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Observed Concentrations of CO2 Cross 400 parts per million Threshold at Several Global Atmosphere Watch Stations
14/ 05/ 2013

Observed Concentrations of CO2 Cross 400 parts per million Threshold at Several Global Atmosphere Watch StationsObserved concentrations of carbon dioxide (CO2) in the atmosphere have exceeded the symbolic 400 parts per million (ppm) threshold at several stations of the World Meteorological Organization's Global Atmosphere Watch network. This is a wakeup call about the constantly rising levels of this greenhouse gas, which is released into the atmosphere by fossil fuel burning and other human activities and is the main driver of climate change. Carbon dioxide remains in the atmosphere for thousands of years, trapping heat and causing our planet to warm further, impacting on all aspects of life on earth.

On May 9, 2013, the daily mean concentration of carbon dioxide in the atmosphere of Mauna Loa, Hawaii, recorded a reading of 400.03 ppm, according to the U.S. National Oceanic and Atmospheric Administration. Mauna Loa is the oldest continuous atmospheric measurement station in the world and so is widely regarded as a benchmark site in the Global Atmosphere Watch.

Several other Global Atmosphere Watch stations have also reported CO2 concentrations exceeding the 400 ppm threshold during the seasonal maximum. This occurs early in the northern hemisphere spring before vegetation growth absorbs CO 2.

The threshold was first crossed at stations in the Arctic. A monthly average value exceeding 400 ppm was registered at Barrow, Alaska, USA (71.3N) for the first time in April 2012, as well as at Alert, in Canada (82.5N). From the beginning of 2013, measured CO 2. values at another GAW Global station, in Ny-Ålesund, Norway, (at 78.9N) also exceeded 400 ppm. This threshold has now also been crossed at stations closer to the Equator. Izaña, (Canary Islands, Spain), reported daily mean values exceeding 400 ppm at the end of April 2013. This was followed by Mauna Loa, which has been carrying out measurements since 1958.

The Global Atmosphere Watch coordinates observations of CO2 and other heat-trapping gases like methane and nitrous oxide in the atmosphere to ensure that measurements around the world are standardized and can be compared to each other. The network spans more than 50 countries including stations high in the Alps, Andes and Himalayas, as well as in the Arctic, Antarctic and in the far South Pacific.

Carbon dioxide is the single most important greenhouse gas emitted by human activities. It is responsible for 85% of the increase in radiative forcing ? the warming effect on our climate - over the past decade. Between 1990 and 2011 there was a 30% increase in radiative forcing because of greenhouse gases. Radiative forcing is calculated relative to the pre-industrial level of key greenhouse gases.

According to WMO's Greenhouse Gas Bulletin, the amount of CO2 in the atmosphere reached 390.9 parts per million in 2011, or 140% of the pre-industrial level of 280 parts per million. The pre-industrial era level represented a balance of CO2 fluxes between the atmosphere, the oceans and the biosphere. The amount of CO2 in the atmosphere has increased on average by 2 parts per million per year for the past 10 years.

At the current rate of increase, the global annual average CO2 concentration is set to cross the 400 ppm threshold in 2015 or 2016.

Further Resources
UNFCCC Executive Secretary on crossing of 400 pm CO2 threshold
Climate and Clean Air Coalition - UNEP
UNEP Climate Change