This greenhouse gas normally shows its annual peak in the rising saw-tooth pattern in mid may, with the net annual peak level rising steadily year on year. It peaks before spring growth in the northern hemisphere starts to absorb the gas, releasing it during the northern winter. On April 25th the level reached 399.72ppm, and hourly readings have topped 400ppm six times in the last week. Charles Keeling, whose curve describes the rise in CO2 over the last half century said that if the line isn't passed in this year's peak, then it certainly will be next year. With two weeks to go before the annual peak, it seems likely that it will. The consensus safe level of atmospheric CO2 in the climate change community is considered to be 350ppm in order to keep Earth from warming less than two degrees Celsius. It has been over two million years since levels were this high, and the Pliocene world was much warmer than ours.
Image credit: NOAA.