More than anything else, climate change is a water problem. Scientists expect more coastal flooding and possibly more inland flooding. They expect higher temperatures and greater evaporation to deplete water resources, creating risks for the food supply. They believe sea-level rise will eventually render some regions uninhabitable.

As the global population reaches 7 billion, ecological distortions are becoming widespread. Among the changes are a drop in fresh water supply in more than two dozen of the largest African nations and a net loss of forest in South America and Australia. Nations with stable or declining populations, such as the United States, have seen a rise in acreage and density of temperate forests.

During the wet season, the city of L.A. sends 100 million gallons of stormwater into the Pacific each day. That water had, for many years, been handled as pollution, since the water produced in rainstorms picks up various effluents that then flush into the ocean.

Paved sidewalks and driveways keep shoes clean and cars out of the mud. But environmentally speaking, too much paving is a disaster, blanketing the ground and preventing rainwater from soaking in. Runoff, especially after strong storms, erodes soil and carries oily residue into streams and eventually into sensitive estuaries. Pervious paving, though, is friendlier.