Toward the end of this century, if current trends are not reversed, large parts of Bangladesh, the Philippines, Indonesia, Pakistan, Egypt and Vietnam, among other countries, will be under water. Some small island nations, such as Kiribati and the Marshall Islands, will be close to disappearing entirely. Swaths of Africa from Sierra Leone to Ethiopia will be turning into desert. Glaciers in the Himalayas and the Andes, on which entire regions depend for drinking water, will be melting away. Many habitable parts of the world will no longer be able to support agriculture or produce clean water.

The people who live there will not sit passively by while they and their children starve to death. Tens or hundreds of millions of people will try very hard to go somewhere they can survive. They will be hungry, thirsty, hot - and desperate. If the search for safety involves piling into perilous boats and enduring miserable and dangerous journeys, they will do it. They will cross borders, regardless of whether they are welcome. And in their desperation, they could become violent: Forced migration can exacerbate ethnic and political tensions. Studies show that more heat tends to increase violence.

The United Nations says the maximum tolerable increase in global average temperatures is 3.6 degrees Fahrenheit above pre-industrial conditions. (Small island nations argued for a much lower figure; at 3.6 degrees, they'll be gone.) But the promises that nations are making ahead of the U.N. climate summit in Paris in December would still, according to the International Energy Agency, lead the average temperature to rise by about 4.7 degrees before the end of the century. Those promises are voluntary and nonbinding, and if they aren't kept, the thermometer could go much higher. Which means our children and grandchildren will be confronting a humanitarian crisis unlike anything the world has ever faced.

CartoonDavies' latest cartoon: Those I Love NY signsCommentSubmit your letterReader essaysGet published in Newsday

Absent the political will to prevent it, the least we can do is to start planning for it.

Rather than leaving vast numbers of victims of a warmer world stranded, without any place allowing them in, industrialized countries ought to pledge to take on a share of the displaced population equal to how much each nation has historically contributed to emissions of the greenhouse gases that are causing this crisis. According to the World Resources Institute, between 1850 and 2011, the United States was the source of 27 percent of the world's carbon dioxide emissions; the European Union, 25 percent; China, 11 percent; Russia, 8 percent; and Japan, 4 percent.

To make calculating easy, let's assume that 100 million people will need new homes outside their own countries by 2050. (That number could be way off in either direction - we won't know until it happens.) Under a formula based on historic greenhouse gas emissions, the United States would take in 27 million people; Europe, 25 million; and so on. Even as a rough estimate, this gives a sense of the magnitude of the problem: The United States has been granting lawful-permanent-resident status to only about 1 million people a year for several decades.

advertisement | advertise on newsday

None of this would be popular, but it would be fair. Climate change results from the cumulative emissions of greenhouse gases all over the world, because the gases stay in the atmosphere for a century or more. International law recognizes that if pollution crosses national borders, the country where it originated is responsible for the damages. That affirms what we all learned in the schoolyard: If you make a mess, you clean it up. The countries that spewed (or allowed or encouraged their corporations to spew) these chemicals into the air, and especially the countries that grew rich while doing so, should take responsibility for the consequences of their actions. If they want to reduce the number of people in need of new homes, they should reduce their emissions.

Finding suitable land for resettlement will be immensely difficult, because it is not only a matter of responsibility or acreage. A population that needs to move may want to go to a place that is geographically similar to where it came from and where it can make the same sort of living as before, such as from fishing, farming or herding. Its members may also wish to go together and re-create their old communities. Yet most of the habitable places on Earth are already inhabited, and moving a sizable population into an area that is already populated is not so easy. The most prominent example of such a movement in modern history is Israel - a project that has not gone smoothly. Technologies such as desalination can make more areas habitable, but they typically take a great deal of money and energy, the very resource we have failed to conserve in the first place.

This problem will also require a new legal solution: Under current law, those displaced by climate change have no recognized legal status. The 1951 Refugee Convention applies only to people who are fleeing because of a well-founded fear of persecution. Nonbinding guidelines have appeared on the treatment of people who cross borders as a result of climate change (the Nansen Principles) and who are displaced internally (the Peninsula Principles), but these have no force of law. A few countries have special arrangements to admit people from certain other countries. They aren't specifically for climate-change refugees, but they could be used in that situation. For example, the United States has "compacts of free association" with the Marshall Islands, Micronesia and Palau to allow their citizens to come here. Australia and New Zealand have very small guest-worker programs. Temporary protected status or humanitarian visas might be available to some people for a limited time.

Sign up for The Point

Go inside New York politics.

Assuming that most nations aren't actually interested in taking in orders of magnitude more migrants than they do now, the vast majority of those who will be displaced by climate change will simply have no place outside their own countries where they can go. The largest number of displaced people is likely to be from Bangladesh, but it's hard to imagine that they will be welcomed in India, which has built a barbed-wire fence along parts of the border.

Just southwest of India is the low-lying island nation of Maldives. Before its president, Mohamed Nasheed, was deposed by a military coup in 2012, he rose to global prominence as a voice of endangered island nations by staging an underwater cabinet meeting to highlight his country's likely fate. Last year, just before the military imprisoned him again, he told me about his message to developed nations. "You can drastically reduce your greenhouse gas emissions so that the seas do not rise so much," he said. "Or when we show up on your shores in our boats, you can let us in. Or when we show up on your shores in our boats, you can shoot us. You pick." Tragically, if today is any foretaste, the most likely outcome is that we will let many of them drown. Witness the spectacle of hundreds of thousands of people fleeing civil war in Syria and repression in Eritrea; making their way to a country without much of a government, Libya; and there being recruited by unscrupulous traffickers who put them on boats pointed to Italy. Thousands perish on these unsafe, overpacked vessels (whose crews often abandon them), and those who survive the passage are not exactly welcomed with open arms. Europe is in a furor over who will take them in, and anti-immigrant fervor tends to rise with the number of people trying to enter, making a resolution especially difficult.

Likewise, many people fleeing poverty in Bangladesh and oppression in Burmaare launching boats to Indonesia, Malaysia and Thailand, and at least initially were being turned away. Australia - a logical destination for people from the small Pacific islands - intercepts incoming boats and sends their occupants to camps it has established in Papua New Guinea, Nauru and (lately) Cambodia.

Maybe the idea of assigning refugees to the nations that caused the climate to change can spur a less-pessimistic future. If we don't want millions of people seeking haven here - or dying while they try - then the United States and other industrialized countries need to become far more aggressive in cutting their greenhouse gas emissions. There is still a little time to reduce the damage. If not, it won't just be environmental; it will be human, too.

Michael B. Gerrard, associate faculty chair at Columbia University's Earth Institute, is the Andrew Sabin professor of professional practice and director of the Sabin Center for Climate Change Law at Columbia Law School.