In a recent New York Review of Books, environment writer Bill McKibbon reintroduces the nonscientific world to James Hansen, who 17 years ago warned the public of the presence of global warming. Mr. Hansen was subsequently vilified and heralded, his work as a NASA scientist refuted then supported. Seven years after his testimony before the Senate Committee on Energy and Natural Resources, more than 1,000 scientists supported his conclusion that the Earth was warming due to the greenhouse effect.

Last month, as Mr. McKibbon relates, Mr. Hansen in a presentation to the American Geophysical Union, provided a vivid description and a timeline for a coming harm that is difficult but necessary to contemplate. Mr. Hansen described what happens as the Earth nears, but has not yet passed, a tipping point of climate change.

“Ocean levels will increase slowly at first, as losses at the fringes of Greenland and Antarctica due to accelerating ice streams are nearly balanced by increased snowfall and ice sheet thickening in the ice sheet interiors. But as Greenland and West Antarctica ice is softened and lubricated by meltwater, and as buttressing ice shelves disappear because of a warming ocean, the balance will tip toward the rapid disintegration of the ice sheets.

“The Earth’s history suggests that with warming of two or three degrees, the new sea level will include not only most of the ice from Greenland and West Antarctica, but a portion of East Antarctica, raising the sea level by 25 meters, or 80 feet. Within a century, coastal dwellers will be faced with irregular flooding associated with storms. They will continually rebuild above a transient water line.

“This grim scenario can be halted if the growth of greenhouse gas emissions is lowed in the first quarter of this century.”

Anyone living near the coast might imagine what would happen environmentally and economically if the sea level rose by 80 feet. The costs of the aftermath of Katrina could seem minor in comparison, and yet unlike levees bursting, melting ice gives humankind both ample time to react and a false sense of safety against what will someday arrive.

Some nations – ostentatiously, the United States – reject doing anything dramatic about climate change out of fear of harming their economies. Easier to imagine than ice melting in West Antarctica is a politician bloviating about the economy as the sea rises around his ankles.