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One of the strongest pieces of evidence for human-induced climate change is the consistent rise in carbon dioxide in modern times, as measured at NOAA's Mauna Loa Observatory in Hawaii, where carbon dioxide has been tracked since 1958. In early 2015, the seasonally adjusted concentration of CO2 in Earth’s atmosphere was close to 400 parts per million (ppm), with a recent growth rate of between 2 and 4 ppm per year.
Around this seasonally adjusted average, the concentrations rise during northern spring and summer and drop during autumn and winter. The weekly average at Mauna Loa first rose above 400 ppm early in 2013 before falling back later in the year. The last time Earth's atmosphere held this much carbon dioxide was at least 3 million years ago.