By PAUL KRUGMAN

A couple of weeks ago the Northeast was in the grip of a severe heat wave. As I write this, however, it’s a fairly cool day in New Jersey, considering that it’s late July. Weather is like that; it fluctuates.

And this banal observation may be what dooms us to climate catastrophe, in two ways. On one side, the variability of temperatures from day to day and year to year makes it easy to miss, ignore or obscure the longer-term upward trend. On the other, even a fairly modest rise in average temperatures translates into a much higher frequency of extreme events — like the devastating drought now gripping America’s heartland — that do vast damage.

On the first point: Even with the best will in the world, it would be hard for most people to stay focused on the big picture in the face of short-run fluctuations. When the mercury is high and the crops are withering, everyone talks about it, and some make the connection to global warming. But let the days grow a bit cooler and the rains fall, and inevitably people’s attention turns to other matters.

Making things much worse, of course, is the role of players who don’t have the best will in the world. Climate change denial is a major industry, lavishly financed by Exxon, the Koch brothers and others with a financial stake in the continued burning of fossil fuels. And exploiting variability is one of the key tricks of that industry’s trade. Applications range from the Fox News perennial — “It’s cold outside! Al Gore was wrong!” — to the constant claims that we’re experiencing global cooling, not warming, because it’s not as hot right now as it was a few years back.

How should we think about the relationship between climate change and day-to-day experience? Almost a quarter of a century ago James Hansen, the NASA scientist who did more than anyone to put climate change on the agenda, suggested the analogy of loaded dice. Imagine, he and his associates suggested, representing the probabilities of a hot, average or cold summer by historical standards as a die with two faces painted red, two white and two blue. By the early 21st century, they predicted, it would be as if four of the faces were red, one white and one blue. Hot summers would become much more frequent, but there would still be cold summers now and then.

And so it has proved. As documented in a new paper by Dr. Hansen and others, cold summers by historical standards still happen, but rarely, while hot summers have in fact become roughly twice as prevalent. And 9 of the 10 hottest years on record have occurred since 2000.

But that’s not all: really extreme high temperatures, the kind of thing that used to happen very rarely in the past, have now become fairly common. Think of it as rolling two sixes, which happens less than 3 percent of the time with fair dice, but more often when the dice are loaded. And this rising incidence of extreme events, reflecting the same variability of weather that can obscure the reality of climate change, means that the costs of climate change aren’t a distant prospect, decades in the future. On the contrary, they’re already here, even though so far global temperatures are only about 1 degree Fahrenheit above their historical norms, a small fraction of their eventual rise if we don’t act.

The great Midwestern drought is a case in point. This drought has already sent corn prices to their highest level ever. If it continues, it could cause a global food crisis, because the U.S. heartland is still the world’s breadbasket. And yes, the drought is linked to climate change: such events have happened before, but they’re much more likely now than they used to be.

Now, maybe this drought will break in time to avoid the worst. But there will be more events like this. Joseph Romm, the influential climate blogger, has coined the term “Dust-Bowlification” for the prospect of extended periods of extreme drought in formerly productive agricultural areas. He has been arguing for some time that this phenomenon, with its disastrous effects on food security, is likely to be the leading edge of damage from climate change, taking place over the next few decades; the drowning of Florida by rising sea levels and all that will come later.

And here it comes.

Will the current drought finally lead to serious climate action? History isn’t encouraging. The deniers will surely keep on denying, especially because conceding at this point that the science they’ve trashed was right all along would be to admit their own culpability for the looming disaster. And the public is all too likely to lose interest again the next time the die comes up white or blue.

But let’s hope that this time is different. For large-scale damage from climate change is no longer a disaster waiting to happen. It’s happening now.