Why does a man who knows about global warming and rising sea levels live by a saltwater canal where his back yard ends twenty inches above the water? Because the location is too beautiful to give up. And because I don’t know if the canal will rise to the doorstep during my lifetime.

We had a preview in 2009 when hurricane Faye drove Atlantic waters against the coast and into the lagoons. My dock and seawall were under water. No one could remember the canal ever rising that high.

Photo by neighbor Chase Patterson, who was watching in case the boat floated off the lift. My wife and I were out of town.

In case it rises over the seawall again and stays there, I plan to apply an old strategy to the new problem. Floridians have long excelled at marketing slightly damp real estate to out-of-town buyers. My ideal customer would be a global warming denier. There seem to be plenty of them, and I only need one. I would proceed something like this:

"Good morning, Mr. de Neyer. I’ll be glad to show you the house. Let’s look around the back yard before we go inside."

"Flood insurance? Sure, we have it. With a good company too — they always pay our claims right away. I’d suggest you stay with ‘em."

"Where is the seawall? Sir, in this neighborhood we do not believe in seawalls. They just ain’t natural enough. Here we don’t need any docks or boat lifts — you just pull your canoe right up on the grass. It’s wildlife-friendly, too. There’s nothing cooler, or greener, than to look out your kitchen window and see a five-foot gator sunning himself on the lawn!"

"That roof on pilings out in the canal? Oh — I’m so glad you asked. That is a fish shelter. Most of the neighbors have them too. The mullet and sea trout just love the shade and the underwater structure. You cast out by that corner piling around dusk, and you’ll get a hit every time!"

"You can’t believe all that global warming stuff, buddy, but I’ll tell you what I know for sure: the fishing in this neighborhood just keeps getting better!"

But I probably won’t need to make this pitch. Judging from the latest IPCC report, our canal will average six inches below the seawall by 2035 in the worst case. Waves may slap the sides of the dock or splash over it, but won’t flood the back yard. In the best case, sea level will rise at about the present rate and bring the canal up about seven inches, just enough to keep the larger dock joists wet. Hurricane surge will be a wild card, as always.

Another wild card is the West Antarctic ice shelf, the contributor to sea level rise whose behavior is least understood. If it collapses, the canal could reach the top of my seawall.

Scientists have modeled the Greenland ice sheet and verified the models against real data. Its behavior is included in the IPCC sea level projection. But the behavior of Antarctica’s western region, with its ice shelves and its glaciers grounded below sea level, could not be projected as well.

The consensus estimate for year 2100 puts sea level ten to forty inches higher than today, provided the West Antarctic ice shelf doesn’t collapse. If it does, sea level would rise higher, perhaps fifty to seventy inches higher than today.

IPCC bases its worst case estimates on its worst case projection of future greenhouse gas emissions: "business as usual," with no effort to reduce emissions.

On a world map, a forty inch rise doesn’t appear to change the coastline much. But some regions will be strongly affected, such as my back yard. Millions of homes are on equally low ground. Along the Gulf coast, many areas of the Mississippi delta will be under water. Much of the Netherlands and Belgium will be below sea level — much more than today — as will a hundred-mile stretch of the Italian coast centered on Venice.

Not that the sea will stop rising in 2100 — it’s expected to keep on for centuries, slowly responding to the warming air and melting icecaps. One projection says the amount of carbon dioxide already in the atmosphere has committed us to 50 to 75 inches of sea level rise.

On the same basis, the global temperature maximum that many nations have agreed to as the target for mitigating climate change commits us to an eventual sea level rise of about 200 inches.

Those projections are based on one research team’s modeling of the major causes of sea level rise. They summarized their findings in a single relationship: for every degree Celsius that global temperature rises — every 1.8 degrees Fahrenheit — an eventual sea level rise of about 90 inches is "locked in."

Squeezing the myriad interactions of the earth-sea-atmosphere system into a one-variable dose/response model seems like a stretch. But some basic physical facts support it. Carbon dioxide lasts a long time in the atmosphere and makes it retain more heat in a fairly predictable way. And across the ages, where we have evidence, periods of global temperature more than 2 degrees Celsius above the present also experienced sea levels at least 200 inches higher than today.

In some futuristic novels, the Greenland and Antarctic ice caps have melted entirely, over a period of centuries or millenia. Sea level is at its maximum, Memphis is a port on the Gulf of Mexico, and Florida is only a legend — a distant echo of the Celtic Land-Under-Waves. You can’t talk about prediction or projection that far out, but an ice-free world is possible. It’s happened repeatedly in earth history.

When I studied climatology as a grad student half a century ago, global warming was only a suspicion, and studies of paleoclimate were just gathering steam. It’s good to see how far this work has progressed, and how much practical guidance is being developed from its findings.