On Monday, October 31st, millions of people will parade around in ghoulish Halloween costumes mocking death. But this year, October 31st will also be a notable day in the history of human life.
The UN estimates that it's the day when human population will reach 7 billion people for the first time.
That number – 7 billion – is again stirring warnings about overpopulation. But the story of the human impact on the planet isn't simply about numbers.
"There's nothing special about 7 billion," says Robert Engelman, director of the World Watch Institute in Washington D.C. "It's just a big round number."
What big round numbers like these do, Engelman says, is give us an opportunity to revisit discussions about the impacts of a rapidly growing human population.
"We hit six billion just in 1999, and here we are twelve years later, and we are at seven billion, raising questions about whether we will successfully get to eight billion, nine billion, or beyond without seeing rising death rates related to food insecurity, disease, conflict."
Alarm bells have been rung about all those possible results of the growing number of people for at least two centuries.
Around 1800, demographer Thomas Robert Malthus warned that food supplies wouldn't be able to keep pace with growing numbers of people. And he predicted that at a certain threshold–which he set at ONE billion–there would be massive disease outbreaks, famine, and a crash in human numbers.
And yet today, human population is seven times Malthus' crisis threshold. Granted, disease outbreaks and famine remain a serious problem in many parts of the world. But the average human is living a longer, healthier life than in Malthus's time.
That's led some people to dismiss Malthus's theory.
Not so fast, says Jonathan Foley, the director of the University of Minnesota's Institute on the Environment.
"Humans aren't exempt from Malthus's observations. We just postponed it again and again and again through innovation."
Foley says the industrial revolution, the green revolution and other new technologies boosted agricultural productivity far beyond what Malthus predicted.
"But at the end of the day, Malthus's ghost is still lurking above us," he adds.
We may have innovated our way out of mass starvation and other problems so far, but those very innovations have created a host of environmental problems that threaten our future, says Foley.
Take agriculture, for example.
"Feeding seven billion people takes a lot of land, a lot of land, a lot of water, and a lot of energy," he says. "Forty percent of all the land on earth is devoted to growing food, 70 percent of all the water we consume is used to irrigate crops. And about a third of all greenhouse gas emissions come from agriculture."
That makes agriculture the single biggest contributor to climate change, loss of species, and global water shortages, all of which scientists warn are reaching critical points. And Foley says we're already running into several limitations.
"The limit of our ability to feed the world, our ability to stabilize our climate, the ability to keep the biosphere intact, and the ability to keep our water resources intact. We can't invent our way out of every limit. We can push closer and closer to the physical limits of a planet, but we're running very close to the edge already."
So what does this mean for our future? And how many more people can the planet support?
For population expert Laurie Mazur, the answer is: it depends.
Mazur directs the non-profit, Population Justice Project. She says it's not just about the number of people, but also about how much each person is consuming.
"If everybody on the planet ate like people in India, mostly vegetarian diet, the world's agricultural production today could feed about 10 billion people," she says.
In other words, there would be no food shortage.
"But on the other hand if we all ate like those of us in the United States, a very meat intensive diet, current agricultural production would feed only two and a half billion people."
That's because 40 percent of today's crops are used as animal feed, making dairy and meat production one of the most inefficient kinds of farming.
Meanwhile, most consumption today is by people in developed countries. But that's changing fast, as big countries like China and India–develop. And Mazur says that's not a bad thing.
"Half of the world's population now lives in poverty, on less than $2 a day, but that should not remain the case."
All of these people have a right to more comfortable lives. But she agrees that the challenge of providing a better life to more people would be easier with fewer of us on the planet.
Besides, she says, the task of reducing our numbers just isn't that difficult.
"This is something we absolutely know how to do," says Mazur.
The average number of kids per woman has fallen from five in 1950 to 2.5 today, thanks to successful family planning programs.
"You don't have to control anyone to slow population growth. The best way to slow growth is by educating girls, by empowering women, by assuring access to reproductive health services, including family planning."
Educated and empowered women willingly choose to have fewer kids, as seen in countries like Bangladesh.
Besides, Mazur points out education and good health services also improve people's lives.
"In other words, everything we need to do to slow population growth, is something we should be doing anyway."