ABOARD THE PAPAL PLANE (Reuters, AFP) - Pope Francis said the recent spate of hurricanes should prompt people to understand that humanity will "go down" if it does not address climate change and history will judge those who deny the science.

"If we don't turn back, we will go down," Pope Francis told reporters on Sunday (Sept 10) on a plane returning from Colombia to the Vatican City. Francis strongly backed the 2015 Paris agreement on reducing global warming, from which the United States withdrew this year.

Pope Francis spoke as hurricane Irma pounded central Florida as it carved through the state with high winds, storm surges and torrential rains that left millions without power, ripped roofs off homes and flooded city streets.

Pope Francis was asked about recent hurricanes, including Irma and Harvey, and if political leaders who do not want to work with other countries to stem global warming should be held morally responsible for future effects on the planet.

"You can see the effects of climate change and scientists have clearly said what path we have to follow," he said, referring to a consensus by scientists that global warming is caused by human activity such as fossil fuels.

"All of us have a responsibility, all of us, small or large, a moral responsibility. We have to take it seriously. We can't joke about it," he said. "Each person has their own. Even politicians have their own."

Ahead of the Paris summit in 2015, Pope Francis wrote a major encyclical, or papal letter, on the care of the environment which backed the gradual elimination of fossil fuels to stem global warming.

The accord, agreed on by nearly 200 countries, aims to cut emissions blamed for global warming.

The United States committed to reducing its own by 26 to 28 per cent, compared with 2005 levels, by 2025. Many world leaders criticised US President Donald Trump for pulling out.

"If someone is doubtful that this is true, they should ask scientists. They are very clear. These are not opinions made on the fly. They are very clear. Then each person can decide and history will judge the decisions," he said.

The pope has already clashed with Mr Trump over immigration issues, such as the president's intention to build a wall on the border with Mexico and deport undocumented aliens. Mr Trump withdrew from the Paris agreement shortly after visiting the Vatican in May. The Vatican had urged him to stay in the accord.

A Vatican official said at the time that the US move was a"slap in the face" for the pope and the Vatican.

The pope also said he hoped Mr Trump would re-think his decision to end a programme protecting undocumented immigrant children, saying it was important for young people to have roots.

"One hopes that it is re-thought somewhat," Pope Francis said in response to a question on the plane returning from Colombia about the programme started by Mr Trump's predecessor and known as Deferred Action on Childhood Arrivals (DACA).

He spoke about DACA and the problems of immigrant families in general, adding that he wanted to learn more about Mr Trump's decision.

Mr Trump's move this month to end the five-year-old programme instituted by former President Barack Obama plunged almost 800,000 young people, known as "Dreamers," into uncertainty.

It drew criticism from business and religious leaders, mayors, governors, Democratic lawmakers, unions and civil liberties advocates. Mr Trump said during his election campaign he would end DACA if he became president.

Last week, 15 states and the District of Columbia filed a lawsuit challenging Mr Trump's decision. It claims his decision was "motivated, at least in part, by a discriminatory motive" against Mexicans, who are the largest beneficiary of the programme.

It points to his statements from the 2016 presidential campaign.

"In the end, young people feel like they have no hope. And who robs them of hope? Drugs, other addictions, suicides - youth suicides are very high - and this happens when they are torn from their roots," Pope Francis said.

"The relationship between a young person and his roots is very important. Young people who have been uprooted today are asking for help," he said.

He appeared to take a swipe at Mr Trump when he said: "The president of the United States... presents himself as a man who is pro-life. If he is a good pro-lifer, he understands that the family is the cradle of life and you have to defend its unity."

Pope Francis, the first Latin American pope, has made defence of immigrants a major plank of his papacy. The pontiff said during the presidential campaign that a man who thought about building walls and not bridges was "not Christian".

Mr Trump, who grew up in a Presbyterian family, shot back saying it was "'disgraceful" for the pope to question his faith.

Pope Francis on Monday appealed to the United Nations to help Venezuela end a deadly economic, political and social crisis.

"The most painful is the humanitarian problem. There are so many people fleeing or suffering," the pontiff told journalists aboard the papal plane on his return trip from a five-day visit to Colombia.

"It's a humanitarian problem that needs to be addressed, and I believe the UN must make itself heard, to help," he said.

On Sunday, Pope Francis issued a special prayer for a peaceful end to Venezuela's "grave crisis", which has caused food and medicine shortages, and clashes at anti-government protests that left 125 people dead from April to July.

The South American country's legislative speaker had called on the pope to plead with President Nicolas Maduro to allow food aid to reach the destitute.

He noted that donations sent to Venezuela in recent months by organisations such as Caritas Chile have not been accepted by the Maduro government.

The Vatican was one of the facilitators of a month-long dialogue between the Venezuelan government and the opposition, which collapsed in mutual recrimination.

"I don't know what Maduro is thinking, I don't know what's going on in his head," Pope Francis admitted to reporters on his plane.