According to United Nations projections, Earth’s human population crossed the 7-billion mark on Monday, Oct. 31, 2011, and continues to climb, possibly topping 10 billion by the end of the century. What is the toll of this growth on the environment and on already-strained areas of the world?

With a three-month-old baby at home (estimated to be the 6,976,533,115th person in the world), social demographer Leah VanWey is optimistic about the future. Affiliated with both the Population Studies and Training Center and the Environmental Change Initiative at Brown, VanWey focuses on the frontier settlement in the Brazilian Amazon, posing the question of how we can simultaneously protect the Amazon’s precious environmental resources while promoting equitable social and economic development. Here, she extends this challenge to the rest of the world.

Yes, the world can support 7 billion and beyond; the important question that we as a human race need to answer is what we want our future to look like. Specifically, what standard of living do we want for people, how much inequality are we willing to tolerate, and how much of nature are we willing to sacrifice to fuel ourselves and our machines.

The key to increasing the size of the economy and also to increasing equality by improving living standards in much of the world is education. With education, health improves, productivity improves, quality of life improves, and we see a more rapid transition away from dangerous and unpleasant jobs. Without education, the global economy can, of course, support 7 billion or more, but at high levels of inequality and with potentially disastrous political consequences. As we have seen in the past several months, inequality is a strong motivation for protest and potentially for revolution.

Environmentally, the answer is much the same — it depends on what we are willing to live with. With currently imaginable technology, 7 billion people living like Americans would not be possible. It simply requires too much land and too much fuel to live in a world of single family homes, eating meat every day, and hopping in the car. To avert catastrophic climate change, we must move away from fossil fuels, and no currently imaginable technology will provide enough energy to allow more than 7 billion people to live like us. This is not to say that I am not optimistic, because I am.

What about previous arguments that we’re reaching the limits of the Earth’s ability to support human life?

Such arguments proved wrong because of humanity’s tremendous capacity for innovation. We can now live in places we couldn’t before, because of technology that overcomes heat or cold or that controls pests and disease. Similarly, we have witnessed a revolution in the productivity of agriculture in large portions of the world, and we have untapped potential for further improvements in Africa and in other crops around the world. At the same time, some increases in production would come at the expense of natural lands and therefore at the expense of biodiversity and other ecosystems services. If we eliminate too much forest or wetlands, we lose the important roles they play in cycling water and cleaning water, further changing local weather on top of the impacts of global climate change.

What are the major consequences of this level of growth?

It depends. ... Faster growth generally poses problems of institutions and infrastructure keeping up with population growth. This is everything from having enough labor and delivery rooms to enough schools to a large enough and dynamic enough economy to absorb entering workers. At a population level, higher natural growth rates (due only to births outnumbering deaths, rather than to migration) means younger populations and therefore a high burden borne by parents and society to feed, clothe, and educate children. But growth rates are not uniform across the world and the variation is more important than the global experience.

Much of the developing world, especially much of Africa, faces a high fertility rate, creating a challenge for societies trying to educate and create jobs for these children. In the last decade or so, many demographers and economists have started pointing out, however, that this is good as the children from high fertility times move into adulthood. As large cohorts of people move into adulthood, a country gets a “demographic dividend” in which there is a large group of working age adults before there is a large group of elderly needing support.

In contrast to this story, much of Europe is facing the prospect of declining population size due to extremely low fertility. This is an equal challenge, and many argue that it provides the answer to the challenge faced by the developing world. Labor migration from the developing world to Europe and Japan can solve labor shortages in places with low or negative rates of natural population growth and labor surplus in places with high growth rates. This is only a temporary solution to either problem, but provides important relief to both migration origins and destinations.

What are the consequences for specific parts of the world that you work on, such as the Brazilian Amazon?

Population change through fertility and mortality affects the population of the Brazilian Amazon much less than does migration. People are still moving into the Amazon in pretty sizable numbers, and people are moving around a lot. Cities are growing at the expense of population in rural areas. The impact of global population growth on the Amazon acts through economic forces primarily. Growing demand for animal feed to raise animals to feed a growing and increasingly affluent population in Europe and China spurred the expansion of the Brazilian soybean industry. The growing of soy for export is changing the landscape in some of the areas I work, with pastures turning into fields and forests turning into pastures or fields. This is a counter-intuitive pattern, with population growth associated with larger and more productive farms. In parts of East Africa where I and colleagues work the opposite is true. Population growth leads to the division of lands with each generation until farms are too small to produce enough food for a family. The difference has to do with the ability to get crops to market and not to any inherent differences between Brazilians and Africans. Many Brazilians simply have the good fortune to have connections to regional and global markets.

In your view, is this milestone a looming disaster or is there any room for optimism?

I am optimistic because population growth rates have been declining since about 1970. I am optimistic because best estimates of the number at which population will peak are lower than they were when I was in college in the mid-1990s. I am optimistic because humans have a history of innovating to overcome potential limits to continued growth (both population and economic). I am also optimistic because what we see today in science looks to me like the coordinated effort necessary to find sustainable paths into the future.