Here at 450-by-2050 we're all about breaking it down and writing out the basics. So here's what the 450 ppm thing is all about:
The basic idea: Carbon dioxide in the atmosphere can be a good thing. It feeds our plants and traps heat in the atmosphere to keep us warm. And it doesn't just stay there - carbon dioxide can be recycled through plants, animals, the oceans, some types of rock; there's a ton of cycles at work on the Earth.
Normally, there's between 150-300 molecules of carbon dioxide for every million air molecules. And it changes, it's constantly changing - every winter the concentration goes up by about 25 or so, there's more of it in the air because trees have dropped their leaves and stopped breathing. It changes over long periods of time - like between warm periods and ice ages. And that's okay - the Earth has ways of dealing with it.
Everything in excess is bad, though.
We're currently at about 380 parts per million. We started the industrial revolution at about 280 - and we're putting more of the stuff out every day. To make things worse, we've also been cutting down tons of forests - now there are fewer trees to breathe carbon dioxide in and exhale oxygen.
And the Earth's other cycles are getting overwhelmed, too. There's just too much of the stuff, over too short a time frame - a human amount of time, instead of the geologic thousands of years that the Earth is used to.
It all adds to needing to do something right now. It's going to be too late soon.