The conventional wisdom is that the chances of Congress passing a bill that puts both a cap and a price on greenhouse gases are somewhere between terrible and nil. President Obama can start to prove the conventional wisdom wrong by making a full-throated case for a climate bill in his State of the Union speech this week.

Washington has been forecasting the likely death of a climate bill with renewed certainty since Massachusetts elected a Republican senator who promised to block pretty much anything Mr. Obama wants. But even before then we were hearing two reasons why a bill could not pass: The Senate won’t have any strength left when it finishes with health care, and the nation cannot afford a bill that implies an increase in energy prices.

The first reason is defeatist, the second greatly exaggerated. The climate change bills pending in the Senate would not begin to bite for several years, when the recession should be over. The cost to households, according to the Congressional Budget Office, would be small. A good program would create more jobs than it cost.

The list of reasons to pass a climate bill, on the other hand, is long and persuasive.

Start with timing. The long-term trend in greenhouse gas emissions is up (the decade ending in 2009 was the warmest on record), and the sooner emissions decline, the better. The bill passed by the House last year calls for emissions in 2020 to be 17 percent lower than they were in 2005. This is the bare minimum required to give the industrialized world a fighting chance of achieving an 80 percent reduction by midcentury, which most mainstream scientists think will be necessary to avert the worst consequences of global warming.