(Reuters) - The amount of climate-warming carbon dioxide in the atmosphere topped 400 parts per million at a key observing station in Hawaii for the first time since measurement began in 1958, the National Oceanic and Atmospheric Administration said on Friday.
To many scientists, crossing the 400 ppm threshold, which means that there are 400 molecules of carbon dioxide for every million molecules in the air, is a bit like the Dow Jones Industrial Average rising above 15,000 points. "It's important mainly as a milestone that marks a steady progress of increasing carbon dioxide in the atmosphere," said James Butler of NOAA's Earth System Research Laboratory.
The threshold has become an important marker in U.N. climate change negotiations, tagged as a dangerous level by most climate scientists. For many years scientists have said that concentrations need to be kept below, or pushed back to, 350 ppm for countries to meet an international target of keeping the average temperature increase below 3.6 degrees Fahrenheit (2 degrees Celsius) this century.
Dozens of observing stations around the world monitor atmospheric carbon dioxide. But Mauna Loa, a volcanic mountain on the Big Island of Hawaii, is regarded as the benchmark site, NOAA said. Two instruments at Mauna Loa showed carbon dioxide at 400.03 ppm on Thursday. Certain arctic observing stations exceeded 400 ppm more than a year ago, and the global average of atmospheric carbon dioxide could break the 400 ppm barrier in the next year or so, Butler said by telephone from Boulder, Colorado.
Whether or not that occurs, Earth's atmosphere hasn't had this much carbon dioxide in it for at least 800,000 years, and possibly for as long as 5 million years.
