One is the difficulty that all democracies face with decisions that impose costs today while promising benefits tomorrow. The shift toward a lower-carbon economy could produce compounding advantages in the form of new industries, new jobs, and, not inconsequentially for the politicians making these decisions, new campaign contributors. It could also prevent environmental hazards that would otherwise occur in a warming world. Yet for many political leaders, all of that has seemed less compelling than the jobs (and contributions) tied to the existing fossil-fuel infrastructure.

On this front, though, the balance looks to be shifting toward environmentalists. Scientific evidence is strengthening the case that not acting on climate carries its own costs—not someday, but now.

The federal National Climate Assessment released last week catalogued current-day consequences linked to a shifting climate that range from heat waves, droughts, and extreme weather (more high-intensity hurricanes along the Atlantic Coast and a nearly 40 percent increase in heavy downpours in the Midwest) to rising sea levels pressing against coastal cities. Scientists followed that cannon shot with the release of new studies this week showing that climate change is accelerating an apparently irreversible melting in the West Antarctic ice cap that will raise sea levels worldwide.

Yet even as the price of inaction grows more tangible, a second structural barrier impedes legislative action. Much like gun control, climate is an issue that unites Republicans by ideology but divides Democrats by geography. Even if Democrats can build a bigger Senate majority through the next few election cycles—they are positioned to add seats in 2016 even if they lose control in 2014—such gains probably won't produce the 60 votes needed to break a filibuster against legislation to limit carbon emissions.

The Democrats' problem is that they cannot build a big Senate majority without winning seats in states heavily dependent on coal, which would suffer the most from limits on carbon. Democrats now hold 21 of the Senate seats in the 19 states that rely on coal to produce a majority of their electricity and half of the seats in the 10 states (some overlapping) that mine the most coal. Resistance from some coal-state Democrats doomed climate legislation in 2009, even when the party controlled 60 Senate seats and then-Speaker Nancy Pelosi narrowly muscled a cap-and-trade bill through the House. Senate Democrats such as North Dakota's Heidi Heitkamp and Indiana's Joe Donnelly remain equally unenthusiastic today.

Theoretically, those Democratic votes could be replaced by Republican votes from states less reliant on coal. But Republicans face overwhelming ideological pressure to oppose action on climate change and even to reject the scientific consensus that it is occurring, as Senator Marco Rubio from vulnerable Florida demonstrated in his dismissal of the federal climate report. Republican unity and Democratic division promises a permanent legislative stalemate over climate.