All the headlines of the 400 ppm barrier being broken were based on cheating and may be wrong as well.
Here’s today Keeling Curve tweet:
399.46 parts per million (ppm) CO2 in airMay 11, 2013 keelingcurve.ucsd.edu
— Keeling_Curve (@Keeling_curve) May 12, 2013
Someone should have told them they need to hold their breath when they stand upwind of the sensor.