The world's air has reached what scientists call a troubling new milestone for carbon dioxide (CO2), the main global warming pollutant. The concentration of CO2 in the arctic reached 400 parts per million (ppm) this spring, according to NOAA measurements, the first time a monthly average measurement for the greenhouse gas attained the 400 ppm mark in a remote location. CO2, emitted by fossil fuel combustion and other human activities, is the most significant greenhouse gas contributing to climate change. Read more »
