Brexit was caused in part by climate change, former US Vice-President Al Gore has said, warning that extreme weather is creating political instability “the world will find extremely difficult to deal with”.

Mr Gore, speaking at an event in which he previewed a sequel to his landmark 2006 documentary, An Inconvenient Truth, said the “principal” cause of the Syrian Civil War had been the worst drought in 900 years, which forced 1.5 million people to move from the countryside to the cities.

There they met a similar number of Iraqis who had fled the conflict in their homeland, creating powder keg conditions that Syrian government officials privately feared would explode.

The resulting war brought more refugees into Europe, causing political instability and helping convince some in the UK to vote to leave the European Union.

One of the most controversial Leave campaign posters showed a queue of refugees stretching into the distance with the caption “Breaking point: The EU has failed us all”. The then-Chancellor, George Osborne, described the poster as “disgusting and vile” and, like others who explicitly compared it to Nazi propaganda, said it had “echoes of literature used in the 1930s”.

Mr Gore, whose new film An Inconvenient Sequel: Truth to Power is due out in the UK in August, told an audience at the Advertising Week Europe event in London: “This collision between the power of industrial civilisation and the surprising fragility of the Earth’s ecosystem now poses a great danger that could even threaten the future of human civilisation itself.

“One of the lines of investigation [scientists] have been pursuing has led them to the conclusion that significant areas of the Middle East and North Africa are in danger of becoming uninhabitable.

“And, just a taste of this, to link it to some of the events that the UK and European Union are going through – think for a moment about what happened in Syria.

“Before the gates of hell opened in Syria, what happened was a climate-related extreme drought.

“From 2006 to 2010, 60 per cent of the farms in Syria were destroyed… and 80 per cent of the livestock were killed. The drought in the eastern Mediterranean is the worst ever on record – the records only go back 900 years, but it’s historic.

“And 1.5 million climate refugees were driven into the cities in Syria, where they collided with refugees from the Iraq War.

“Wikileaks revealed the internal conversations in the Syrian government where they were saying to one another ‘we can’t handle this, there’s going to be a social explosion’. There are other causes of the Syrian civil war, but this was the principal one.”

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The war broke out in 2011 as people took to the streets during the Arab Spring protest movement against dictators in the Middle East. President Bashar Assad responded by sending in the troops.

Mr Gore said this had produced an “incredible flow of refugees into Europe, which is creating political instability and which contributed in some ways to the desire of some in the UK to say ‘whoa, we’re not sure we want to be part of that anymore’”.

He said this kind of political instability was being felt by a number of countries around the world.

“Some countries have a hard time even in the best of seasons but the additional stress this climate crisis is causing really poses the threat of some political disruption and chaos of a kind the world would find extremely difficult to deal with,” Mr Gore said.

Inside the classroom with Chios's child refugees Show all 13 1 /13 Inside the classroom with Chios's child refugees Inside the classroom with Chios's child refugees Inside the classroom with Chios's child refugees Volunteers walk a group of refugee children towards their school on the island of Chios AFP/Getty Inside the classroom with Chios's child refugees Inside the classroom with Chios's child refugees Refugee children pose at a makeshift camp on the island of Chios AFP/Getty Inside the classroom with Chios's child refugees Inside the classroom with Chios's child refugees A Syrian Kurd mother combs Roza's hair, as she prepares to go to a volunteer-run school in a refugee camp on the island of Chios AFP/Getty Inside the classroom with Chios's child refugees Inside the classroom with Chios's child refugees Refugee children attend an English language class at the volunteer run school on the island of Chios AFP/Getty Inside the classroom with Chios's child refugees Inside the classroom with Chios's child refugees Refugee children carry vegetables in a refugee camp on the island of Chios AFP/Getty Inside the classroom with Chios's child refugees Inside the classroom with Chios's child refugees A newly arrived Syrian refugee, 13, holds her sister, 2, in a makeshift camp on the island of Chios AFP/Getty Inside the classroom with Chios's child refugees Inside the classroom with Chios's child refugees Refugee children pretend they go to school as they play in a refugee camp in the island AFP/Getty Inside the classroom with Chios's child refugees Inside the classroom with Chios's child refugees A Greek girl walks past a graffiti on her way to a school on the island of Chios AFP/Getty Inside the classroom with Chios's child refugees Inside the classroom with Chios's child refugees A Malian refugee child poses from behind a fence in a makeshift camp AFP/Getty Inside the classroom with Chios's child refugees Inside the classroom with Chios's child refugees A Syrian Kurd mother combs Roza's hair, as she prepares to go to a volunteer-run school in a refugee camp on the island of Chios AFP/Getty Inside the classroom with Chios's child refugees Inside the classroom with Chios's child refugees A Syrian family from Aleppo newly arrived to Greece sits in a makeshift camp on the island of Chios AFP/Getty Inside the classroom with Chios's child refugees Inside the classroom with Chios's child refugees A child carries a broken blackgammon game in a makeshift camp on the island of Chios AFP/Getty Inside the classroom with Chios's child refugees Inside the classroom with Chios's child refugees A child holds onto her mother, Djeneba from Mali, before attending school in a refugee camp on the island of Chios AFP/Getty

This is a view shared by the current US Defence Secretary, General James “Mad Dog” Mattis.

Despite Donald Trump’s apparent dismissal of the problems being caused by climate change, the US military is taking the issue seriously.

Mr Mattis made this clear in comments to US Senators as he sought their confirmation of his appointment.

“Climate change is impacting stability in areas of the world where our troops are operating today,” he said.

“It is appropriate for the combatant commands to incorporate drivers of instability that impact the security environment in their areas into their planning.

Refugees welcome here: Protesters demand UK resettle more migrants in response to refugee crisis Show all 7 1 /7 Refugees welcome here: Protesters demand UK resettle more migrants in response to refugee crisis Refugees welcome here: Protesters demand UK resettle more migrants in response to refugee crisis The Solidarity With Refugees group said Saturday’s protest aimed to “show our Government and the world that Britain is ready to welcome more refugees”. Rex Features Refugees welcome here: Protesters demand UK resettle more migrants in response to refugee crisis People march through central London as they take part in a protest rally organised by Solidarity with Refugees in a bid to urge the Government to take more action on the migrant crisis Press Association Refugees welcome here: Protesters demand UK resettle more migrants in response to refugee crisis The protest comes days before world leaders meet to discuss crisis at UN General Assembly Press Association Refugees welcome here: Protesters demand UK resettle more migrants in response to refugee crisis Demonstrators made their way from Park Lane to Parliament Square in London on Saturday afternoon Press Association Refugees welcome here: Protesters demand UK resettle more migrants in response to refugee crisis Marchers chanted “refugees are welcome here” and waved banners reading “no-one is illegal” and “let’s help people” Press Association Refugees welcome here: Protesters demand UK resettle more migrants in response to refugee crisis The march was supported by charities and groups including the Red Cross, Asylum Aid, Save the Children, Hope Not Hate, Oxfam and the UN Refugee Agency Rex Features Refugees welcome here: Protesters demand UK resettle more migrants in response to refugee crisis In the wake of Alan’s death, David Cameron pledged to resettle 20,000 Syrian refugees in the UK over the coming five years but there have been additional calls to re-home those who have already reached Europe, as well as asylum seekers coming from other conflict zones such as Iraq and Afghanistan Rex Features

“Climate change can be a driver of instability and the Department of Defence must pay attention to potential adverse impacts generated by this phenomenon.

“I agree that the effects of a changing climate – such as increased maritime access to the Arctic, rising sea levels, desertification, among others – impact our security situation.