Today, the world celebrates the 40th anniversary of Earth Day.

Comparing the state of our environment today with the condition of our planet in 1970 clearly shows we have a lot to celebrate.

In the years since Maine Sens. Edmund Muskie and George Mitchell helped pass the Clean Water and Clean Air acts, Maine’s rivers, which were little more than open cesspools 40 years ago, are attracting fish and other wildlife once again.

We’ve been able to confront threats like acid rain and the depletion of the ozone layer, and save species like the American bald eagle from the brink of extinction.

But in the same span of time, the population of the Earth has almost doubled, to 7 billion people. Nearly a third of our brothers and sisters around the world do not enjoy reliable clean water and sanitation.

And the environmental problems of the past were relatively easy to address compared to global climate change. Like the proverbial frog in a slow-boiling kettle, we can see that the world is gradually getting warmer, but we might not muster the initiative to do anything about it before it’s too late.

The stakes are high. Whether we choose denial or concerted, effective action will make a difference to billions of lives on Earth.

Often, the most vulnerable communities — the world’s poorest people, the most fragile species and ecosystems — are the ones who suffer the most from severe changes in climate because of global warming. This makes facing up to the challenge not only a matter of environmental concern, but also a matter of justice.

Still, the environmental successes of the past four decades have taught us that with education, awareness and willingness to work together toward public and private solutions, major devastation to the environment can be avoided and reversed.

On this 40th anniversary of Earth Day, we have a golden opportunity not only to address the threats of climate change, but also to build a stronger, more secure economy in the process.

Leaders at every level of government — and millions of citizens — are getting serious about creating new energy policies and making energy use choices designed to reduce dependence on foreign oil, make air cleaner and reduce the risks we face from climate change.

But the biggest opportunity is in Washington, D.C., where lawmakers are about to release a new plan to tackle the threats of global warming and promote energy independence.

For the first time in the history of the United States, we could have a meaningful energy policy that’s focused on clean, homegrown energy, instead of on dirty fossil fuels that largely come from overseas.

Our state and its workers have a lot to gain from a sensible energy policy. We can reduce the amount of money we spend on energy, and create new jobs and businesses — especially in rural areas.

Sens. Olympia Snowe and Susan Collins have both worked hard for a comprehensive policy to address energy and climate change. We hope that they’ll continue to look out for Maine’s best interests by standing tough against out-of-state fossil fuel interests and supporting strong controls on the pollutants that are threatening our atmosphere.

Global climate change isn’t simply an environmental issue: It’s an economic and moral issue. But it’s a challenge we can meet with innovations, policies and collaborations that involve both nations and neighborhoods.

On Earth Day 2010, we can celebrate the progress and leadership that Maine’s people, businesses and political leaders have already achieved in leading the way toward a new energy economy.

This Earth Day, let’s not give up until we’ve put sustainable solutions in place that will benefit us all, and all life, on the only planet we call home.