On Earth Day, back in April, I highlighted a very interesting chart from the Environmental Protection Agency illustrating how much cleaner the nation’s air has become over the past four decades. This achievement comes despite the fact that a number of trends – energy consumption, population, vehicle miles traveled – have increased considerably.

The conventional wisdom holds that more people driving more miles and using more energy should result in more air pollution. But they haven’t. Our air has been getting cleaner instead … much cleaner.

Along comes more evidence from the federal government – this time from the National Aeronautics and Space Administration. Its satellite images make a similar point in dramatic fashion.

The space agency’s Aura satellite has been monitoring the planet since 2004. Among its findings, according to NASA, is that “people in major U.S. cities are breathing less nitrogen dioxide – a yellow-brown gas that can cause respiratory problems.”

Nitrogen dioxide – or NO2 – is among the six common pollutants regulated by EPA under the Clean Air Act. It is produced largely through the combustion of gasoline in motor vehicles and coal in plants generating electricity.

NASA’s images show just how significantly NO2 pollution has decreased from 2005 to 2011.

What accounts for this visible drop-off in NO2 emissions? According to NASA, “The shift is the result of regulations, technology improvements, and economic changes, scientists say.”

That statement is correct, but could use a little clarification.

I’ll note that the 2005 to 2011 period Aura measures roughly parallels the period during which America’s shale gas revolution got underway. The abundance of natural gas produced from the nation’s shale plays has helped lead to a dramatic switch among many utilities. They are moving from coal to cleaner-burning natural gas to generate electricity.

Numbers from the Energy Information Administration tell the tale. As the amount of electricity generated by coal has fallen during the period in question, about 90 percent of that shortfall has been made up by soaring production of electricity from natural gas.

As many others have noted, this has resulted in fewer greenhouse gas emissions nationwide.  And fewer emissions of pollutants like nitrogen dioxide as well.

For those who follow energy issues, this is just another chapter in the remarkable story of how shale energy is benefiting people and our environmental quality. It’s a story that more people should know.