Last week, The Boston Harbor Association released new maps of Boston’s future floodplain; you can see the worst-case scenario map in this Boston Globe blog. If you’re curious about Boston’s past floodplain, take a look at the Mapping Boston site’s Composite Map of Boston, which shows where land was filled since 1630. Actually, you don’t need to examine both maps. One will do. In short, Boston’s made land—including the entire Back Bay, the South End, half of South Boston, North Station, and Faneuil Hall, not to mention a huge swath of East Cambridge and Charlestown—will probably go underwater by 2100, and perhaps much sooner.

Mind you, they’re (fairly) dry today. The Boston Harbor Association (TBHA) maps track future flooding due to climate change. When the world warms up, glaciers melt, adding more water to the ocean, and the ocean itself expands as the water heats. It’s obvious that this warming is happening. What isn’t clear is just how much bigger the ocean will get, and exactly when that will happen. Scientists estimate that the Boston harbor is going to rise somewhere between 2.5 and 5 feet by 2100.

By itself, a 2.5 foot ocean rise doesn’t seem like such a big deal. Hey, Boston’s daily tidal range is about 10 feet—and the map of the day-to-day effects of a 2.5 foot ocean rise shows a few wharves under water, and not much else. Big deal. You’d think the New England Aquarium could deal with having their piers sink under the sea.

The problem is that the Boston Harbor’s water isn’t level. The ocean has tides, and sometimes tides are higher than others. In Boston, the year’s highest tides come in the spring… when Boston has a lot of rain. In a hundred-year-storm—a storm which has a one percent chance of occurring in a given year—Boston’s harbor water already rises by five feet during a 100-year-storm surge. If that surge happens when it’s low tide, no one cares—it doesn’t even reach high tide level. If it happens at high tide, there’s a flood.

The TBHA’s maps show what will happen when a 100-year-storm strikes at low tide, at high tide, and at the highest spring tide after the harbor has risen 2.5 feet. Once the harbor has risen 2.5 feet a five-foot storm surge makes the New England Aquarium a bit damp, but life goes on. Things look worse if the storm surge hits at high tide; now, South Boston, the Rose Fitzgerald Kennedy Greenway, Faneuil Hall and stretches of Route 93 are under water (including the exits under the Rose Fitzgerald Kennedy Greenway).

If you want to see the outline of the original pre-Boston Shawmut peninsula, though, take a look at the map of what would happen if a 100-year-storm hits during spring high tide and overflows the Charles River dam. And there it is; the dry land shows the area the Reverend William Blackstone deeded to the Puritans in 1630 (minus the three mountains Bostonians tore down to start making new land in the mudflats). I’m not the only one to notice: commenters on the Boston Globe site saw the connection too.

What is to be done? Remember, these projections are for the best-case scenario of the Boston Harbor rising just 2.5 feet; it could rise twice that much, as Architecture Boston showed earlier this year. We can’t just load the Kennedy Greenway onto Noah’s Park Ark to save it from being deluged with salt water—much less all of South Boston. Apart from the devastation to homes, businesses, Route 93 and the MBTA, I doubt that Boston’s historic trees will take kindly to sitting in salty water seasoned with motor oil for days. Also, parks aren’t a high priority after a disaster. New Orleans lost more than 100,000 trees after Hurricane Katrina, and just got around to replanting 4,000 trees this year.

The Massachusetts Department of Health has a Center for Disease Control (CDC) grant to assess the effects of climate change; we might be spared the worst human wreckage of a flood. Architects are trying to imagine how to plan buildings that might last 75 years, so they might be in good shape. But our landscape will change. We may engineer some way to divert the waters—a seawall? Dutch-style pumps?— which I suspect will be resisted as strenuously as Cape Wind’s turbines, or we may simply witness the destruction of our city’s tree canopy, parks, and historic buildings. Take your photos for the grandkids now: this is the “before” era. Before what, I’m not sure.