Pope Francis warned time was "running out" to save the planet while expressing regret at his absence from climate talks in Glasgow, in a letter published Thursday.
The COP26 summit, he said in the letter to Catholics in Scotland, was "meant to address one of the great moral issues of our time: the preservation of God's creation."
"Time is running out; this occasion must not be wasted, lest we have to face God's judgement for our failure to be faithful stewards of the world he has entrusted to our care," wrote Francis.
The pontiff said in the letter dated Tuesday, but published by the Vatican on Thursday, that he had hoped to take part in the COP26 climate talks underway in Glasgow.
But "I regret that this did not prove possible."
The 84-year-old is a staunch advocate for the environment and has called climate change an "unprecedented threat" requiring urgent action.
The crunch COP26 summit -- in which negotiators are wrangling over ways to limit global warming to 1.5-2 degrees Celsius compared to pre-industrial levels -- ends Friday.