Main Greenhouse
Gases Reach Highest Level Ever Since Pre-Industrial Time

Levels of most greenhouse gases continued to
increase in 2008,  says the World Meteorological
Organization’s (WMO) in its 2008 Greenhouse Gas Bulletin published
today. This confirms the continued trend of rising atmospheric
burdens of greenhouse gases since 1750, according to he WMO.

Global concentrations of carbon
dioxide, methane and nitrous oxide, which are the main long-lived
greenhouse gases in the atmosphere, have reached the highest levels
recorded since pre-industrial times. Since 1990, the overall
increase in radiative forcing caused by all long-lived greenhouse
gases is 26% and the increase was 1.3% from 2007 to 2008.

Greenhouse gases trap
radiation within the Earth’s atmosphere causing it to warm. Human
activities, such as fossil fuel burning and agriculture, are major
emitters of greenhouse gases which scientists widely recognize as
drivers of global warming and climate change.  After water
vapour, the four most prevalent long-lived greenhouse gases in the
atmosphere that are directly influenced by humans are carbon
dioxide, methane, nitrous oxide and halocarbons. WMO, through its
Global Atmosphere Watch (GAW) Programme, coordinates the
observations of these gases in the atmosphere through a network of
stations located in more than 50 countries.

The globally averaged mixing ratio
of carbon dioxide (CO2) in 2008 was 385.2 ppm (number of molecules
of the gas per million molecules of dry air), with an increase of
2.0 ppm from the previous year, continuing the tendency of
exponential increase. CO2 is the most important human-emitted
greenhouse gas in the atmosphere, contributing 63.5% to the
increase in overall radiative forcing since 1750.

Its atmospheric abundance was
nearly constant at about 280 ppm before industrialization. During
the time period 1979-1984 CO2 contributed 56% of the increase in
radiative forcing caused by long-lived greenhouse gases. Since then
CO2 has gained importance and during the five-year period from 2003
to 2008 CO2 was responsible for 86% of the increase in radiative
forcing, which is more than four times superior to all other
long-lived greenhouse gases combined. Since 1750, atmospheric CO2
has increased by 38%, primarily because of emissions from
combustion of fossil fuels, deforestation and land use change.

The globally averaged mixing ratio
of methane (CH4) in 2008 was 1797 ppb, which means an increase of
7ppb from the previous year. While the concentration of CH4 was
stable for seven years (from 1999 to 2006), both 2007 and 2008 show
a significant increase. Methane contributes 18.2% to the increase
in overall global radiative forcing since 1750. 60% of CH4
emissions come from anthropogenic sources such as ruminants, rice
agriculture, fossil fuel exploitation, landfills and biomass
burning. Before the industrial era, atmospheric methane was about
700ppb. Increasing emissions from anthropogenic sources are
responsible for the 157% increase in the CH4 concentration since
1750.

The globally averaged mixing ratio
of nitrous oxide (N2O) in 2008 was 321.8 ppb, 0.9 ppb higher than
in 2007, and 19% above the pre-industrial level. N2O contributes
6.2% to the increase in the overall global radiative forcing since
1750. The atmospheric abundance of N2O prior to industrialization
was 270 ppb. N2O is emitted into the atmosphere from natural and
anthropogenic sources, including oceans, soil, biomass burning,
fertiliser use and various industrial processes.

The combined radiative forcing by
halocarbons is nearly double that of N2O. Some halocarbons such as
chlorofluorocarbons (CFCs), previously used as refrigerants, as
propellants in spray cans and as solvents, are decreasing slowly as
a result of the phase-out of these compounds through the Montreal
Protocol on Substances that Deplete the Ozone Layer. However,
concentrations of other gases such as HCFCs and HFCs, which are
used to substitute chlorofluorocarbons, are increasing rapidly.
These two classes of compounds are very potent greenhouse gases and
together with sulphur hexafluoride (SF6) they contributed 8.9% to
the increase in radiative forcing from 2003 to 2008, which is more
than the contribution from N2O during this period.

This year’s Greenhouse Gas Bulletin
is the fifth in the series, reporting data since 2004. The
Bulletins provide critical information on the global state of the
atmosphere in a concise manner and highlight recent accomplishments
of research and technology application. The 2008 Bulletin precedes
the 15th session of the United Nations Framework Convention on
Climate Change (Copenhagen, 7-18 December 2009).

WMO prepares and distributes the
annual Greenhouse Gas Bulletins in cooperation with the GAW
Scientific Advisory Group for Greenhouse Gases. The measurement
data are archived and distributed by WMO’s World Data Centre for
Greenhouse Gases (WDCGG), hosted by the Japan Meteorological Agency
(JMA).