The threshold of 400 carbon dioxide molecules per million molecules of Earth's atmosphere is an arbitrary but still significant milestone, reflecting a near 50% increase in the concentration of the greenhouse gas since humanity first started burning fossil fuels for industry. Sure, the Earth has experienced hotter chemistry before, but Peter Gleick says it all the in the title of his post: The Last Time Atmospheric CO2 was at 400 parts per million Humans Didn’t Exist. The Arctic was also free of ice, and CO2 levels were changing 1000 times slower than they are today. Not that we can't survive and even thrive in the midst of an economy that daily poisons and deadens the Earth, but it will be a much uglier place we have paid for.
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