Rising sea levels threaten the homes and livelihoods of hundreds of millions around the world. The public has barely registered what this means, but climate scientists have been thinking about it for years. They remember the old story of King Canute, who showed his courtiers that for all his power he couldn’t keep the tide from coming in.

The scientists have considered various options. One is to stand our ground, building walls high and thick enough to hold off the rising seas. Just recently, we’ve learned how the Romans built sea walls with concrete that’s lasted for 2,000 years.

It’s an attractive technological solution because we love to live near the sea, and we have whole cities that could soon be under water. The cost of losing them would be in the multi-trillions, and the walls would cost only multi-billions.

But a more realistic idea is called “managed retreat,” when communities at risk of submergence simply move to higher ground — like a tsunami alarm in slow motion.

Well, it’s realistic if both governments and coastal residents think it is. A recent study, published in the journal Nature Climate Change, gives us a sense of what would be involved.

After studying cases around the world that have resettled over a million people, the researchers found that a lot depends on who wants the resettlement. If the government wants it and the residents don’t, trouble will follow. But if the residents want to move and the government agrees, everyone benefits.

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This model works fine if you’re dealing with a small Alaskan island or a patch of Dutch farmland. It doesn’t work so well if the sea is threatening some very expensive real estate, like Vancouver’s. A 2016 study looks at climate change adaptation in Manila, Lagos, and Vancouver. All three cities are susceptible to both rising sea level and serious storm surges.

In Vancouver, we’re especially at risk along the shores of the North Arm of the Fraser, False Creek, and the harbour itself. No one wants to write off the billions invested in those neighbourhoods.

Then again, the hobby farms of Southlands and the condos of False Creek may be uninsurable by, say, 2040. About that time, YVR will be a tidal marsh, and maybe the world’s biggest clam garden.

The authors of the Nature Climate Change study argue that the broader society and the displaced residents should all benefit from retreat. But what’s a benefit when Grannie in Richmond has to move to Langley or North Vancouver, and no housing is available? Do we abandon the Fraser Delta while leaving toxic chemicals in the ground to poison the rising Salish Sea? Can we afford to collect those chemicals and find somewhere on high ground to store them?

Bear in mind that climate change also means more storms and storm surges, threatening still more property. Even a managed retreat will still need to involve strengthening the bridges over Burrard Inlet and the Fraser, not to mention the docks. By then we likely won’t be shipping any oil at all from Burnaby, but we’ll need some kind of foreign trade to keep our economy going. And any new housing in the Fraser Valley will have to be sturdy enough to survive hurricane-force winds now and then.

The boreal forest at risk

B.C.’s Lower Mainland is likely to find itself between the devil and the deepening blue sea. Last year saw the Fort MacMurray fire, with almost 100,000 displaced people. This year it’s our turn, with over 45,000 displaced so far this summer. As future summers become even hotter and drier, the whole boreal forest — and the species it supports — will be at risk.

Northern communities that depend on logging, farming, or mining will find themselves in retreat also, managed or not. Those displaced will mostly return to rebuild — until they’re burned out once too often, and retreat south for good.

Governments will be hard-pressed to support those displaced by sea and fire alike, and struggling to fund social services like health care and education. And of course the flow of refugees from worse-off countries will keep growing... countries like the United States.

As bleak as this prospect may seem, it’s based on the near term and a global temperature rise of about two degrees by the end of this century.

But we can’t console ourselves that temperatures and sea level will then stop. More likely they will continue to rise, even if we stop carbon emissions at once. A 2010 study warned that rising temperatures will lead to “degassing” of methane from the seabed and permafrost, plus release of carbon dioxide from burning forests: “Thus while central estimates of business-as-usual warming by 2100 are 3 to 4ºC, eventual warming of 10ºC are quite feasible and even 20ºC is theoretically possible.”

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That would mean that large parts of the planet, supporting half the human population plus their livestock and crops, would endure prolonged temperatures of more than 35ºC. That would make much of the planet literally uninhabitable: “We conclude that a global-mean warming of roughly 7ºC would create small zones where metabolic heat dissipation would for the first time become impossible, calling into question their suitability for human habitation. A warming of 11 to 12ºC would expand these zones to encompass most of today’s human population.”

Such a warming, the authors say, would likely require some 300 years. They would likely be the most savage and violent three centuries in the 300,000-year history of Homo sapiens, with billions in a totally unmanaged retreat from the lands that had supported them — and a few lucky millions trying to hold the still-inhabitable parts of the world.

Seen in this light, the current migrations from Africa, the Middle East, and Latin America are just the first of many unmanaged retreats to higher, cooler ground. Our initial response has been surprise and anxiety, when we should have seen the refugees coming and made suitable preparations. Whether or not we learn from our mistakes could determine the future of the planet.