For the moment, we have an opportunity to make fundamental changes to the way our country deals with the environment, changes that would lock in public acceptance of climate change. But we have to act quickly — not only because the crisis is urgent, but because one good blizzard could change a lot of minds.

For now, though, Americans, long cynical about global warming, are confronting the facts. According to a survey conducted in July by the University of Texas, 70 percent of Americans believe the climate is changing, compared to 65 percent in March, and only 15 percent say it isn’t. Party affiliation continues to divide public opinion, but today most Republicans, 53 percent, believe in climate change, as do 72 percent of independents and 87 percent of Democrats.

Perhaps that’s because this year’s extreme weather has afflicted residents of red and blue states equally. The United States Drought Monitor, based at the University of Nebraska, reports that moderate to severe drought conditions this summer are affecting 64 percent of the lower 48 states, leading to domestic food inflation and record high prices for grain.

The Midwest is becoming a Dust Bowl, the Southwest and Rocky Mountains a tinder box. Lakes and rivers across the South are drying up. And a series of brutal heat waves, severe storms and prolonged power failures has punished residents of the Northeast, generating widespread concern that the region’s infrastructure is woefully unprepared for the strange weather that’s become our new norm.

But the most visible human drama of climate change is happening in cities. Cities are not merely the population centers where dense concentrations of people are trapped and exposed during dangerous weather events. They are also “heat islands,” whose asphalt, brick, concrete and steel attract the heat while pollution from automobiles, factories and air-conditioners traps it. City dwellers experience elevated heat at all hours, but the difference matters most at night, when the failure of high temperatures to fall deprives them of natural relief. For the most vulnerable people, these “high lows” can be the difference between life and death.

Americans began to take urban heat seriously after 1995, when a record-breaking heat wave — three days of triple-digit heat — baked Chicago. Ordinarily, heat waves fail to produce the kind of spectacular imagery we see in other disasters, like earthquakes, tornadoes, hurricanes and floods. Heat doesn’t generate much property damage, nor does it reveal its force to the camera or naked eye. Heat waves are invisible killers of old, poor and other mostly invisible people. Until the summer of 1995, medical examiners and media outlets often neglected to report heat-related deaths altogether.

But the great Chicago heat wave changed things. It caused so much suffering that at one point nearly half the city’s emergency rooms closed their doors to new patients. Hospitals were not the only institutions stretched beyond capacity by the heat. Streets buckled. Trains derailed. The power grid failed. Water pressure diminished. Ambulances were delayed.

There were “water wars” in poor neighborhoods, where city workers cracked down on residents who opened fire hydrants for relief. There were surreal scenes at City Hall, where members of the mayor’s staff declined to declare a heat emergency, forgot to implement their extreme heat plan and refused to bring in additional ambulances and paramedics.

And there was Mayor Richard M. Daley, telling reporters: “It’s hot. It’s very hot. But let’s not blow it out of proportion,” while the morgue ran out of bays and the medical examiner had to call in a fleet of refrigerated trucks to handle the load. When the temperatures finally broke, 739 Chicagoans had died as a result of the heat wave.

Chicago learned from the disaster, and today it is a national leader in planning for the next acute heat emergency. The city compiles a list of old, isolated and vulnerable residents, and public workers contact them when dangerous weather arrives. City officials and community organizations promote awareness and encourage residents to check in on one another. The local news media treat heat waves as true public health hazards. Everyone knows how perilous the new climate can be.

Unfortunately, Chicago keeps getting reminders. In the early July heat wave, despite its improved emergency response system, Chicago reported more heat deaths than any other city or state. And this week the Union of Concerned Scientists released “Heat in the Heartland,” a study that reports an increased incidence of dangerous hot weather throughout the Midwest in the past 60 years, including elevated evening temperatures and more heat waves lasting three days or longer. Along with Chicago, the report singles out St. Louis, Detroit, Minneapolis and Cincinnati as being at risk, but also cites public health research predicting more heat waves in towns and cities throughout the Midwest and Northeast.

There are likely to be more such reports this summer, like a recent Environment America Research and Policy Center study finding more extreme downpours as a result of global warming. If a particularly strong hurricane hits, climatologists will once again explain that rising sea levels increase the likelihood of more, and more severe, storms, and perhaps even more Americans will begin believing in climate change. Then again, in our event-driven culture, a cold winter may well reverse the trend.

Americans’ growing concerns about global warming will mean nothing if our national leaders are unwilling to seize the moment and do something about it. No city wants to be the next Chicago, and no urban government should go without the kind of acute-heat emergency plan that Chicago has today. But the question that may determine the life and death of future generations is whether we will face up to the larger heat emergency before the political climate changes once again.