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MaunaLoaCO2
Via Climate Progress: NOAA reported Friday that the daily mean concentration of CO2 in the air around Mauna Loa, Hawaii, surpassed 400 parts per million this week
The difference between 399 parts per million of carbon dioxide and 400 ppm isn't much, but new data from NOAA shows that that threshold has now been crossed. The last time earth saw levels this high it was a very different place:
This period of Arctic warmth “coincides, in part with a long interval of 1.2 million years when the West Antarctic Ice sheet did not exist.” Indeed, sea levels during the mid-Pliocine were about 25 m [82 feet] higher than today!

It is worth noting that a 2009 analysis in Science found that when CO2 levels were this high 15 to 20 million years ago, it was 5° to 10°F warmer globally and seas were also 75 to 120 feet higher.

Paleo-climate Professor Michael Mann, author of The Hockey Stick and the Climate Wars (and today, a debut DKos diarist), emailed noting that "The last time we're confident CO2 levels breached 400ppm, was the mid Miocene, around 10 million years ago, though it could have been a bit later, e.g. 4 million years. In either case, we know that the globe was quite a bit warmer than today, with much less ice around, and much higher sea level." When asked, Mann added he could think of no significant factor present now and not present then that might dampen warming.

The new data and recent work solidly linking past CO2 levels to much warmer temperatures lends further credence to the views expressed by climate scientists like NASA's James Hansen. That even maintaining CO2 at current levels, let alone raising them further through ongoing carbon emissions, could lead to dangerous, irreversible changes in the climate.

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