First the bad news: The “safest” herbicide in the history of science may be harming us in ways we’re just beginning to understand. And now for the really bad news: Because too much is never enough, the Environmental Protection Agency just raised the allowable limits for how much of that chemical can remain on the food we eat, and the crops we feed to animals – many of which end up on our plates as well. If you haven’t guessed its identity yet, it’s Monsanto’s RoundUp, a powerful weed killer.

Here’s the thing: As farmers adopted Monsanto’s genetically modified seeds in droves — the majority of corn, soy and cotton grown worldwide include the company’s RoundupReady trait — there has been an explosion in the use of the pesticide for which the trait is designed: You guessed it, RoundUp.

In the U.S. alone, it’s estimated that over 200 million pounds of the stuff are spread on fields and farms every year. That’s almost triple the amount used in 2001. (These numbers, by the way, are all estimates, since the USDA doesn’t precisely track glyphosate use because MONSANTO!)

Here at Grist, climate change is our bread and melting butter. But this month, we’re feeling especially hot and bothered. As part of our in-depth look at the warming planet, we’ve compiled a list of the U.S. cities that we think will be in the hottest water as the mercury rises — in some cases, up to their foreheads.

Phoenix, Ariz.: The founders of Phoenix spotted a particularly dry stretch of desert and thought, “You know what this place could use? Golf courses.” Unfortunately, this town of 4.5 million has been getting hotter by almost a degree a decade since 1961; in 2011 Phoenix had 33 days over 110. In heat like that, air conditioning is a life-and-death issue, and that A/C runs on America’s electric grid. That’s scary enough, but the power on that grid comes from dams on the Colorado River — the same shrinking river that wets Phoenix’s enormous whistle. Then again, Phoenicians named their town after a bird that periodically bursts into flames, so they must have seen this coming.