Cities are driving the human-caused climate change that threatens the global environment, but also offer hope.

That is the overriding message of a chapter devoted to cities in a major U.N. report on climate change released on Monday, providing city planners around the world with guideposts aimed at avoiding climate catastrophe.

"The fact that cities are responsible for more than two-thirds of global greenhouse gas emissions means that if cities do something, they can solve two-thirds of the problem. So that's pretty exciting," said Karen Seto, a professor at Yale University's School of the Environment and one of two coordinating lead authors of the report's chapter on cities.