Two American scientists, James Galloway and Harold Mooney, have won this year's Tyler Prize for environmental achievement. VOA's Mike O'Sullivan spoke with the winners about their work, and about the state of the earth's fragile ecosystems.

The two men work on opposite sides of the country and have different specialties. Mooney is an environmental biologist at Stanford University in California. Galloway began his career as a chemist and teaches environmental sciences at the University of Virginia.

They are part of the growing field of environmental science, which developed at universities in the 1970s as specialists from different disciplines applied their expertise to environmental problems.

Galloway began his career by studying acid rain, which is caused by sulfur and nitrogen compounds that are converted into acids in the atmosphere. He says the form of nitrogen that reacts with other elements is increasing in the environment because of nitrogen-rich fertilizers and the burning of fossil fuels. The nitrogen cascades through ecosystems, accumulating in rivers, causing a buildup of ozone in the lower atmosphere, leading to acid rain and coastal dead zones, contributing to buildup of greenhouse gases in the atmosphere and depleting protective ozone in stratosphere.

It all starts with a two-atom molecule called elemental nitrogen, or N2. It makes up 80 percent of the earth's atmosphere, where it is stable, but creates problems in compounds such as nitrous oxide.

"There's a bad-news good-news side of nitrogen," said Galloway. "First of all, the fact that humans can convert N2 into nitrogenous fertilizer allows more people to be supported on the surface of the earth. We need that fertilizer to grow food. And so that's a great story."

He says nitrogen-rich fertilizer can support growth of the earth's population from its current 6.5 billion people to the nine or 10 billion expected by mid-century.

The bad effects of nitrogen can be reduced. For example, we can use technology to restrict the release of nitrogen into the environment, and also make food production more efficient, so less fertilizer is needed. Galloway says the challenge is growing, however, as more of the world turns to a Western meat-based diet. He notes that meat production requires large amounts of grain for animal feed, and so uses more fertilizer and nitrogen in the process.

Harold Mooney of Stanford says globalization is changing the planet in both good and bad ways. On the positive side, he says the global standard of living is going up in many regions.

"But now, with rapid transport, things are brought around the world purposely between these regions, but then also inadvertently," he said. "So we're getting outbreaks of diseases and pests, human diseases as well as diseases of crops."

He says invasive water plants are clogging rivers and newly introduced species are reducing biodiversity.

But the scientists are optimistic that worldwide collaborations of environmental experts can identify the problems and suggest solutions.

Linda Duguay, a marine scientist with the University of Southern California and executive director of the Tyler Prize, says the earth's complex systems are connected, and scientists today study them that way.

"Basically, what people have realized is the environment isn't just about the biology or the chemistry or the physics or the weather - that really to understand the earth's system, as we've come to call it, we need to bring people in from all these different areas to work together on the problems," she said.

Harold Mooney says scientists are getting a better understanding of the challenges facing our planet. He says scientific knowledge is one part of the equation.

"But then we've got to get the policy community engaged in doing something about it, but you can't have that happen unless the public is well aware of what is happening and puts pressure on the policy community, at least shows them that they care about it," he said.

The hardest part, says James Galloway, is persuading governments to act now on long-term problems.

"The challenge to the scientist is to be able to take the information that they have and put it in a language that a policy maker can understand," he said.

The honorees say awards like the Tyler Prize bring needed attention to environmental issues, and to the research on the environment being done by scientists around the world.

Previous winners include the primate anthropologist Jane Goodall and former U.S. Surgeon General C. Everett Koop.

In the 35 years since the prize was inaugurated, Mooney and Galloway say we have gained a better understanding of the human impact on the planet. That is the good news. They say there is also bad news: environmental problems are getting more complicated.