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I think the science indicates that ~350ppm was the threshold for the Arctic ice to stay put. We passed that in about 1987-88.
We had 391.03ppm at Mauna Loa in Hawaii in October 2012. This level "breathes" up and down by ~5-6ppm in the northern hemisphere due to all the trees and other land plants that grow and then either drop their leaves or die each year. The level peaks in about March and hits it's minimum in about October.
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