evidence: The small fraction of CO2 in the atmosphere can be misleading: consider instead the mind-boggling amount of CO2 we’ve added. [...] What’s 424 ppm of this bottle of air? It’s 11.4 billion billion molecules of CO2, of which almost 4 billion billion were added by human activities. And any infrared radiation trying to get through our bottle—which is how the Earth gives off heat into space—will have to run the gauntlet of all those CO2 molecules, each one of which can absorb infrared light and keep it from escaping to space. [...] So don’t get distracted by how small a percentage 424 ppm is: this is still a mind-boggling amount of CO2 we’ve added to the atmosphere, and a monumental change from the 280 ppm humanity has experienced for most of our history. "These molecules are strong infrared absorbers," says Kroll, "and their concentration is going up."