I've never understood why we get so worked up about numbers that are multiples of 10. I mean, we're only one mutated hox gene from having Simpsons-like hands and counting in base-8. Better still, if we were all like the guy who killed Inigo Montoya's father, we would use base 11 and I'd be turning 37. Anyway, I suppose we have to declare our milestones somehow, and 400 (base 10) is the number of the day. Yes, folks, the level of CO2 in the atmosphere just crossed 400 ppm for the first time in human history. And by human history, I don't mean Roman or Greek times, I mean during the whole time you could find something on this planet and call it human.
Think about that. The last time the Earth had an atmosphere with this much CO2 was 3 million years ago. Back then, there were no humans. There were no Neanderthal. Our beloved genus Homo would not appear for several hundred thousand years. Our closest ancestor was Australopithecus afarensis (aka Lucy):
I don't mean to suggest that high CO2 will suddenly favor poor language skills, longer toes, and robust body hair; however, it is very hard to predict what our new climate will mean for us. We know the big picture of what will happen--higher temperatures, changing precipitation patterns, less ice, higher sea level, and a more acidic ocean. These will be major challenges to our civilization that has built major cities along the coasts and in places with limited freshwater (I'm talking about you, Phoenix). Our big brains and five-fingered hands give us an amazing ability to adapt to our environment. We have the ability to reign in CO2 emissions, and maybe even accelerate the rate at which is is removed from the atmosphere. We also can choose to reinforce or relocate our cities and citizens. While I'm bummed that we hit this milestone, I'm more disappointed that we (I'm talking to you, USA), can't seem to have a rational, science-based discussion of the risks of the climate change and the choices we face.