USA Today: For the first time in human history and likely for the first time in at least 800,000 years, the average level of carbon dioxide (CO2) in Earth's atmosphere topped 400 parts per million for an entire month.
Carbon dioxide is the greenhouse gas considered to be most responsible for global warming, according to the National Oceanic and Atmospheric Administration's (NOAA) Earth System Research Lab.
Scientists at the Scripps Institution of Oceanography in San Diego reported that April's average...