They're saying that these clouds have blunted the impact of CO2 emissions. Most of the CO2 increase (to ~420ppm) above pre-industrial levels (from ~275ppm) has

They're saying that these clouds have blunted the impact of CO2 emissions. Most of the CO2 increase (to ~420ppm) above pre-industrial levels (from ~275ppm) has occurred over the past 100 years; about half of it has only been since ~1990 (then ~350ppm).