Climate Central: This week is a big one for our world. Atmospheric carbon dioxide (CO2) levels climbed above the 400 parts per million (ppm) at the Mauna Loa Observatory and it's distinctly possible they won't be back below that level again in our lifetimes.
Humans have burned enough fossil fuels to drive atmospheric CO2 to levels that world hasn't seen in at least 400,000 years. That's driven up temperatures, melted ice and caused oceans to acidify. Some extreme weather events around the world have become more...