Text by Bill McKibben

As the world's population hits 6 billion, Lester Brown keeps watch-and worries about what will happen if all 6 billion of us start eating steak and eggs. 

Imagine yourself the ceo of the planet. Like any good ceo, you need a few numbers you can watch closely, numbers that will alert you to signs of trouble and opportunity. Making decisions for the planet, though, is not like running a business, where there are always a few numbers that tell the tale: sales per square foot, volume of product shipped, stock price. Instead, there are literally millions of numbers you could look at-so many they could paralyze you.

 Or you could call Lester R. Brown. Brown, an unassuming and meticulous man of 65, spends his days in the modest Washington, D.C., office of the Worldwatch Institute, which he founded in 1974 and where he serves as president. It has offices, a receptionist, a conference room: It could be an insurance agency. And in a way it is. Brown's life is devoted to tracking what he calls the vital signs: the stock price of the earth, the sales per square foot of the planet. 

For 25 years Worldwatch has issued papers on topics from "conserving mountain ecosystems" to "reinventing cities." Every year its researchers comb government reports and corporate databases to issue a volume called State of the World. Six times a year its staff publishes World Watch magazine. No one tries harder to figure out who's winning in the battle between people and the planet. So Audubon sat down with Brown this summer to ask what numbers he'd choose as the most important of all, the five or six pieces of data to track every month, every year, if you want to know how the planet's doing.

 He had dozens of candidates. You could chart the rise of the hyperconsumer culture, for instance, by looking at the number of liposuctions performed annually, which topped 400,000 last year in the United States alone. Or take a look at the average vehicle speed in London, which has now slowed to the point that it matches the number for horse-and-buggy days-proof, Brown says, that the car culture has gone out of control. But as we talked he whittled his list down to the basics, the numbers that give the best chance of predicting whether the planet will just squeak through the next few decades or plow into a series of walls.

Q: Where should we begin? What number is keeping you up at night?
A: Water use. We're using water faster than it can be replenished. Worldwatch has just released a big study of the water that's available for irrigation [Pillar of Sand], and what it shows is the tremendous extent of aquifer depletion. In many areas of India, for instance, water tables are falling one to three meters a year. My colleague Sandra Postel, the author of this study, estimates that the annual aquifer depletion in just four areas-the United States, the Middle East, China, and India-totals 160 billion cubic meters of water. That's the water it takes to produce about half the U.S. grain harvest. 
  The problem with water, though, is that the shortfalls don't show up until the very end. You can go on pumping unsustainably until the day you run out. Then all you have is the recharge flow, which comes from precipitation. This is not decades away, this is years away. We're already seeing huge shortages in China, where the Yellow River runs dry for part of each year. The Yellow River is the cradle of Chinese civilization. It first failed to reach the sea in 1972, and since 1985 it's run dry for part of each year. For 1997 it was dry for 226 days. 
Q: But most of us have no real way to keep track of how much water China is pumping from its aquifers. What number could we use as a proxy? 
A: The price of wheat is a good number to keep track of. It's dropped from $240 a ton in 1950 to $120 this year. If we were managing our water supplies on a sustainable basis, wheat prices would be far above where they are now. And since wheat is the best single indicator of world food supplies, I think we're going to see a situation where water scarcity leads to food scarcity. 
Q: In much of the world, however, it's abundance that's on the rise, isn't it? 
A: Sure. And if everyone in the world consumed meat, eggs, and cheese the way we do in this country, it would take about three planets the size of this one to satisfy our needs. It takes a lot of grain to grow all that meat. But that's the direction the world is going in. When income goes up, one of the first things that happens in every society is that the consumption of livestock products goes up. Last year the world ate 26 percent more meat than it did just eight years ago. 
  There are also numbers that don't do you much good anymore, like the world's fish catch. Everyone concedes that it's topped out and overfishing will probably permanently reduce the amount of fish in the sea. In 50 years we've seen a doubling of the seafood catch per capita; the next generation may well see a halving. There's not much we can do about that one. 
Q: Are there other areas where we've already crossed the line-where the upheavals are actually under way? 
A: Well, look at climate. When we sat down to graph the earth's average temperature for 1998, instead of going up by a few hundredths of a degree, it jumped by an amazing one-third of a degree above the previous record. It literally went off the chart; we had to extend the vertical axis of our graph just to make it fit. [See "You're Getting Warmer . . . " ] 
  Higher temperatures mean more evaporation. And the more water goes up, the more comes down. That's basic physics. So it should come as no surprise that weather-related damage from things like floods and storms totaled $92 billion in 1998. The previous record, in 1996, was $60 billion. That number didn't just go off the top of the chart, it went off the top of the page. 
  The third climatic indicator is that last year weather-related events drove 300 million people from their homes-some for days, some for weeks, some forever. In Florida an entire county was evacuated because of a drought-fed wildfire. 
  What all those numbers mean is that climate instability may be accelerating in ways even the computer models don't anticipate. It reminds me of one time when a scientist went to Capitol Hill to testify about the greenhouse effect. "It won't be a smooth, gradual process," he told the senators. "There will undoubtedly be some surprises." One senator immediately asked him, "What will the surprises be?" If you knew, they wouldn't be surprises. 
Q: Are there any numbers that are improving? For instance, if we were looking for signs that people were starting to wean themselves from fossil fuels, what should we keep track of? 
A: One of the most encouraging things that's happened in the 1990s is that world coal use has not gone up at all. The economic decline in Eastern Europe and the former Soviet Union has reduced coal use there; in China they've sharply cut the subsidies for coal, and that's led them to use it much more efficiently. At the same time, we've seen a 22 percent annual growth in wind power this decade. Solar cell sales are increasing at 16 percent a year. 
  The United States still produces the most photovoltaic cells, but not by much-the Japanese are a close second. And U.S. wind-power use is not increasing as fast as others'. We added 226 megawatts of wind capacity in 1998; Denmark added 308 megawatts. But one gets the feeling that with both wind and solar we're looking at some fairly dramatic advances in the years immediately ahead. China, for example, opened its first wind farm last year, in Inner Mongolia, which is the Saudi Arabia of wind. China could double its electricity generation through wind alone. In the United States, the Department of Energy shows we could meet our national electricity needs with wind power from Texas and the Dakotas alone. At the moment, however, they supply only nominal amounts. 
  But the more enterprising corporate leaders are starting to smell real investment opportunities, I think. Mike Bowlin, who heads ARCO, said in a speech this winter that we were in the beginning of "the last days of the age of oil." He thinks we're going toward a hydrogen-based economy, with fuel cells and the like. British Petroleum has committed $1 billion to the development of solar power and wind energy. 
Q: Human numbers have exploded in this century, from a billion and a half in 1900 to 6 billion this year. But now fertility rates are falling worldwide, leading some to say that the population bomb has been defused. Is the number of people on the planet still one of the vital signs? 
A: The first thing to say is, Don't forget how fast we've been growing. There's been more growth in world population since 1950 than during the preceding 4 million years, since we first stood upright. 
  At first glance, a slowing in the rate of growth may be reassuring. But ecologists look not just at the growth rate but at absolute numbers-the demands those people put on a particular ecosystem. If the demand for food or energy or whatever is already above what the system can supply, the question is not, How do we slow the rate of growth a bit? but, How do we reestablish a truly workable balance between population size and water, population size and grain, population size and seafood? There's no one answer, of course. How many people the earth can support depends on how those people live. If they eat like Americans, the world can grow enough grain to support only 2.5 billion of them. The planet could support 5 billion Italians-they eat low enough on the food chain that they use only half the grain that we do. Or the world could feed 10 billion Indians, who eat mostly starchy foods. 
  There's one more thing you need to realize. Some of the projections about slowing population growth have to do with increases in mortality-with the HIV epidemic in developing countries. In South Africa, new data show that 22 percent of the adult population is HIV-positive. By contrast, in the developed world the infection rate has never topped 1 percent. 
  Barring a medical miracle of some sort, HIV will claim more lives in the early part of the next century than World War II did in this one, most of them in sub-Saharan Africa. To see a region's population so decimated by infectious disease you have to go back to the Indians of the New World when they were first exposed to smallpox, or to the 14th-century plague in Europe. This is simply a staggering thing, and it's not the way anyone wants to see population brought under control. 
  And there's something else those numbers tell you. Many nations in the developing world are entering a phase of what I call demographic fatigue. Their populations have been growing so fast for half a century that they've just been overwhelmed by the need to educate the growing number of kids, to create jobs for all the young people, and to deal with the environmental consequences such as deforestation and soil erosion. When a new threat like AIDS emerges, they simply don't have the energy to respond-they're spread so thin fiscally that they can't deal with it. 
Q: Is there any unmitigatedly good news on the environmental front, besides all the advances in renewable energy? 
A: I think that the moratorium on building roads in the national forests that the U.S. Forest Service imposed last year is a remarkably positive development. For decades the government has been subsidizing the cost of clear-cutting trees on public land. Also in 1998 the Chinese made the decision to stop cutting trees in the upper reaches of the Yangtze basin and start planting them instead. When Beijing says trees are worth three times as much standing as they are cut down, then you know the world is starting to change. 
Q: If you add up all the numbers, do you end up with a score? Can you tell if we're winning or losing? Is that even the right way to think about it?
A: People always ask, "Is it too late?" Is it too late to save the Aral Sea? Yeah, it's gone. Is it too late to save the Indonesian rainforest? Maybe. Is it too late to avoid a rise in global temperature? Apparently, though we can still ameliorate the rise. Is it too late to keep the glaciers in Glacier National Park from melting away? Probably.
  I remember talking to someone who had just read our State of the World 1999, and she said she had two reactions: The first is that a lot of exciting things were happening on the environmental front-suddenly the Japanese are coming up with solar roofing materials, suddenly hybrid electric cars are available in dealerships. But she also said the evidence that we're in serious trouble is more convincing than ever before. And I think that sort of captures it. 
  If I seem to be especially worried about water and food, it's because we don't have the same cushion there that we do elsewhere-there's nothing like wind energy we could switch to that could perhaps get us out of our problems. We seem to be tied to the system of photosynthesis; we've not figured out a way around that yet. The reality is, cropland productivity has been rising much more slowly since 1990 than in the preceding decade. We're dealing with physiological constraints now-even with biotechnology, it's hard to dramatically increase yields. A stalk of corn yields only so many ears, no matter what you do. 
  I use the analogy of running the mile-there's a number we're familiar with. By the first modern Olympics, we'd broken the five-minute mile. Roger Bannister broke four minutes in 1954. But even with all the advances in sports medicine, training techniques, sports psychology, there's no one talking about breaking the three-minute mile. There are some numbers it's just really hard to change. 
Q: Is there anything we can do? 
A: We have to restructure the global economy. If we stay on the current path, environmental deterioration will eventually lead to economic decline, as it did for the Sumerians, the Mayans, and other civilizations whose archeological sites we now study. 
  But we don't have much time. We have to restructure the tax system, and to do that, individuals must become politically active. We must reduce income taxes and increase taxes on environmentally destructive activities, such as burning fossil fuels, using virgin raw materials, and generating waste. The purpose of environmental taxes is to get the economy to tell the truth. For example, when we buy a gallon of gasoline, we pay only the cost of pumping the oil, refining it, and distributing the gasoline to a service station. We do not pay the cost of the disruptive change in climate that results from burning gasoline, nor do we cover the health-care costs of those suffering from the resulting air pollution. 
  This economic restructuring, in effect building an economy that is environmentally sustainable, amounts to a revolution, one comparable in scale to the agricultural revolution and the industrial revolution. The big difference is that whereas the agricultural revolution was spread out over 10,000 years and the industrial revolution has been under way for two centuries, the environmental revolution must be compressed into a few decades. We can do this in the time available only if we become politically active.

Bill McKibben's most recent book is Maybe One: A Personal and Environmental Argument for Single-Child Families. His book The End of Nature, about how humans have altered the very nature of the earth, was republished this fall in a 10th-anniversary edition. 
 

It's not just the number of people that weighs on the earth but how those people live. Consider France and Thailand, two countries with roughly the same population. The average French person requires 13 acres of land and ocean to support her consumption: the food she eats, the energy she uses, the products she buys. In Thailand, by contrast, one person requires 5 acres. This number is the country's average "ecological footprint," according to Mathis Wackernagel of the San Francisco-based think tank Redefining Progress. By considering consumption habits as well as population size, we can discover the true impact of a nation's people on the planet.