|Changes in atmospheric radiative forcing by long-lived greenhouse gases and the 2007 update of the NOAA Annual Greenhouse Gas Index (AGGI). 1990 is the reference year. Click to enlarge.|
The globally averaged mixing ratios of carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O) reached new highs in 2007 with CO2 at 383.1 ppm (up 0.5% from 2006); CH4 at 1,789 ppb (up 0.34% from 2006); and N2O at 320.9 ppb (up 0.25% from 2006). These values are higher than those in pre-industrial times (before 1750) by 37%, 156% and 19%, respectively.
Using the NOAA Annual Greenhouse Gas Index (AGGI), the total warming effect of all long-lived greenhouse gases was calculated to have increased by 1.06% from the previous year and by 24.2% since 1990. However, levels of chlorofluorocarbons (CFCs) continue to slowly decrease, a result of emission reductions under the Montreal Protocol on Substances that Deplete the Ozone Layer.
The Bulletin notes that CO2 is the most important infrared radiation absorbing, anthropogenic gas in the atmosphere and is responsible for 63% of the total radiative forcing of Earth by long-lived greenhouse gases. Its contribution to the increase in radiative forcing is 87% for the past decade and 90% for the last five years.
Measurements beginning in 1958 show that the average increase of CO2 in the atmosphere corresponds to approximately 55% of the CO2 emitted by fossil fuel combustion. The remaining fossil fuel-CO2 has been removed from the atmosphere by the oceans and the terrestrial biosphere.
Globally averaged CO2 in 2007 was 383.1 ppm and the increase from 2006 to 2007 was 1.9 ppm. This growth rate is larger than the observed average for the 1990s (~1.5 ppm/yr), mainly because of increasing emissions of CO2 from fossil fuel combustion.—2007 Greenhouse Gas Bulletin
The growth rate for N2O during 2007 was 0.8 ppb from the year before. The mean growth rate has been 0.77 ppb per year over the past 10 years.
While the atmospheric concentrations of CO2 and N2O are increasing steadily, the growth rate of methane concentrations has slowed over the past decade with some variations from one year to the next. The 6 ppb rise from 2006 to 2007 is the highest annual methane increase observed since 1998.
It is still too early to state with certainty that this latest increase is the start of a new upward trend in methane levels, according to the WMO. Human activities, such as fossil fuel exploitation, rice agriculture, biomass burning, landfills and ruminant farm animals, account for some 60% of atmospheric methane, with natural sources, such as wetlands and termites, responsible for the remaining 40%.
After water vapour, the four most prevalent greenhouse gases in the atmosphere are carbon dioxide, methane, nitrous oxide and chlorofluorocarbons. The WMO Global Atmosphere Watch (GAW) coordinates the measurement of these gases in the atmosphere through a network of observatories located in more than 65 countries.
Since the mid-18th Century, carbon dioxide concentrations in the atmosphere have risen 37%. Population growth and urban development worldwide continue to increase the use of fossil fuels, such as oil, coal and natural gas. At the same time, the clearing of land for agriculture, including deforestation, is releasing carbon dioxide into the air and reducing carbon uptake by the biosphere.
This year’s Greenhouse Gas Bulletin is the fourth in the series, the three previous ones providing results for 2004, 2005 and 2006 respectively. WMO prepares and distributes the annual Greenhouse Gas Bulletins in cooperation with the GAW Scientific Advisory Group for Greenhouse Gases, with the assistance of the NOAA Earth System Research Laboratory and WMO’s World Data Centre for Greenhouse Gases (WDCGG). The measurement data are archived and distributed by the WDCGG, hosted by the Japan Meteorological Agency (JMA).