Mayor Bill de Blasio pledges to cut New York carbon emissions by 40% by 2030

• De Blasio among several US mayors at Vatican climate change summit
• Mayor says papal encyclical 'is not a call to arms, it is a call to sanity'

Bill de Blasio, the mayor of New York City, has pledged to reduce his city's carbon footprint by 40% by 2030 in a speech in which he praised Pope Francis - who he called the "highest moral authority" - for calling on the world to take bold action on climate change.

The remarks were presented at a rare Vatican meeting of mayors from around the world, who gathered to discuss the environmental challenges facing their cities and the correlation between global warming and modern slavery.