50 parts per million. That's the figure scientists agree is the "safe" upper limit of carbon dioxide in our atmosphere. Beyond that point, it's increasingly likely that our planet will become inhospitable for the humans that cling to its surface. Crops will fail, sea levels will rise and millions, if not billions, will die in catastrophic weather events. The National Oceanic and Atmospheric Administration's Mauna Loa Observatory reports that levels have now reached 409.9 parts per million.
Thanks to our ceaseless desire for more power, that number isn't going to shrink any time soon. The International Energy Agency's 2018 report found our energy demand has risen by 2.3 percent, the highest rise in a decade. Unfortunately, much of that demand was met by burning more fossil fuels, releasing even more CO2 into the atmosphere.