claim: “[ Carbon dioxide ] just crossed 400 parts per million, and high-end estimates extrapolating from current trends suggest it will hit 1,000 ppm by 2100.
evidence: Measurements of CO 2 from the Mauna Loa observatory show that concentrations have increased from about 313 parts per million ( ppm ) in 1960, passing the 400 ppm milestone on May 9, 2013.