June’s Rio + 20 Conference on Sustainable Development left environmentalists little to cheer about. Kumi Naidoo, executive director of Greenpeace International, called the meeting “a failure of epic proportions.” And Washington Post reporter Juliet Eilperin said the conference “may produce one lasting legacy: convincing people it’s not worth holding global summits.”

For those who care about population and sustainability, the meeting was especially disappointing. Despite advocacy efforts by scientists and NGOs from around the world, including a high-profile lead-up report by the UK’s Royal Society, the outcome document failed to recognize the environmental implications of population dynamics and to connect the dots between demography, reproductive health, and sustainable development.

How did this happen? The reasons are many, but here, I’ll focus on one that is fundamental to the global stalemate on environmental issues in general and to population and environment issues in particular: inequality.

The Gap

Around the world, there is a vast – and widening – gulf between the “haves” and the “have-nots.” A 2008 study by the UN University’s World Institute for Development Economics Research found that the richest one percent of adults now own 40 percent of global assets. The bottom half of humanity, in contrast, owns barely one percent of all wealth.

This appalling gap in wealth is matched by disparities in environmental impact: The relatively affluent citizens of industrialized countries consume a far greater share of the planet’s resources and emit a greater quantity of waste than their counterparts in the developing world. According to the Global Footprint Network, if everyone on Earth lived the lifestyle of an average American, we would need five planets to support everyone.

These disparities, and the injustice they represent, are a major obstacle to global consensus and action on environmental problems. Take climate change; because industrialized countries have historically contributed the lion’s share of heat-trapping carbon dioxide emissions, those countries lack the moral authority to call for reductions elsewhere. And developing countries, which are struggling to lift their people from abject poverty, reasonably fear that emissions limits will stunt their development.

Inequality also effectively shuts down discussion of the environmental impact of population growth. It is well established that environmental impact is shaped by a combination of resource use, technology, and population size. The impact of any group of people depends on how they use resources (their systems of production and consumption) and on how many people are doing the producing and consuming.

But, in the context of global inequality, many believe that a focus on population growth is a dodge, a way of shifting attention away from industrialized countries’ supersized consumption habits. As environmental activist and journalist George Monbiot puts it, “population is the issue you blame if you can’t admit to your own impacts: it’s not us consuming, it’s those brown people reproducing.”

All Things Being Equal, Population Matters More

Inequality makes it difficult to address – and even acknowledge – the environmental impact of population growth. But that doesn’t mean that population is irrelevant to sustainability. Indeed, in the more equitable world that many of us would like to inhabit, population dynamics would be more important, not less.

Again, consider climate change. Today, human beings collectively emit more than 30 billion tons of the greenhouse gas carbon dioxide each year. That aggregate figure masks vast disparities in per capita emissions, both between and within countries. Americans are among the highest per-capita emitters on Earth, at about 17 tons per person, per year. Europeans emit about half that and most Indians and sub-Saharan Africans come in at a ton or less. Today, the poorest people in the world make a negligible contribution to climate change.

But let’s imagine a world in which wealth and its attendant environmental impacts are distributed more equitably. Let’s say Americans, through behavior and/or technological change, are able to cut carbon emissions by two-thirds and Europeans cut theirs in half. While we are at it, let’s conjure a massive redistribution of resources and technology, which enables everyone on Earth to converge at an emissions level of five tons per person, per year – about the aggregate level of China today.

Against this backdrop, consider the range of possibilities for future population growth. The UN Population Division predicts that world population will grow from 7 billion today to anywhere between 8 and nearly 11 billion by 2050.

In our equitable world, if world population reaches nine billion (slightly below the UN’s medium variant projection), global carbon dioxide emissions would rise to 45 billion tons of CO 2 per year. This would represent a 50 percent increase over our current level. In the equitable world scenario, the difference between a world population of 8 billion and 11 billion would be about 15 billion tons of CO 2 per year – half our current emissions and quite possibly the margin between a manageable climate crisis and catastrophe.

In the short term, the difference is less dramatic: by 2025, our equitable world would emit 38 billion tons of carbon a year at the low population variant; 42 tons at the high variant. And, of course, it is in the short term that we must drastically reduce our carbon emissions if we are to keep global warming below two degrees Celsius – the threshold set in the Copenhagen Accord to prevent the most disastrous effects of climate change. But there is a clear divergence between the emissions impact of a slow-growing population and a fast-growing one, which widens over time.

In an equitable world, then, population matters. In fact, the only scenario in which population doesn’t matter (much) is one where the current inequitable divide between rich and poor remains fixed for all time. Fortunately, that scenario is unlikely, because development is proceeding, if haltingly, even in the poorest countries.

The Moral Challenge of Our Time

So, what does this mean for the inequitable world in which we actually live?

First, if we take seriously the twin imperatives of social justice and sustainability, we must address inequality by fostering human and economic development in less developed countries. Of course, that will entail greater resource use in these countries. And, since the planet cannot sustain 7 billion people living as we do in the industrialized world – much less a future population of 8 or 11 billion – it is crucial that we simultaneously reduce resource consumption in the industrialized countries and find ways to meet human needs at less environmental cost. One model is the “safe and just space for humanity,” envisioned by Oxfam International, in which all people have the resources they need to fulfill their human rights, while ensuring that humanity’s collective use of natural resources does not exceed sustainable boundaries.

Finally, we should aim for the lower end of the UN population projections. We can do this by educating girls, empowering women, and ensuring universal access to family planning and reproductive health services. These cost-effective interventions not only slow population growth, but, equally if not more important, they have other enormous benefits for women and families. Moreover, by freeing up resources for productive investment, slower population growth can jumpstart development and help reduce inequality. A wealth of evidence suggests that a world population of 8 billion would be better than 11 billion, for human beings and for the natural systems that sustain all life.

Inequality is the moral challenge of our time; it casts a deep shadow over the global debate on sustainability, obscuring other priorities. In fact, the challenges of inequality, overconsumption, and population growth are all vitally important, and profoundly interconnected. All deserve the most urgent priority if we are to build a world that is sustainable and just.