For the first time, someone has put a price on what Americans are willing to pay to protect their children and grandchildren from the effects of global warming: $18.75 a month.

The effort began with a Washington Post/ABC News poll last week that found that 62 percent of Americans think the government should regulate greenhouse gases even if regulation raises the costs of things they buy. The pollsters then briefly described the cap-and-trade legislation that passed the House on Friday. Fifty-two percent of those polled said they would support it.

The poll then asked if respondents would be willing to pay $10 a month extra on their electric bills to cover higher costs if cap-and-trade significantly reduced greenhouses gases. Fifty-six percent said they would; 42 percent said they wouldn’t.

Then the pollsters asked if respondents would be willing to pay $25 a month extra. Support dropped to 44 percent and opposition increased to 54 percent.

Nate Silver, a respected poll analyst who runs the numbers-crunching website FiveThirtyEight.com, then plotted the poll results on a graph and came up with what he called the “environmental indifference point,” the price point at which the majority of Americans no longer feel the effort is worth it: $18.75 a month.

The Congressional Budget Office estimated the cap-and-trade bill would raise the average annual household electric bill $14.58 a month by 2020, well below the indifference point. Electricity would cost more because utility companies that burn coal and other fossil fuel would have to buy pollution permits if they exceed their emissions caps.

The CBO’s numbers, too, are squishy. They don’t include the impact that higher energy prices would have on the rest of the economy. Nor do they extend beyond the year 2020, when the cost of emission permits is expected to be much higher. By 2020, the House-passed bill mandates a 17 percent reduction in greenhouse gases, rising to 83 percent by the year 2050. It’s entirely likely that the cost of meeting those goals will exceed the indifference point.

On the other hand, what’s the alternative?

The consensus among climate scientists is that greenhouse gas emissions have caused, are causing and will continue to cause dangerous changes to the planet. Indeed, many climatologists believe the cap-and-trade bill is a Band-aid on a sucking chest wound. NASA scientist James Hansen, often called the “father of global warming,” calls the bill a “monstrous absurdity.”

But it is what is politically possible now — if only barely. If four House members had changed their votes, it would have failed, and there’s no guarantee it will pass the Senate. It is just the first step on a long road, and it’s going to cost all of us more than we’re comfortable with.

To pretend that it will not is disingenuous. But to ignore the threat — even for the noble goal of promoting economic growth — is suicidal. Sooner or later, if you put off repairs on your roof, it will come crashing down on your children’s heads.