Echoing the virtually unanimous findings of mainstream scientists, Pope Francis fixes the blame squarely on humans and their burning of fossil fuels, while warning of an “unprecedented destruction of ecosystems, with serious consequences for all of us.” The hardest-hit — here again he echoes mainstream thinking — will be the poorest citizens of the poorest countries, those least able to adapt to the rising seas and devastating droughts and floods that are likely to occur even in this century without swift remedial action.

The timing of “Laudato Si” could not have been better. In December, delegates from nearly 200 nations will gather in Paris to make one more attempt at a global arrangement that would commit all nations to reduce their emissions of carbon dioxide and other greenhouse gases, before atmospheric concentrations reach what some believe is the point beyond which truly intolerable consequences are inescapable. Some countries, including the United States and the European Union, have already published their commitments — President Obama has pledged to reduce America’s greenhouse gas emissions by 26 percent below 2005 levels by 2025. Other big emitters, including China and Brazil, are expected to do so well before Paris.

Although the Vatican has not said so, Pope Francis must surely have intended the document as a nudge to individual governments; Brazil, for instance, has a large Catholic population and may see his words as cover for what could be unpopular policies to limit development and the clearing of forests for agricultural purposes — a restriction that is essential to any effective climate policy.

Sadly, the encyclical, compelling as it is, is unlikely to have a similarly positive effect on American politics. The Republican presidential candidates, on the whole, have avoided discussing climate change, at least when they’re not dismissing it completely as an issue. Meanwhile, in Congress, by straight party-line votes, both the House and Senate appropriations committees have passed bills that would make it impossible for President Obama to limit carbon dioxide emissions from coal-fired power plants — the centerpiece of the strategy he intends to present to the negotiators in Paris.

Even though Mr. Obama is virtually certain to veto such measures, the fact that a majority in Congress does not support him cannot help his credibility as a world leader on the climate issue. But of course, that matters not a bit to the likes of Mitch McConnell, the Senate majority leader. What concerns Mr. McConnell is whether the Environmental Protection Agency will impose new rules on the coal mining industry in Kentucky.