Concentrations of carbon dioxide in the atmosphere have reached the 400 parts per million threshold two months earlier this year than last, an indication that the planet will soon experience the 400 ppm level year-round, according to scientists from the Scripps Institute of Oceanography. Last year was the first time in hundreds of thousands of years that the 400 ppm threshold was crossed. Scripps scientists expect CO2 levels to hover around 400 ppm for the next two months, when the Northern Hemisphere spring will go into full bloom and plants will suck CO2 from the atmosphere until going dormant in the fall. “It’s just a matter of time before it stays over 400 forever,” said Ralph Keeling, who took over the CO2 monitoring program from his father, Charles David Keeling, who started it in 1958. Since then, atmospheric CO2 levels have risen steadily from 313 ppm as the world continues to burn fossil fuels. Scientists estimate it’s been 800,000 to 15 million years since the planet has seen concentrations this high.