The pontiff was welcomed to America on the first full day of his trip there by President Barack Obama who commented the Lord had made a beautiful day.
"Holy Father, on behalf of Michelle and myself, welcome to the White House," he said, joking that his back yard was not typically that crowded.
A crowd of 15,000 watched as the pope and president stood to attention on a red-carpeted platform decorated with red, white and blue bunting for the national anthems of the Holy See and the United States.
Francis said: "When it comes to the care of our common home we are living at a critical moment of history.
"We still have time to make the change needed to bring about a sustainable and integral development, for we know that, things can change.
"Such change demands on our part a serious and responsible recognition, not only of the kind of the world we may be leaving to our children but also to the millions living under a system which has over looked them."
Francis has been fending off conservative criticism of his economic views. He told reporters on his flight from Cuba that some people may have an inaccurate impression that he is "a little bit more left-leaning".
"I am certain that I have never said anything beyond what is in the social doctrine of the church," he said.
As for conservatives who question whether he is truly Catholic, he added: "If I have to recite the Creed, I'm ready."
The White House said Mr Obama was hoping to find common ground with the pope on efforts to combat climate change and fight income inequality.
But the two differ on other issues such as abortion and same-sex marriage.
Mr Obama told the pontiff: "We support your call to all world leaders to support the communities most vulnerable to changing climates and to come together to preserve our precious world for future generations.
"Your Holiness in your words and deeds you set a profound moral example and in these gentle but firm reminds of our obligations to God and to one another you are shaking us out of our complacency."
He added: "You remind us that the Lord's most powerful message is mercy, that means welcoming the stranger with empathy and an open heart.
"From the refugee who flees war torn lands to the immigrant who leaves home in search of a better life.
"It means showing compassion and love for the marginalised and the outcast."
The 78-year-old pontiff is due to meet with America's 450-strong bishops' conference at the Cathedral of St Matthew the Apostle.
