Apparently everyone knows that global warming only makes climate more extreme. A hot, dry summer has triggered another flood of such claims. And, while many interests are at work, one of the players that benefits the most from this story are the media：the notion of “extreme” climate simply makes for more compelling news.

Consider Paul Krugman, writing breathlessly in the New York Times about the “rising incidence of extreme events.” He claims that global warming caused the current drought in America’s Midwest, and that supposedly record-high corn prices could cause a global food crisis.

But the United Nations climate panel’s latest assessment tells us precisely the opposite： For “North America, there is medium confidence that there has an overall slight tendency toward less dryness. ” Moreover, there is no way that Krugman could have identified this drought as being caused by global warming without a time machine: Climate models estimate that such detection will be possible by 2048, at the earliest.

And, fortunately, this year’s drought appears unlikely to cause a food crisis, as global rice and wheat supplies remain plentiful. Moreover, Krugman overlooks inflation： Prices have increased six-fold since 1969，so, while corn futures (期货)did set a record of about $8 per bushgl (蒲式耳)in late July, the inflation-adjusted price of corn was higher throughout most of the 1970s, reaching $ 16 in 1974.

Finally, Krugman conveniently forgets that concerns about global warming are the main reason that corn prices have skyrocketed since 2005. Nowadays 40 percent of corn grown in the United States is used to produce ethanol (乙醇），which does absolutely nothing for the climate, but certainly distorts the price of corn—at the expense of many of the world’s poorest people.

Bill Mckibben similarly worries in The Guardian about the Midwest drought and corn prices. He confidently tells us that raging wildfires from New Mexico and Colorado to Siberia are “exactly” what the early stages of global warming look like.

In fact, the latest overview of global wildfire incidence suggests that fire intensity has declined over the past 70 years and is now close to its preindustrial level.

When well-meaning campaigners want us to pay attention to global warming, they often end up pitching beyond the facts. And, while this may seem justified by a noble goal, such “policy by panic” tactics rarely work, and often backfire.

Remember how, in the wake of Hurricane Katrina in 2005, A1 Gore claimed that we were in store for ever more destructive hurricanes? Since then, hurricane incidence has dropped off the charts. Exaggerated claims merely fuel public distrust and disengagement.
   That is unfortunate, because global warming is a real problem, and we do need to address it.