The carbon dioxide concentration in the atmosphere increased in 2013 at the fastest rate in almost 30 years, spurring concerns about global warming ahead of a United Nations climate summit later this month.

According to the annual Greenhouse Gas Bulletin of the World Meteorological Organization (WMO), released on 9 September, the average amount of CO 2 in the atmosphere reached 396.0 parts per million in 2013 — 2.9 parts per million above the 2012 level. This is the largest annual increase since 1984. Averaged over an entire year, the global annual CO 2 concentration is now expected to pass the symbolic threshold of 400 parts per million in 2015 or 2016.

The report is based on observations from the WMO’s Global Atmosphere Watch network. Preliminary data suggest that reduced CO 2 uptake by plants and soils might add to the worrying increase in atmospheric CO 2 concentrations resulting from fossil fuel burning.

Atmospheric methane, the second most important long-lived greenhouse gas, also reached a new high of about 1,824 parts per billion last year, mostly due to increased emissions from cattle breeding, rice farming, fossil fuel mining, landfills and biomass burning.

Between 1990 and 2013, radiative forcing by long-lived greenhouse gases — the main cause of global warming — has increased by 34%, according to the WMO report.

The question remains, however, of why the rise in global mean temperatures near the surface has apparently slowed, after a series of exceptionally warm years in the 1990s. Scientists have suggested a number of possible explanations for the global warming pause. According to the latest hypothesis, regularly occurring changes in circulation patterns in the Atlantic and Southern Ocean may have caused an increased volume of relatively warm water to sink to the depth of the ocean, thus reducing the amount of ocean heat escaping to the atmosphere.

The world’s oceans take up one-fourth or so of CO 2 emitted into the atmosphere. As emissions have been steadily rising for decades, the corresponding changes in ocean chemistry are dramatic: the current rate of ocean acidification seems to have been unprecedented in at least over the last 300 million years, according to an analysis included in the WMO report.

World leaders are set to discuss steps to reduce greenhouse gas emission at the United Nations climate summit on 23 September in New York.