Fewer than half of Catholics in the United States are aware of the Pope's encyclical on the environment, according to a new survey today.

Four in ten Catholics and three in ten of all adults in the US knew of the encyclical, according to a poll by Associated Press and Yale University.

Even fewer, under a quarter, had heard it mentioned in church.

The environment is expected to be one of the subjects he addresses in speeches and sermons when Pope Francis visits Philadelphia for the World Meeting of Families at the end of September.

The visit comes as forest fires have raged this summer in states such as California, where once plentiful reservoirs are becoming little more than puddles of water as a three-year drought intensifies. A paper published in the Geophysical Research Letters paper today argues that while the drought is probably due to natural variations in the weather, it could become acute because of global warming. Scepticism on climate change is strong in parts of the US.

In his encyclical, Laudato Si, published in June, Pope Francis argued that global warming was driven by a "structurally perverse" world economic system and an unfettered pursuit of profit that exploited the poor and risked turning the Earth into an "immense pile of filth." He urged people of all faiths and no faith to save God's creation for future generations.

Just 43 per cent of Catholics and 39 per cent of all adults told Associated Press that global warming was a moral issue. Few believed that climate change had any connection to religion or poverty.

Rather than focus on environmental issues, Catholic bishops in the US have preferred to speak to issues such as marriage, abortion and restoring lapsed Catholics to faith. The issue of clergy sex abuse has also dominated the agenda for several years.

Some have taken up the baton of climate change however. Bishops in Iowa, Illinois and Ohio have held news conferences Laudato Si, echoing the Pope's call for leadership on the issue and and pledging to cut carbon emissions. Orange diocese in California ran a conference on the theology of the encyclical. Miami Archbishop Thomas Wenski has backed President Barack Obama's clean power plant rules announced this month.