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India is building a fence along its porous 2,500-mile border with Bangladesh, hoping to stop the flow of migrants, many displaced by changes in climate.

Semantics has become part of the equation, as politicians debate what to call victims of global warming - refugees or migrants. Governments tend to prefer migrants, while international aid and environmental groups opt for refugees.

The debate over the definitions has real consequences, said Kathleen Newland of the Migration Policy Institute in Washington.

“This distinction between the obligation owed toward a refugee in contrast to other people who are in trouble - even desperate trouble - is why the terminology matters,” she said.

It will be difficult for wealthier countries with lots of space to open their doors to people running from climate change, Ms. Newland added.

“I think it is much more likely that, if the rich do anything, they will try to work through governments and international organizations to meet their humanitarian needs in the short run and to help people adapt to the changed circumstances in the long run,” she said. “If climate change is very rapid and extreme, these efforts are likely to be far from adequate.”

Activist groups argue that wealthy countries have a moral obligation because they produce the most greenhouse-gas emissions, which most, but not all, scientists say causes global warming.

Pacific island nations contribute less than half a percent of global emissions, yet they are three times more vulnerable to climate change than other countries, according to the IPCC.