When Sen. Gaylord Nelson of Wisconsin sponsored the first Earth Day 38 years ago, our country's environmental outlook was grim. Smog choked the air in Los Angeles and New York, and many cities dumped raw sewage into our greatest rivers, including the Mississippi.

The years since then have taught us two important lessons. First, progress is possible if we have imagination and commitment. Thanks to the Clean Air Act of 1970, we have reduced the amounts of carbon monoxide, lead and other pollutants in the air we breathe. Thanks to the Clean Water Act of 1972, cities have installed modern wastewater treatment plants. The Mississippi River is healthier today than it was 30 years ago.

The second lesson is that environmental stewardship can go hand in hand with economic growth. This lesson is especially important to remember as we confront the most pressing environmental challenge of our time: global climate change.

Just a few years ago, climate change was not widely discussed. Now it's a concern among hunters across Minnesota, who see the effect on our wetlands, and among businesses in Duluth, which are concerned about the falling level of Lake Superior and the impact on Great Lakes shipping.

The science is clear. Global temperatures are up 1 degree in the last century. That doesn't sound like much -- but to put it in perspective, they have risen only 5 degrees since the height of the Ice Age. The Environmental Protection Agency predicts that temperatures could rise another 3 to 7 degrees in the next 100 years.

So the challenge is clear. But with challenge comes opportunity -- opportunities to create new jobs, new industries and a new green economy.

Just drive past the massive wind turbines at Buffalo Ridge and you'll understand what wind power can mean to Minnesota. Every wind turbine creates good jobs in construction and manufacturing, and spins off thousands of dollars to the local economy.

In Starbuck, Minn., a 10-employee company called Solar Skies is producing high-quality solar panels and creating good middle-class jobs. In St. Cloud, the city is getting ready to install a new ice rink using geothermal energy.

The question is: Does the United States want to be a leader in creating the green industries of the future? Or are we going to sit back and watch the opportunities pass us by?

In at least two important clean-energy fields, we have already fallen behind. We rank third in wind-power production, behind Denmark and Spain. We rank third in solar power installed, behind Germany and Japan.

These countries surpassed us largely by adapting technologies that we first developed. We came up with the right ideas, but we didn't capitalize on them with the right policies.

How can we regain leadership? As Minnesota's own Tom Friedman noted in a recent article in the New York Times, the market alone won't work. To spur investment, the government must play a role -- creating the right incentives and setting the highest standards, then letting the market respond. Such incentives must be available over the long term, because investors need a stable, predictable investment horizon to make long-term decisions.

For years we have celebrated Earth Day as a mostly symbolic event -- a day of teach-ins at our schools and rallies on the lawns of state capitols. This year, let's go back to the idea of Earth Day's founders: Let's take action to leave a cleaner, healthier world for the generations who follow us.

Amy Klobuchar, D-Minn., is a member of the Senate Environment and Public Works Committee. She has introduced the American Renewable Energy Act, which would expand and extend tax credits and other incentives for private investment in solar power, wind power and other forms of renewable energy.