Carbon concentration in the atmosphere veers dangerously close to the dubious milestone of 400 part per million. (The revised reading came out at 399.89.) “For the previous 800,000 years, CO2 levels never exceeded 300 ppm, and there is no known geologic period in which rates of increase have been so sharp. The level was about 280 ppm at the advent of the Industrial Revolution in the 18th century, when the burning of fossil fuels began to soar.”