New York has 88 endangered or threatened species, 87 Superfund sites, one of the world's largest garbage dumps, and about two billion gallons of untreated sewage that pour into the Long Island Sound each year.

New York also has 19.5 million people. They own 10.8 million registered vehicles and 8.1 million houses. About eight million people live in New York City alone, each producing on average about three pounds of trash each day.


For various reasons, linking the world's rapid population growth to its deepening environmental crisis, including climate change, is politically taboo. In fact, in the United States, Europe and Japan there is public hand-wringing over falling birthrates, and government policies to encourage child-bearing.

But those declining birthrates mask explosive growth elsewhere in the world.

In less than a lifetime, the world population has tripled, to 7.1 billion, and continues to climb by more than one and a half million people a week.

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A consensus statement issued last May by scientists at Stanford University, signed by more than 1,000 scientists, warned, "Earth is reaching a tipping point."

An array of events under way — including what scientists have identified as the sixth mass extinction in the earth's history — suggest that human activity already exceeds earth's capacity. Climate change is but one of many signs of environmental stress. "The big connector is how many people are on earth," said Anthony Barnosky, a UC Berkeley integrative biologist.

The world population is expected to reach 9.6 billion by mid-century. The United States is expected to grow from 313 million people to 400 million. Economies have expanded many times faster than population, vastly increasing consumption of goods and services in wealthier countries.

"The combination of climate change and nine billion people to me is one that is just fraught with potential catastrophes," said John Harte, a UC Berkeley ecosystem scientist.


California Gov. Jerry Brown had the report translated into Chinese and sent it to China's president in June.

The Anthropocene

So complete is human domination of earth that scientists use the term Anthropocene to describe a new geological epoch.


Residues from 100 million tons of synthetic chemical compounds produced each year are so pervasive that they commonly appear in polar bear tissues, whale blubber and the umbilical cords of babies. Each year, humans appropriate up to 40 percent of the earth's biomass, the product of photosynthesis, earth's basic energy conversion necessary to all life.

Humans have converted more than 40 percent of the earth's land to cities or farms. Roads and structures fragment most of the rest. Humans appropriate more than half the world's fresh water. Ancient aquifers in the world's bread baskets, including the once-giant Ogallala in the Great Plains, are being drained.

Only 2 percent of major U.S. rivers run unimpeded. California's Bay Delta has been entirely re-engineered.


A quarter of known mammal species, 43 percent of amphibians, 29 percent of reptiles and 14 percent of birds are threatened. African elephants may be extinct within a decade. A third of world fisheries are exhausted or degraded. Ocean acidification, a product of fossil fuel burning, is dissolving calcifying plankton at the base of the food chain.

A garbage gyre at least twice the size of Texas swirls in the Pacific.



Falcons in skyscrapers


Take food. The World Resources Institute, an environmental think tank, estimates that by mid century the world will need 70 percent more food, because as people grow wealthier they eat more meat, requiring more grain to feed livestock.

But that will require converting more land to crops, even as urbanization destroys prime farmland. Farms are a big source of deforestation, which removes carbon sinks, and a big emitter of greenhouse gases that cause climate change. Climate change reduces yields by increasing the frequency of droughts and floods. Lower yields will require conversion of more land to farms.


"We can't just continue dumping nitrogen into the ocean at the same rate and expect everything to be fine," Marvier said. "The good news though, is that when we do clean up our act, we tend to see some pretty amazing bounce back."



Yet humans have always invented new technologies, such as the plant breeding breakthroughs of the Green Revolution, to forestall disaster. Ehrlich said he was right because at least two billion people are malnourished.

Reducing population growth was central to the U.S. environmental movement at its birth in 1970, spurred in part by the Ehrlichs' book and the Club of Rome's "Limits to Growth." Most environmental groups now steer clear of the subject.

Population and consumption both drive ecological damage.

"Even in poorer nations that don't have the impact that the average American has on the planet, population, as it grows, squeezes out other species, because people need space to live and the other species need space to live," said Jeffrey McKee, an anthropologist at Ohio State University who found a high correlation between human population growth and wild species extinctions. "At some point they come into juxtaposition, and something has to give. So far, it hasn't been us."

Population momentum

Plummeting fertility rates, from 4.9 births per woman worldwide in the 1960s to the current 2.6, led many to believe that worries were overblown.

The drop surprised demographers. Half the world, including Japan and Western Europe, but also China, Vietnam, Brazil and many other emerging economies are below the 2.1 replacement fertility rate. The U.S., the world's third largest country behind China and India, and the only rich country still growing rapidly, recently saw its birth rate fall to 1.9.

News coverage has stressed a "birth dearth" that threatens economic growth and elderly retirements. Some have worried that the entire human species could contract to just one billion by the year 2300 because of a failure to reproduce.

An important exception to falling fertility rates is sub-Saharan Africa, along with such places as Afghanistan and Yemen, where birth rates remain exceptionally high. United Nations demographers sharply raised their population projections last year, adding another billion people by century's end, to nearly 11 billion, because African fertility rates have stalled at high levels, over five births per woman.

Africa's population is expected to more than double by 2050 to 2.4 billion, accounting for nearly half of worldwide growth. Nigeria is expected to reach one billion people by the end of the century, and surpass the United States before mid-century.

From now until 2050, poor countries will add the equivalent of a city of one million every five days, said a report last year by the Royal Society, a British scientific organization.

Population momentum ensures that absolute numbers will keep rising for decades despite falling birth rates. That's because the exponential growth that took just 12 years to add the last billion in 2011 — and will take just 14 more years to add the next billion — means growth is building from a large base of people, many in their child-bearing years.

Falling birth rates have lulled people into complacency, said J. Joseph Speidel, a professor at UC San Francisco's Bixby Center on Global Reproductive Health. "The annual increment is rising quite dramatically," he said. "We are still adding about 84 million people a year to the planet."

Unintended births


Half of U.S. pregnancies, about three million a year, are unintended, according to the National Campaign to Prevent Teen and Unplanned Pregnancy.

About half of these end in abortion.

An estimated 222 million women in poor countries want to limit their family size but have no access to contraception. Across cultures, from Iran to Thailand to California, voluntary access to contraception has slashed fertility rates, Speidel said. But discussion of population growth remains taboo.


Campbell argued that voluntary contraception is not coercive, but blocking women from controlling how many children they have is. When given a chance, she said, women across cultures choose to provide a better life for fewer children.


"What many of us really worry about is that there will be this crash landing, from a planet with nine billion, rapidly down to five or so," said ecologist Harte. "The landing will result from methods of population reduction that none of us want to see, like famine, disease and war. I don't think anybody has described a workable trajectory that gets us up to nine and then softly back down to five."

Carolyn Lochhead is the San Francisco Chronicle's Washington correspondent. Clochhead@sfchronicle.com