You won't have been able to feel it, but on Friday the levels of carbon dioxide in the atmosphere hit a milestone amount.
For the first time in recorded history, CO2 levels averaged 400 parts per million (ppm), or 0.04 percent, at a monitoring station on Mauna Loa, Hawaii.
And, say scientists, it's very much down to the way we live: our gadgets, our homes ... and our cars.
400 ppm is a significantly higher average than at any other point since records began half a century ago, and it's a much higher level than readings taken from air bubbles trapped in millenia-old ice.
According to The New York Times, Antarctic ice cores going back 800,000 years contain air bubbles with measurements between 180 and 280 ppm, correlating with cool and warm periods in the earth's history.
Carbon dioxide in the atmosphere traps the sun's heat, warming the planet and affecting the climate--so as the readings show, higher concentrations have a measurable effect on average global temperatures.
Levels have risen rapidly in the last 50 years or so.
When measurements began at Mauna Loa in the late 1950s, CO2 was at 315 ppm. Prior to the Industrial Revolution, it was nearer the 280 ppm limit.
Are cars to blame?
Cars are often scapegoat number one on the list of carbon dioxide sources, and with more than one billion on the planet, it's true their contribution is vast.
The huge strides automakers have made to increase fuel economy and reducing CO2 have certainly cleaned up modern vehicles--and other pollutants have gone down too.
But with countries with huge populations like India and China increasingly taking to four wheels, it's an uphill struggle to reduce CO2 overall.
And large countries are larger emitters of CO2--China and the U.S. together produce more than 40 percent of global CO2 emissions.
That isn't just from cars, of course. It includes power generation, air travel, sea travel, and more. Very little you own or use doesn't contribute in some way to the carbon load--and we simply live in a consumptive society.
What can be done?
400 ppm is essentially just another number. Its capacity to shock is psychological, since CO2 levels have been at equally troubling figures in the 390s ppm for some time now.
To illustrate: 200 mph is very fast, but so is 199 mph. It's mainly shocking on a symbolic level--and in comparison to historical levels.
However, as a symbolic number, it once again becomes news.
Fewer people will be able to ignore it, and there'll be added emphasis on reducing it--or, at least, preventing further rises.
The question has long since become--even if we were to fill our roads with zero-emission vehicles and radically boost efforts to clean up our power grids--whether we have the ability to prevent that number climbing past the point of no return.
Whatever that may be.