The papal encyclical on climate change added a welcome moral dimension to what has been a mostly scientific and political issue. Global policymakers, especially in the U.S., should hear and heed Pope Francis, who wrote that “a very solid scientific consensus indicates that we are presently witnessing a disturbing warming of the climactic system. … Humanity is called to recognize the need for changes of lifestyle, production and consumption, in order to combat this warming or at least the human causes which produce or aggravate it.”

This stark assessment has discomfited some U.S. conservatives. Republican leaders often express skepticism, or denial, or even claim that global warming is a hoax. The wide partisan divide extends to everyday Americans, according to a July 1 poll from the Pew Research Center. Ideological “silos,” Pew reports, have resulted in a 44-percentage-point gap between Republicans and Democrats on those who say the Earth is warming due to human activity.

This split freezes the climate change debate in the U.S., but the world won’t wait. Globally, the first six months of 2015 were the hottest ever recorded, according to the National Oceanic and Atmospheric Administration.

The political calendar is about to heat up, too, culminating in a major U.N. climate change conference in Paris this December. The U.S. can lead by example and by persuading nations to curb carbon emissions through conservation, using alternative energy and other methods. Instead, however, the conference will likely coincide with GOP presidential candidates pledging to roll back the relatively limited response President Obama has achieved.

U.S. political dynamics are unique, if not singular, said Minneapolis Mayor Betsy Hodges, who this week attended a Vatican summit on climate change and human trafficking. “I noticed that, probably about halfway through today, and just realized that climate change deniers cast a pall on the conversation in the United States that as far as I can tell does not extend anywhere else in the world,” Hodges said from Rome on Wednesday. “The sensation here was very much, ‘Well, this is happening so what are we going to do about it?’ ”

The “we” were mostly mayors whose cities can lead. “Cities are the most important place to go for solutions around climate change because cities are also places that have factors that most significantly contribute to climate challenges, but also are a source of innovation to respond to those challenges as well,” Hodges said.

The Vatican’s message, with its moral clarity and acknowledgment of accepted science, is a galvanizing event. Yet U.S. policy changes happen through the political process, meaning that for real progress more Republicans will need to heed the conservative principle that the planet must be protected.