hansen posited in a 2007 paper that 350 ppm (parts per million) of carbon dioxide in the atmosphere is a safe upper limit to avoid a climate tipping point. the current record level is 389 ppm of co2, an almost 40-percent increase from the pre-industrial revolution level of 278 ppm. in 1988 the earth's atmosphere surpassed the 350 ppm mark, while global co2 emissions per capita rose.