For nearly every day of April, from every recording center around the world, the amount of carbon dioxide in the atmosphere was over 400 parts-per-million — and it's not clear if it will ever decrease.
April, as Slate's (excellent) climate reporter Eric Holthaus wrote on Thursday, is the first month in recorded history when the monthly average has topped 400. In fact, it's almost certainly the first time in at least 800,000 years that there has been as much atmospheric carbon dioxide as this. Not to mention all of the other warming gases in the atmosphere: methane, ozone, nitrous oxide. The more of those gases in the atmospheric blanket, the less heat that escapes into space.
The 400 ppm number is mostly symbolic, representing the trend more than anything. Holthaus links to a graph from UC San Diego. That vertical line at the far right isn't an axis, it's how CO2 levels have spiked since we started burning oil and coal.
That's the trend. Passing 400 ppm by itself simply means we've seen a certain amount of warming. It's the passing that's important, not the number being passed.
In its recently-released report amalgamating climate science, the UN's Intergovernmental Panel on Climate Change made predictions of how climate change will evolve based on four carbon dioxide scenarios.
That green path is the one that the UN and environmentalists and anyone else who is paying attention to climate change is hoping we'll take. (The red one, representing unchecked use of fossil fuels over the century, is essentially catastrophic.) But even on that path, we don't get below 400 ppm any time within the next 100 years. And that requires at least a significant and dramatic reduction in the use of oil and coal and probably the development of ways to extract carbon dioxide from the atmosphere.