breeding grounds. 

Several U.S. cities including New York, Chicago, and Boston pump millions of dollars each year into their respective battles against a rat population explosion. But scientists predict that a global warming of "2 degrees" Celsius, or 3.6 degrees Fahrenheit, will occur across the globe by the end of this century. This increase of warmer winters and hotter summers provides the most fertile breeding ground for rats to reproduce at staggering rates.