In the centuries to come, history books will likely look back on September 2016 as a major milestone for the world's climate. At a time when atmospheric carbon dioxide is usually at its minimum, the monthly value failed to drop below 400 parts per million.
That all but ensures that 2016 will be the year that carbon dioxide officially passed the symbolic 400 ppm mark, never to return below it in our lifetimes, according to scientists.
Because carbon pollution has been increasing since the start of the Industrial Revolution and has shown no signs of abating, it was more a question of "when" rather than "if" we would cross this threshold. The inevitability doesn't make it any less significant, though.
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September is usually the month when carbon dioxide is at its lowest after a summer of plants growing and sucking it up in the northern hemisphere. As fall wears on, those plants lose their leaves, which in turn decompose, releasing the stored carbon dioxide back into the atmosphere. At Mauna Loa Observatory, the world's marquee site for monitoring carbon dioxide, there are signs that the process has begun but levels have remained above 400 ppm.
Since the industrial revolution, humans have been altering this process by adding more carbon dioxide to the atmosphere than plants can take up. That's driven carbon dioxide levels higher and with it, global temperatures, along with a host of other climate change impacts.
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