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Scientists are warning the planet has now reached a grim climate milestone not seen for two or three million years. The National Oceanic and Atmospheric Administration says the amount of heat-trapping carbon dioxide in the atmosphere has topped 400 parts per million. The 400 ppm threshold has been an important marker in U.N. climate change negotiations, widely recognized as a dangerous level that could drastically worsen human-caused global warming.

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On May 9, the daily mean concentration of carbon dioxide in the atmosphere of Mauna Loa, Hawaii, surpassed 400 parts per million (ppm) for the first time since measurements began in 1958. Independent measurements made by both NOAA and the Scripps Institution of Oceanography have been approaching this level during the past week. It marks an important milestone because Mauna Loa, as the oldest continuous carbon dioxide (CO2) measurement station in the world, is the primary global benchmark site for monitoring the increase of this potent heat-trapping gas.

Its concentration has increased every year since scientists started making measurements on the slopes of the Mauna Loa volcano more than five decades ago. The rate of increase has accelerated since the measurements started, from about 0.7 ppm per year in the late 1950s to 2.1 ppm per year during the last 10 years.

“That increase is not a surprise to scientists,” said NOAA senior scientist Pieter Tans, with the Global Monitoring Division of NOAA’s Earth System Research Laboratory in Boulder, Colo. “The evidence is conclusive that the strong growth of global CO2 emissions from the burning of coal, oil, and natural gas is driving the acceleration.”

Before the Industrial Revolution in the 19th century, global average CO2 was about 280 ppm. During the last 800,000 years, CO2 fluctuated between about 180 ppm during ice ages and 280 ppm during interglacial warm periods. Today’s rate of increase is more than 100 times faster than the increase that occurred when the last ice age ended. http://researchmatters.noaa.gov/news/Pages/CarbonDioxideatMaunaLoareac
hes400ppm.aspx

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Just two centuries ago, that number was only 280 parts per million. So if we continue to add carbon to the atmosphere at current rates, we’ll reach a doubling of the pre-industrial levels of CO2 within the next few decades.

If we continue to burn fossil fuels at accelerating rates, if we continue with business as usual, we will cross the 450 parts per million limit in a matter of maybe a couple decades. We believe that with that amount of CO2 in the atmosphere, we commit to what can truly be described as dangerous and irreversible changes in our climate.

We saw Arctic sea ice diminish to the lowest level we’ve ever seen, and it’s on a trajectory where there will be no ice in the Arctic at the end of the summer in perhaps a matter of 10 years or so. We will see more destructive tropical storms. We’ll see more flooding. We’ll see more drought. And that’s just the tip of the iceberg, because, remember, we’ve only just crossed 400 now. We will reach 450 ppm in a matter of a couple decades if we continue with business as usual.

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