By Environment Correspondent Deborah Zabarenko
WASHINGTON (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.
Carbon dioxide, a heat-trapping greenhouse gas, remains in the atmosphere for hundreds of years. It is emitted by fossil-fueled vehicles and coal-fired factories and power plants as well as by natural activities such as breathing.
Carbon dioxide concentrations at Mauna Loa are documented in a graph known as the Keeling Curve, named for Charles Keeling, who began measurements there in 1958, when the level was 317 ppm. Information on the Keeling Curve is available at http://keelingcurve.ucsd.edu.
During the last 800,000 years, the level of atmospheric carbon dioxide fluctuated between 180 ppm and 280 ppm. With the widespread burning of coal and oil during the Industrial Revolution, the concentration of carbon dioxide rose to about 290 ppm by the end of the 19th century, Butler said.
In the 20th century, the rate of increase accelerated, with levels between 370 and 380 ppm by the year 2000. An animated graph that shows the history of atmospheric carbon dioxide is online at http://www.esrl.noaa.gov/gmd/ccgg/trends/history.html .
(Reporting by Deborah Zabarenko; Editing by Ros Krasny and Xavier Briand)