The Mall has monuments to heroism, freedom and sacrifice. Pretty soon it will also have a monument to failure.

Drive on 17th Street NW, just south of Constitution Avenue, and you’ll see concrete footings, a mound of dirt and jersey barriers — all part of an oft-delayed project to build a floodwall to protect downtown Washington from a rising Potomac River.

The flood wall, and similar initiatives elsewhere, amount to tacit acknowledgments that the fight against climate change, the cause celebre of the environmental movement for more than a decade, has failed in its primary purpose. In the race to prevent disaster, it’s already too late.

Among climate-change activists, the realization is spreading that the combination of political inaction on greenhouse gases, plentiful new petroleum supplies and accelerating changes in weather patterns means there is no escaping more life-altering floods, droughts and fires. Although ongoing efforts to reduce carbon emissions could mitigate even worse catastrophe, momentum has shifted in part to preparing for the inevitable consequences of a warmer planet.

Perhaps the most vivid example of this came Tuesday afternoon, when New York Mayor Michael Bloomberg rolled out his $19.5 billion plan to “prepare for the impacts of a changing climate,” with proposals ranging from coastal levees to the protection of hospitals. Last year, Bloomberg cited climate change as his main reason for endorsing President Obama’s reelection, praising Obama’s “major steps to reduce our carbon consumption.” But speaking Tuesday from a Brooklyn greenhouse damaged last fall by Hurricane Sandy, Bloomberg addressed the inevitability that rising temperatures and sea levels would bring even worse.

“By mid-century, up to a quarter of all New York City’s land area, where 800,000 residents live today, will be in the flood plain,” he said, and “40 miles of our waterfront could see flooding on a regular basis just during normal high tides.” We no longer have the luxury of ideological debate, he said. “The bottom line is we can’t run the risk.”

Andrew Light, a global-warming specialist at the liberal Center for American Progress, explained to me the recent shift toward efforts to adapt to climate change rather than merely seeking to prevent it. “We’re starting to see very strong evidence of climate-related extreme events happening sooner than we thought with only a 1-degree [Celsius] rise in temperature,” he said, “and a more refined science saying now that we will more than likely edge up to or cross the 2-degree threshold.”

Climate activists had long sought to limit the temperature rise to 2 degrees, but this now seems both impractical and insufficient. “Our best-case scenario now is we could delay by a couple of decades the point at which we cross the threshold,” Light said. This means that cutting carbon emissions is still important but that it’s also time to prepare for what’s coming.

Among the needed adaptations: floodwalls and expanded coastal wetlands, fortified subway systems, buried power lines, houses with detachable foundations, roads rebuilt on higher ground, drought-resistant crops and changes to hydroelectric facilities and nuclear power-plant cooling systems. States in the Southwest may need pipelines and desalinization plants for drinking water.

Low-lying and poorer parts of the world will have it much worse. But even in the United States, vast coastal areas — New Orleans, the Florida Keys and elsewhere along the Gulf of Mexico, North Carolina’s Outer Banks, parts of Long Island — eventually may need to be abandoned to higher seas. As a start toward depopulating those areas, the federal government may need to cut off disaster insurance.

Obama created an “Interagency Climate Change Adaptation Task Force ” in 2009 to examine everything from agriculture to sewer system failures and public-health consequences, but much of the work remains theoretical. Bloomberg’s new plan, with 250 specific recommendations and a hefty price tag, puts climate-change adaptation into a more concrete realm.

The businessman-mayor called it “a battle that may well define our future for generations to come” and outlined changes to building standards, telecommunications, transportation and a dozen other areas.

“Waves that do reach our shore will find a strong line of coastal defenses, reinforced dunes and widened beaches, levees, floodwalls and bulkheads, and tide gates and surge barriers,” Bloomberg said. “New grade infrastructure will absorb water, it will divert it into higher-capacity sewers, and our critical systems will operate with less interruption throughout the storm and bounce back quicker if they do go down.”

Bloomberg spoke confidently, as if he were a general laying out a military plan. But he was really talking about limiting casualties.

Twitter: @Milbank

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