Earth look a little like Solaris on September 27, 2016.
Earth looking a little like Solaris on September 27, 2016. NASA

Global Mean Atmospheric CO2 Levels Clear 400ppm, Forever: The average concentration of carbon dioxide in our atmosphere fluctuates annually, dropping during the northern hemisphere’s spring and summer and rising in the fall and winter. This pattern is a product of what ecologists refer to as the phenology of northern forests: the timing of key events in their life cycle. As deciduous trees grow new leaves in spring, they increase their capacity to store carbon dioxide and steadily remove it from the atmosphere. Then, as these leaves fall to the ground and decompose prior to the onset of winter, carbon dioxide is re-released, causing concentrations to spike. And so while we’ve exceeded concentrations of 400 parts per million in the recent past, levels have always dropped back down to a more comforting 350-399 ppm average during the September, the cycle’s annual nadir. No longer: for the first time in recorded history, CO2 concentrations remained above 400 ppm for the entire month—meaning they’re unlikely to ever fall below this symbolic milestone again. Climate Central has more.

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