The concentration of carbon dioxide in Earth’s atmosphere hit more than 420 parts per million for the first time in human history on Saturday. The daily average of 421.21 ppm at Hawaii’s benchmark Mauna Loa Observatory means human activities like burning fossil fuels have produced so much CO2 pollution that the atmospheric concentration of CO2 is more than halfway to double preindustrial levels. ā€œIā€™m even more certain CO2 causes global heating than I am that smoking causes cancer,” Kate Marvel, a research scientist at Columbia University told the Washington Post. “The world is already more than 2 [degrees Fahrenheit] warmer than it was before the Industrial Revolution.ā€ (Washington Post $)