The last time atmospheric levels of carbon dioxide reached 400 parts per million, humans didn't exist. Before the start of the Industrial Revolution, global average CO2 was about 280 ppm, and it fluctuated between about 180 ppm and 280 ppm during the past 800,000 years. This month, some daily records touched 400 ppm for the first time since, probably, somewhere between 2 and 4.6 million years ago, when the closest equivalent to our species was Homo habilis. CO2 emissions in 2012 were a record 35.6 billion tons. CO2 concentrations in the atmosphere will decline over the coming months, as northern hemisphere forests take up some of the gas from the atmosphere; but in the northern fall, they will resume their trajectory upward - and upward, and upward.