Pope Francis is once again proving he has absolutely zero patience for climate change deniers.

Climate change is “one of the most worrying phenomena our humanity is experiencing,” the pontiff wrote Thursday in a letter sent to world leaders gathered for the United Nations’ annual climate change conference. He also warned participants against falling prey to certain “perverse” attitudes on the issue, including denial, indifference and resignation.

Such attitudes “certainly do not help honest research or sincere and productive dialogue on building the future of our planet,” Francis wrote, according to a translation provided by Vatican Radio. “We need an exchange that unites us all, because the environmental challenge we are experiencing, and its human roots, regards us all, and affects us all.”

NurPhoto via Getty Images Pope Francis has long been an advocate for action on climate change.

The purpose of this year’s meeting, officially called the 23rd Conference of the Parties, is to hammer out how countries will implement the Paris climate accord. The landmark pact, which nearly 200 countries signed in 2015, aims to reduce greenhouse gas emissions around the world.

Francis didn’t berate any particular nation. But there’s only one country in the world that has remained opposed to the global agreement: the United States.

JOHN MACDOUGALL via Getty Images From left to right: French President Emmanuel Macron; prime minister of Fiji and president of COP 23 Frank Bainimarama; German Chancellor Angela Merkel; and UN Secretary-General Antonio Guterres. The leaders pose on Wednesday before the opening session of the United Nations' conference on climate change in Bonn, Germany.

President Donald Trump promised in June that he would withdraw the U.S. from the pact, claiming it would damage the American economy. The U.S. is one of the world’s top emitters of greenhouse gases.

The Trump administration has also proposed rolling back the Clean Power Plan, an Obama-era federal regulation aimed at limiting greenhouse gas emissions from power plants. The president has instead promised to revive America’s coal industry and increase oil and gas production.

Trump has long been a climate change skeptic, previously describing the phenomenon as “bullshit” and a hoax “created by and for the Chinese.”

Francis, on the other hand, is a staunch advocate for action on the environment. In a 2015 encyclical called “Laudato Si,” he made it clear that he believes humans are partly to blame for the troubles the Earth is facing. He laid out a moral and theological case for addressing climate change and urged Catholics and others around the world to make the issue a top priority.

AFP via Getty Images Pope Francis exchanges gifts with President Donald Trump and first lady Melania Trump at the Vatican on May 24.

When Francis met Trump at the Vatican earlier this year, the pontiff sent the president off with some heavy reading ― a copy of “Laudato Si” was among the many gifts Francis gave Trump.

Judging by the administration’s decision to defend fossil fuels at the COP23 conference, there’s a chance Trump may not have gleaned much from his homework.