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Ice core records show that until the Industrial Revolution, atmospheric CO2 levels remained fairly steady at around 280 ppm. By 1961, CO2 data collected at a monitoring station at the summit of Hawaii's Mauna Loa volcano showed that atmospheric CO2 levels were rising steadily by about 2 ppm per year. Nearly half a century later, in 2005, CO2 concentrations had increased to 380 ppm. Seeing global concentrations above the 400 ppm threshold at a time of year when atmospheric CO2 is typically at its lowest level is a critical turning point. [Source: NASA Climate Change News]
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