Climate change could produce a refugee crisis that is “unprecedented in human history”, Barack Obama has warned as he stressed global warming was the most pressing issue of the age.

Speaking at an international food conference in Milan, the former US President said rising temperatures were already making it more difficult to grow crops and rising food prices were “leading to political instability”.

He said the United States was currently experiencing “floods on sunny days”, increased wildfires and, in Alaska, increased coastal erosion as the ice melts and no country was “immune” to the problem.

If world leaders put aside “parochial interests” and took action to reduce greenhouse gas emissions by enough to restrict the rise to one or two degrees Celsius, then humanity would probably be able to cope.

Failing to do this, Mr Obama warned, increased the risk of “catastrophic” effects in the future, “not only real threats to food security, but also increases in conflict as a consequence of scarcity and greater refugee and migration patterns”.

"If you think about monsoon patterns in the Indian subcontinent, maybe half a billion people rely on traditional rain patterns in those areas,” he told the Seeds & Chips conference, in remarks previously reported by Business Insider.

"If those rain patterns change, then you could see hundreds of millions of people who suddenly find themselves unable to feed themselves, because they're already at subsistence levels.

“And the amount of migration, the number of refugees that could be resulting from something like that, would be unprecedented in human history.”

He noted that some of the worst effects of climate change would be “borne by people in poor nations that are least equipped to handle it”.

The current refugee crisis, which has seen hundreds of thousands of people from war-torn Syria and other places affected by conflict and poverty, travel to Europe would be “just the beginning of the kinds of problems we would see”, Mr Obama said.

“Some of the refugee flows into Europe originate not only from conflict, but also from places where there are food shortages that will get far worse as climate change continues.”

He dismissed sceptics’ claims that climate change is still a matter for debate.

“The only real controversy is ‘how much warmer will it get?’ There’s really no controversy that the planet is getting warmer and the human activity is contributing to the warming,” he said.

“And what is also the conclusion of almost every scientist is if the planet warms at the far end of the potential estimates that it would be catastrophic and at the low end it would still be disruptive.”

Current climate models produce a range of possible outcomes as a result of the doubling of carbon dioxide in the atmosphere, from about 1.5C to 5C of warming. So far CO2 has risen from about 280 parts per million – a level that had remained fairly constant from the end of the last Ice Age to the 1800s – to more than 400ppm today.

And Mr Obama stressed how important he considered the issue to be.

“During the course of my presidency, I made climate change a top priority because I believe, for all the challenges we face, this is the top one that will define the contours of this century more dramatically perhaps than any other,” he said.

“No nation, whether it’s large or small, rich or poor, will be immune from the impacts of climate change.”

The election of the climate-science denying Donald Trump as president was simply “part of what happens in democracy”, Mr Obama said.

Lisa Murray's climate change photography Show all 12 1 /12 Lisa Murray's climate change photography Lisa Murray's climate change photography Dinka cattle herders starting their migration in South Sudan Lisa Murray Lisa Murray's climate change photography A Dinka woman fetches water in South Sudan Lisa Murray Lisa Murray's climate change photography Veronica in South Sudan preparing tea outside her home, recently ravaged by heavy flooding Lisa Murray Lisa Murray's climate change photography Children playing in Vietnam. When it floods, transport to and from school is a major challenge Lisa Murray Lisa Murray's climate change photography Sugeng, a fish and crab farmer from Indonesia who suffers financially every time the area floods Lisa Murray Lisa Murray's climate change photography Tan, a vegetable farmer, learning new methods through Oxfam in Vietnam Lisa Murray Lisa Murray's climate change photography Maluk, a 19-year-old from Tonj South, South Sudan Lisa Murray Lisa Murray's climate change photography Normally this farmer in South Sudan would be harvesting sorghum, but rains are late so the hunger season continues Lisa Murray Lisa Murray's climate change photography Herders bringing home their cattle in Afar, Ethiopia Lisa Murray Lisa Murray's climate change photography Farmers harvesting chilli in Ethiopia Lisa Murray Lisa Murray's climate change photography A woman in Tigray, Ethiopia, scares birds away from her crops with a slingshot Lisa Murray Lisa Murray's climate change photography Irula tribe woman in Tamil Nadu Lisa Murray

Earlier, without referencing Mr Trump, he said: “You get the politicians you deserve.”

But the “good news” was that the private sector had already grasped that the future would “is in clean energy”.

“I do not believe that any part of the world has to be condemned to perpetual poverty and hunger. And I do not believe that this planet is condemned to ever-rising temperatures,” he said.

“I believe these are problems that were caused by man and they can be solved by man.

“I’m fond of quoting the words of Dr Martin Luther King Jr, who believed that there’s such a thing as being too late.