We’re on a similar trajectory with our debt. Mounting deficits have driven America’s debt-to-G.D.P. ratio from 36.2 percent in 2007 to 72.8 percent today. In their widely hailed book on credit crises, “This Time Is Different,” the economists Carmen Reinhart and Kenneth Rogoff argue that countries that allow their debt-to-G.D.P. ratios to exceed 90 percent experience slower growth and greater instability — much like hitting a climate tipping point. Indeed, they note, those who would point to low interest rates today as some kind of “all-clear” for more debt “should remember that market interest rates can change like the weather.”

Image Thomas L. Friedman Credit... Josh Haner/The New York Times

There is another striking parallel. At some point, when we allow so much carbon to build up in the atmosphere, our mightiest efforts to cut emissions through energy efficiency, conservation and new technologies will only enable us to stay in place. They won’t be able bend the curve downward anymore. And 450 p.p.m. is not a place we want to get stuck. And, at some point, the debt will get so large that big tax increases and spending cuts will simply go to pay interest. We also won’t be able to bend that curve anymore, and spending on infrastructure, education and the poor will vanish.

I am struck by how many liberals insist on reducing carbon emissions immediately, but, on the deficit, say there is no urgency because no interest rates rises are in sight. And I am struck by how many conservatives insist we must reduce the deficit immediately, but, on climate, say there is no urgency because, so far, temperature rise has been slight. (Although 2012 was the hottest year on record in the continental U.S.) One reason interest rates are so low is that they are being suppressed by the Federal Reserve’s quantitative easing. That won’t last. As for the climate, well, “Mother Nature doesn’t do quantitative easing,” said Harvey. Beware of nonlinear moves in both.

We can’t go off coal overnight, and we can’t go into recession by cutting spending overnight, but we need to start tapping on the brakes in both realms by agreeing on spending cuts, tax increases and new investments that would be phased in as the economy improves, as well as higher efficiency standards for power plants, buildings, vehicles and appliances that would be phased in, too.

A carbon tax would reinforce and make both strategies easier. According to a September 2012 study by the Congressional Research Service, a small carbon tax of $20 per ton — escalating by 5.6 percent annually — could cut the projected 10-year deficit by roughly 50 percent (from $2.3 trillion down to $1.1 trillion).