Vast swaths of forests in western North America are dead or dying, killed by pine bark beetle. The beetles have been there all along, but prolonged droughts reduced the trees’ ability to defend themselves from the inner bark-munching bugs.

The western slopes of the Sierra Nevada range in California have been especially hard hit by the depredation, just as people who made money in Silicon Valley sought to move their families out of the choked cities and up into the beautiful mountain forests. Now, to mitigate risk of catastrophic fire and the further spread of pests such as bark beetle, landowners must cut down dying and dead trees—the forests that were the very reason they moved there in the first place.

The tragic irony of this situation is similar to one familiar to Orrin Pilkey ’57 PhD, an emeritus professor of geology at Duke University in North Carolina. For years, Pilkey has been arguing, based on evidence that he and many others gathered, that building sea walls to protect the beaches we love actually destroys them.

Beaches are natural shape shifters and, if we build far enough back from the water line, a wide beach is a natural protection against storm surges. But people want to build homes and resorts close to the water. To keep high tides and storm surges out of basements and living rooms, seawalls and revetments are constructed. But this prevents the natural shifting of beach sands—and the sand ends up eroded by wave action and washed out to sea. Soon, the beachfront has no beach—and imported sand, sometimes from an expanding black market—is brought in to rebuild the beach. And the cycle resumes—except now, that process is being exacerbated by rising sea levels.

“I have been very controversial in some circles, especially among developers and the (Army) Corps of Engineers,” Pilkey says. “They say nasty things about me.” A town in North Carolina banned him, so unpopular were his evidence-based claims of inexorable sea level rise.

Or at least they used to be. There is a growing, if grudging, acceptance of the need for what Pilkey in his latest book calls a retreat from a rising sea. Recently, the governor of Louisiana declared a state of emergency along its coasts, as sea level rise erodes the equivalent, according to a local reporter, of one football field per hour. Sea level along the Eastern Seaboard of North America has already risen nearly a foot in the past 100 years.

“Now,” Pilkey says, “editorials (in local papers) are very much anti-beach front development. We’ve made progress, but it’s slow.”

Eventually, people are going to migrate, Pilkey says. The question is, are we going to retreat inland in a rational manner or are we going to maintain a “business-as-usual” mentality and only flee at the last minute, in panic and disarray? There’s a tug-of-war between real estate interests (which don’t want to lost the value of developed land, no matter what it costs to keep it dry) and others who see the expense of trying to maintain the status quo as just too costly. That battle is already playing out in places like Miami.

Pilkey says that microtides have been detected in puddles in downtown Miami. “Miami sits atop highly porous limestone,” he says. That’s why south Florida is so famous for its sinkholes but is also why, according to Pilkey, “Miami has no future. No sea wall, no dike, no levee, nothing is going to stop the sea from coming into that city.”

New Orleans and Sacramento, both situated in vast deltas, are also in grave danger of drowning. The Sacramento River delta is one of the world’s great agricultural areas but is being salinized. “Rising sea level is already ruining farmlands on low portions of the continent,” Pilkey says. In Pilkey’s neck of the woods, farmland has already been lost to salinization in northeast North Carolina; further north, farms around the Delaware Bay, too, are being abandoned due to salinization.

Retreating from low-lying coastal areas is just one source of displacement. The southwest United States, among other areas, is becoming dangerously hot. Even if greenhouse gas emissions are kept in line with the Paris Accord, the number of extremely hot days is going to soar. And when it’s hot, Americans switch on the AC—meaning we use more electricity and, in turn, pump more carbon into the atmosphere in the process of generating that electricity.

Factor in the fact that much of North America’s agricultural regions are in already-warm areas, and we face another problem: Crop productivity declines sharply as temperatures soar. So does worker productivity while, at the same time, crime increases.

For water-rich regions such as the Pacific Northwest, all this adds up to the idea that we should be preparing for an influx of migrants. As University of Washington climate scientist Cliff Mass wrote in a 2014 blog post, “The Pacific Northwest will be one of the best places to live as the earth warms.” (“Forget Florida,” Mass adds, echoing Pilkey.)

In other words, start building bunk beds, because space could get tight as people in San Diego, Los Angeles, Phoenix, and elsewhere from the hot zones move north.

However that shakes out, the movement of people within the United States due to climate-related issues will be nothing compared to what other parts of the world are already experiencing.

Every three seconds someone is forced from their home by conflict or human rights abuse, according to a recent report from the United Nations. A lot of those political and religious conflicts have an underlying environmental cause that results in food insecurity. Over the course of its long-running civil war, half the population of Syria has fled. Syria, along with the rest of the Fertile Crescent, “experienced the most devastating drought in the instrumental record,” according to researchers writing in the Proceedings of the National Academy of Sciences. Washington State University hydrologist Jenny Adams agrees that it is resource depletion that is wreaking havoc in Syria and elsewhere. And senior military experts from the U.S. and around the world warn of a climate-related “humanitarian crisis of epic proportions,” as retired U.S. Marines Corps Brigadier General Stephen Cheney recently said. “There are direct links to climate change in the Arab Spring, the war in Syria, and the Boko Haram terrorist insurgency in sub-Saharan Africa,” Cheney adds.

Pilkey says, “We have extreme movement of people” in the world today, and hundreds of millions more will migrate in the coming decades. “How do you move tens of millions of people? And where do you move them to?” Pilkey points to the situation in India and its neighboring country, Bangladesh.

Some 160 million people live in Bangladesh, a country barely above sea level in the delta of the Ganges and several other major rivers. Storm surges regularly roar up-delta, flooding homes and killing tens of thousands of people. Fearing climate migrants, India has built a wall along 70 percent of its long border with Bangladesh. This wall, though, is largely ineffectual—and so India’s border is bolstered by armed guards who have been known to shoot Bangladeshi migrants.

But most border walls, from the Great Wall of China to the Berlin Wall, are largely ineffectual, according to several scholars. On the centrist thinktank RAND Corporation’s blog, Daniel M. Gerstein writes that the notion of an “‘impenetrable’ walls” is better characterized as a “thin brittle line… with little chance” of stemming the flow of people.

Walls don’t stop movement; they just alter its direction for a while, as Reece Jones writes in a Climate Policy article. Short walls with lots of armed guards—such as those around a prison or a medieval city—are indeed effective. Longer walls, while perhaps temporarily popular symbols of a solution, just force migrants on to new paths, often considerably more dangerous ones that result in much higher fatality rates.

“Resilience is essential,” says Pilkey. “We are late for mitigation.” We’re going to need to marshal a grand army of fact-based strategies to deal with the consequences of rising temperatures and the concomitant climate instability global warming brings. The potential for innovation, both social and technological, are enormous—and potentially enormously profitable.

As risk-assessment expert Dante Disparte writes in the Harvard Business Review, the costs of addressing climate change are great. But the costs of business-as-usual are much greater.