How do scientists know that global warming is true? Because the evidence is overwhelming. To see that evidence, start the video below. For the sources of the slides used in the video, click here. (Note: I made this video in 2012, so the charts are a few years out of date. But each of the dangerous trends I show has continued. For example, in March 2015 monthly atmospheric carbon dioxide concentration topped 400 ppm for the first time. 2014 set a new global temperature record. January-April 2015 were the warmest four month period of any year on record. And so on. See the Green Book for more charts as of late spring 2015.)