It has happened. For the first time, atmospheric carbon dioxide levels at Mauna Loa Observatory in Hawaii passed a daily average of 400 parts per million. There is now more carbon dioxide in the atmosphere than at any point since three million years ago — about 2.9 million years before humans existed.

Carbon dioxide, or CO2, is a naturally occurring gas. It is also a gas that is generated when fossil fuels like oil and coal are burned. And it's a gas that traps heat in the atmosphere that might otherwise escape into space. The more CO2, the thicker the blanket the Earth is wearing. And while passing 400 ppm is largely only symbolic, it suggests that the problem of climate change isn't getting better.

Mauna Loa Observatory, run by the National Oceanic and Atmospheric Administration, has a graph that it updates regularly showing CO2 levels by day and week for the past year. It looks like this.

That up-and-down motion is natural seasonal variation. If you instead look at data year-over-year, the trend is obvious.

That graph only goes back as far as when Mauna Loa started operations in the 1950s. If you include data taken from ice core samples — long rods of ice extracted from thick ice formations which contain trapped bubbles of air — you can plot way farther back. This graph shows CO2 levels since 1 AD. That little spike at the end there is what we're on the tip of. And it starts right around the time that we figured out how to burn oil and coal.

If you're wondering about the blanket effect, here's how CO2 levels compare with temperature shifts for the past 160 years or so. The temperature data is expressed as variance from the average temperature between 1951 and 2000, and is global.

The New York Times explains the significance of the milestone.

The best available evidence suggests the amount of the gas in the air has not been this high for at least three million years, before humans evolved, and scientists believe the rise portends large changes in the climate and the level of the sea. …

From studying air bubbles trapped in Antarctic ice, scientists know that going back 800,000 years, the carbon dioxide level oscillated in a tight band, from about 180 parts per million in the depths of ice ages, to about 280 during the warm periods between. The evidence shows that global temperatures and CO2 levels are tightly linked.

There are no signs that increases in CO2 levels are slowing. Or, stripping out the numbers and history, there are the words used by Maureen Raymo, a scientist at Columbia University interviewed by the paper: "It feels like the inevitable march toward disaster."