The joke goes something like this: “Global warming. I’m all for it. It was colder than a midnight trip to a brick outhouse last winter.”

But the facts about global warming and climate change aren’t funny. There’s a political debate about whether global warming is real and if it possesses a significant threat to the planet. But the scientific debate is settled.

Human activities are changing the atmosphere, warming the planet and altering the world’s climate and weather patterns. That warming is evident around New England, as temperatures rise and rain replaces snow.

Maine is one of the leaders in trying to reduce greenhouse gases. A new law passed this year requires the state to reduce carbon dioxide emissions to less than 1990 levels by 2020.

While important, these state-by-state efforts can only do so much. A coherent national policy that recognizes the science on the subject is crucial if emissions of greenhouse gases are to be significantly reduced.

Sen. Olympia Snowe, along with Sens. John McCain and Joe Lieberman, has proposed a “Climate Stewardship Act,” which would mandate reductions in carbon dioxide emissions nationally.

The plan, which is gaining support in the Senate, is an important response to a real problem.

“There is compelling scientific evidence that warming trends are real, and that they are affecting our nation’s public health,” Snowe said in a release.

It’s difficult to motivate individuals when serious problems are predicted in another 100 years. But only through recognizing that our individual actions have an impact on the world we live in can things be turned around.

Consumers should look for higher fuel standards for the cars they drive. We should explore alternative energy sources to reduce the U.S. dependency on carbon dioxide producing fossil fuels and take seriously how our actions affect the air and the weather.

It will take a coordinated effort on the federal, state, local and individual levels to answer the threats caused by greenhouse emissions and global warming.