EACH of us took turns over the past 43 years running the Environmental Protection Agency. We served Republican presidents, but we have a message that transcends political affiliation: the United States must move now on substantive steps to curb climate change, at home and internationally.

There is no longer any credible scientific debate about the basic facts: our world continues to warm, with the last decade the hottest in modern records, and the deep ocean warming faster than the earth’s atmosphere. Sea level is rising. Arctic Sea ice is melting years faster than projected.

The costs of inaction are undeniable. The lines of scientific evidence grow only stronger and more numerous. And the window of time remaining to act is growing smaller: delay could mean that warming becomes “locked in.”

A market-based approach, like a carbon tax, would be the best path to reducing greenhouse-gas emissions, but that is unachievable in the current political gridlock in Washington. Dealing with this political reality, President Obama’s June climate action plan lays out achievable actions that would deliver real progress. He will use his executive powers to require reductions in the amount of carbon dioxide emitted by the nation’s power plants and spur increased investment in clean energy technology, which is inarguably the path we must follow to ensure a strong economy along with a livable climate.