If politicians are to be believed, migration caused by climate change will cause the world huge problems. One of the latest to repeat the warning is UK shadow immigration minister Chris Bryant, who, like many others before him, said 200 million people may be forced to flee their country. But how reliable is this figure?

In a speech to the Institute for Public Policy Research, Chris Bryant warned that millions of people around the world could be forced to leave their homes over the next few decades and move to countries less affected by environmental problems.

"If we get climate change wrong there is a very real danger we shall see levels of mass migration as yet unparalleled," he said.

"The United Nations (UN) estimates that in 2008 20 million people were displaced by climate change."

In the longer term, he said, "you can imagine that the UN estimates of 200 million such refugees, more than the total number of worldwide migrants today, may be about right".

We cannot comment in any way on the accuracy of a figure we did not produce UN spokesperson

The MP told the BBC it would be inaccurate to say he "warned that climate change will create 200 million migrants". He added, however: "It would be accurate to say that I argued that we have to tackle the push factors that affect migration such as climate change as otherwise the numbers who are made environmental refugees may reach the estimate of 200 million."

But how were these estimates calculated, and is the future really that bleak?

Let's take the 20 million first.

Alex Randall may seem on paper the kind of person who would agree with figures like this - he works for the Climate Outreach and Information Network, a charity which aims to raise awareness of climate refugees and their needs.

In fact he is critical. The 20 million figure, he suggests, is reached by "adding up all of the people who've been displaced by any kind of natural disaster and labelling them climate refugees." And that, in his view, is problematic.

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"It's certainly true that climate change might be making some of those particular disasters more likely. But it's certainly not the case that we can attribute all of those individual displacements to climate change alone," Randall says.

It's also wrong to infer that people affected by a natural disaster will inevitably migrate, he suggests.

"People tend to move short distances for a short period of time," he says. "And then move back."

So, what of the estimate that there could be 200 million climate refugees by 2050?

This figure has travelled far and wide. It was noted in the Stern Review on the Economics of Climate Change, was mentioned by the president of the UN General Assembly in 2008, and has been cited by numerous NGOs.

It comes from research papers by Oxford-based scientist Prof Norman Myers - including one published in 1995 and another in 2005.

The disappearing map Another claim made by Norman Myers, that 50 million people would become climate refugees by 2010, was shown in 2011 to have been wrong - leading the UN Environment Programme to drop a map illustrating the idea from its website. The caption read: "Fifty million climate refugees by 2010... There is general agreement about the current global environmental and development crisis. It is also known that the consequences of these global changes have the most devastating impacts on the poorest..." Initially no explanation was given for the disappearance, just a bizarre error message: "Dear visitor, it seems like the map you are navigating by is maybe not fully up-to-date, or that it might have an error in it, or is it that your GPS is not loaded with the correct data? We are just taking the scenic route, darling! See Honey, We're not lost. I know where we are: This way I think... mmm!" A UNEP spokesman did eventually tell the BBC that the map had been removed because it was wrong. The curious can still download it from the website of the GRID-Arendal centre, established by the Norwegian government.

Some academics, however, have doubted Myers' numbers from the start. Speaking to the BBC in 2011, Stephen Castles from Oxford University's International Migration Institute, suggested that Myers' "objective in putting forward these dramatic projections was to really scare public opinion and politicians into taking action on climate change".

While this was "a very laudable motive", he said, there were major problems with the method Myers had used to make his projections.

In the long run I do believe very strongly that it will better for us to find that we have been roughly right than precisely wrong Norman Myers

"He simply took a map of the world, worked out what areas would be inundated if the sea rose, say by 50cm, and then simply assumed that all the people affected by this sea level rise would have to migrate - and that a lot of them would migrate to developed countries. Really there was no basis for it."

One reason why Myers' figures have been so widely repeated may be that the UN itself has helped give them credibility.

But last week the UN told the BBC that it "cannot comment in any way on the accuracy of a figure we did not produce".

Even the website of the Biodiversity Institute at Oxford University, at which Norman Myers is listed as an associate researcher, states that his work on environmental refugees "is widely viewed as lacking academic credibility".

But speaking to the BBC in 2011 Norman Myers defended his methods.

"It's really difficult to say how many there are and where are they… but in the long run I do believe very strongly that it will be better for us to find that we have been roughly right than precisely wrong."

Prof Myers also makes clear in his research that not everyone he classes as an environmental refugee will flee their country - they could be forced to move somewhere else within the national boundaries.

He also told the BBC that science was "never ever completely final - it's always a bit iffy". He added: "I think it would be much harder to demonstrate that there aren't any of these environmental refugees than to demonstrate that there are environmental refugees."

Equally, Stephen Castles did not deny that some people had been and would in future be forced to move country. In this category he put the inhabitants of some Pacific islands.

But the numbers he estimated not in millions, but in tens of thousands.

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