OPINION: Planet Earth is getting too crowded

William N. Ryerson is president of the Population Institute and the Population Media Center.

Today is the 40th anniversary of the first Earth Day. The first Earth Day marked the beginning of one of the largest grassroots movements that the world has ever seen. In the intervening four decades, much has been accomplished, including the passage of landmark legislation on clean air and clean water, and an international agreement to protect the Earth's ozone layer.

But the planet is at greater environmental risk today than it was 1970. The reason is simple: Population growth has outpaced the green movement. People today are more environmentally conscious, more energy efficient and far more likely to recycle - but there are a lot more people on the planet. Globally, population has grown from 3.7 billion to 6.8 billion since 1970, an increase of 84 percent.

While it's true that birthrates are falling in most parts of the world, we are still adding another billion people to the planet every 12 years or so. And, to paraphrase, Everett Dirksen's famous quip, a billion here and a billion there, and the next thing you know you're talking about real population growth. In my lifetime, global population has already doubled, and it will likely triple, too.

When they hear these numbers, some people shrug their shoulders and opine that the world's a big place. It is, but it's not how many people the Earth can contain, it's how many people the Earth can sustain. And the latest data from the World Wildlife Fund and the Global Footprint Network suggests that we are long past the point of living sustainably. We are devouring resources far faster than the Earth can regenerate them.

Population growth strains the environment and, just as important, stresses the Earth's capacity to provide for our own care. Last year, the United Nations reported that the number of chronically hungry people now exceeds 1 billion. Hunger, in fact, has been rising now for more than a decade. Within 20 years, we may be facing what John Beddington, Britain's chief scientific adviser, calls a "perfect storm": Population growth, combined with climate change and the world's insatiable demand for more food, energy and water, could easily trigger a global crisis.

If we are to avoid this, we must act on multiple fronts. We need to boost food production dramatically, particularly in the developing world. We need to slash emissions of greenhouse gases and become far more energy efficient. We must also address the growing scarcity of fresh water - a threat that may be greater than the impending energy crisis.

But unless we do more to encourage smaller families and prevent unwanted pregnancies, it's hard to envision a good outcome. The heroic gains that we make on agricultural productivity, energy efficiency and reductions of greenhouse gas emissions could easily be swamped by a rising tide of humanity. Unless more is done to reduce birthrates, we will have an additional 2.5 billion more people on the planet by 2050.

If humanity keeps growing, the rest of the world will pay a heavy price. Leading biologists warn that half of all plant and animal species could be in danger of extinction by the end of the century.

If we truly love the Earth, the large unmet need for family planning in the world must be met. But expanding the supply of contraceptives, by itself, is not enough. In many parts of the developing world, women have little or no say in how many children they bear. In rural Yemen, for example, many girls are forced to marry at the age of 8. Early marriage and early pregnancies condemn many young women to lives of poverty and, all too often, an early death.

Empowering women and elevating their status is one of the most important things that we can do for developing countries. It may also be critically important to the future of the planet - and every species that calls it home.