CO2 Levels Reach 400ppm at Mauna Loa For First Time On Record: Titus Andronicus writes "Today, NOAA reported, 'On May 9, the daily mean concentration of carbon dioxide in the atmosphere of Mauna Loa, Hawaii, surpassed 400 parts per million for the first time since measurements began in 1958.' For comparison, over the last 800,000 years, CO2 has ranged from roughly 180 ppm to 280 ppm. 'For the entire period of human civilization, roughly 8,000 years, the carbon dioxide level was relatively stable near that upper bound. But the burning of fossil fuels has caused a 41 percent increase in the heat-trapping gas since the Industrial Revolution, a mere geological instant, and scientists say the climate is beginning to react, though they expect far larger changes in the future.' The last time Earth had 400 ppm was probably more than 3 megayears ago."
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