Pope Francis’s forthcoming encyclical on climate change and nature looks set to be one of the defining statements of his pontificate. In the two years that he has occupied the throne of St Peter, the Argentine pontiff has charted a radical course, departing from his predecessors on key topics such as sexuality and marriage while pursuing reforms to the administration of the Church itself. Now, he is intervening in one of the most heated political and scientific debates of our age in a way that will delight many environmentalists and alarm conservatives among his flock.

In “Laudato Si” (Be Praised), to be published tomorrow but some of whose contents have been leaked in advance, the Pope has thrown his weight behind the nostrums of climate science. He acknowledges the weight of scientific evidence that places the blame on “human action”. That may not seem an earth-shaking conclusion to some. The Vatican has long overcome its historic reticence about accepting scientific advance.

What is striking and courageous, however, is the overt political content of the statement. Its timing is significant, given that the world is preparing for this December’s climate conference in Paris, widely seen as the last chance to secure a deal on global emissions.