Scientists recorded the largest single-year increase in carbon dioxide in 20 years in 2013, and total concentrations of the greenhouse gas are now set to cross the 400 parts-per-million mark as early as next year, the United Nations World Meteorological Organization said in a Sept. 9 report.
Globally, carbon dioxide levels hit 396 parts per million in 2013, an increase of 2.9 ppm, the largest year-to-year gain since 1984, according to the WMO's annual Greenhouse Gas Bulletin.
The UN's Intergovernmental Panel on Climate Change has warned that levels of the heat-trapping gas must be kept below 450 ppm to avoid the most catastrophic impacts of climate change related to rising global temperatures.
Daily concentrations of carbon dioxide already have crested the 400 ppm at observational sites in the Arctic in spring 2012 and at the Mauna Loa Observatory in Hawaii in 2013.
At the current rate of increase, the annual average concentration of carbon dioxide in the atmosphere “is set to cross the symbolic 400 parts per million threshold in 2015 or 2016,” according to the WMO bulletin.
The 400 ppm threshold has been termed a “tipping point” by scientists—including James Hansen, the former director of the NASA Goddard Institute for Space Studies—at which changes to the climate are believed to be irreversible.
According to NASA Goddard Institute's records, in 1880, the carbon dioxide level in the atmosphere was about 285 parts per million; by 1960, it had risen to about 315 parts per million.
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