Over the next ten years, I predict, the mainstream of the environmental movement will reverse its opinion and activism in four major areas: population growth, urbani­zation, genetically engineered organisms, and nuclear power.

Reversals of this sort have occurred before. Wildfire went from universal menace in mid-20th century to honored natural force and forestry tool now, from “Only you can prevent forest fires!” to let-burn policies and prescribed fires for understory management. The structure of such reversals reveals a hidden strength in the environmental movement and explains why it is likely to keep on growing in influence from decade to decade and perhaps century to century.

The success of the environmental movement is driven by two powerful forces—romanticism and science—that are often in opposition. The romantics identify with natural systems; the scientists study natural systems. The romantics are moralistic, rebellious against the perceived dominant power, and combative against any who appear to stray from the true path. They hate to admit mistakes or change direction. The scientists are ethicalistic, rebellious against any perceived dominant paradigm, and combative against each other. For them, admitting mistakes is what science is.

There are a great many more environmental romantics than there are scientists. That’s fortunate, since their inspiration means that most people in developed socie­ties see themselves as environmentalists. But it also means that scientific perceptions are always a minority view, easily ignored, suppressed, or demonized if they don’t fit the consensus story line.

Take population growth. For 50 years, the demographers in charge of human population projections for the United Nations released hard numbers that substantiated environmentalists’ greatest fears about indefinite exponential population increase. For a while, those projections proved fairly accurate. However, in the 1990s, the U.N. started taking a closer look at fertility patterns, and in 2002, it adopted a new theory that shocked many demographers: human population is leveling off rapidly, even precipitously, in developed countries, with the rest of the world soon to follow. Most environmentalists still haven't got the word. Worldwide, birthrates are in free fall. Around one-third of countries now have birthrates below replacement level (2.1 children per woman) and sinking. Nowhere does the downward trend show signs of leveling off. Nations already in a birth dearth crisis include Japan, Italy, Spain, Germany, and Russia—whose population is now in absolute decline and is expected to be 30 percent lower by 2050. On every part of every continent and in every culture (even Mormon), birthrates are headed down. They reach replacement level and keep on dropping. It turns out that population decrease accelerates downward just as fiercely as population increase accelerated upward, for the same reason. Any variation from the 2.1 rate compounds over time.

That’s great news for environmentalists (or it will be when finally noticed), but they need to recognize what caused the turnaround. The world population growth rate actually peaked at 2 percent way back in 1968, the very year my old teacher Paul Ehrlich published The Population Bomb. The world’s women didn’t suddenly have fewer kids because of his book, though. They had fewer kids because they moved to town.

Cities are population sinks-always have been. Although more children are an asset in the countryside, they’re a liability in the city. A global tipping point in urbanization is what stopped the population explosion. As of this year, 50 percent of the world’s population lives in cities, with 61 percent expected by 2030. In 1800 it was 3 percent; in 1900 it was 14 percent.

The environmentalist aesthetic is to love villages and despise cities. My mind got changed on the subject a few years ago by an Indian acquaintance who told me that in Indian villages the women obeyed their husbands and family elders, pounded grain, and sang.  But, the acquaintance explained, when Indian women immigrated to cities, they got jobs, started businesses, and demanded their children be educated. They became more independent, as they became less fundamentalist in their religious beliefs. Urbanization is the most massive and sudden shift of humanity in its history. Environmentalists will be rewarded if they welcome it and get out in front of it. In every single region in the world, including the U.S., small towns and rural areas are emptying out. The trees and wildlife are returning. Now is the time to put in place permanent protection for those rural environments. Meanwhile, the global population of illegal urban squatters—which Robert Neuwirth’s book Shadow Cities already estimates at a billion—is growing fast. Environmentalists could help ensure that the new dominant human habitat is humane and has a reduced footprint of overall environmental impact.

Along with rethinking cities, environmentalists will need to rethink biotechnology. One area of biotech with huge promise and some drawbacks is genetic engineering, so far violently rejected by the environmental movement. That rejection is, I think, a mistake. Why was water fluoridization rejected by the political right and “frankenfood” by the political left? The answer, I suspect, is that fluoridization came from government and genetically modified (GM) crops from corporations. If the origins had been reversed—as they could have been—the positions would be reversed, too.