The Mauna Loa Observatory has the world’s longest record of direct atmospheric carbon dioxide readings. But that’s just part of the story. Credit: Larry O’Hanlon

Four hundred parts per million. It’s just a number, but it speaks volumes. Humanity’s unchecked extraction and release into the atmosphere of ancient, naturally sequestered carbon (a.k.a. fossil fuels) is driving us into uncharted climatic territory. According to the Mauna Loa Observatory, we have hit that milestone, and show no signs of slowing down.

Put another way, we’ve increased the atmospheric concentration of carbon dioxide by 25 percent in 100 years. And if we continue at the present accelerating consumption rate, which all signs indicate we will, we should hit 450 ppm in 2040.

But images speak louder than words. You may have already seen the widely distributed wiggly climbing Keeling Curve comprised of the 55-year record at Mauna Loa Observatory on the Big Island of Hawaii. Here’s what you get when you tack on 250 years of ice core data onto that:

Credit: Scripps Institution of Oceanography, U.C. San Diego

Ouch! This really reveals how extremely rapidly we are messing with the chemistry of our atmosphere since the Industrial Revolution. But if you step back and look at the record from ice cores over the last 800,000 years you get this:

Credit: Scripps Institution of Oceanography, U.C. San Diego

Be sure you look at the far right-hand side of this graph. And consider that some scientists believe we are now at a carbon dioxide level not seen in more than 3 million years. That’s when it becomes very clear that we have something to worry about. Humans have been around only about 2 million years. And while many climate change naysayers will tell you “climate cycles are natural,” this last graph makes it clear we have stepped well beyond what is “natural.” So yes, 400 ppm is just a number, it’s also a wake up call.