GCOS Implementation Plan - WMO

Implementation Plan for the Global Observing System for Climate in Support of the UNFCCC
(2010 Update)
Understanding the sources and sinks for CO 2 and CH 4 is crucial. One of the challenges is to
distinguish between natural and anthropogenic sources, for which accurate global measurements are
required. While the atmospheric burden of CO 2 is increasing quite steadily by about 0.5% per year,
the rise in methane concentration levelled off during the last decade. However, from 2006 to 2007
methane increased by 6 ppb in the atmosphere, the largest rise since 1998, and there was a further
increase from 2007 to 2008, although it is still too early to state with certainty that these increases
represent the beginning of a new upward trend of methane. 72 There are large uncertainties in the
budget of methane, and observations combined with modelling are needed for better understanding of
the sources and sinks.
N 2 O is a greenhouse gas which mainly originates from agriculture, but is also produced by natural
sources, i.e., soils and ocean. These sources are diffusive and therefore it is not easy to obtain from
inverse modelling. There is also a large variability in the stratosphere.
Halocarbons are currently a minor contribution to GHGs but they are potent GHGs and represent a
potential long-term threat. Some of them (chlorofluorocarbons (CFCs) and hydrochlorofluorocarbons
(HCFCs)) are regulated by the Montreal Protocol, since they are also ozone depleting gases, but they
might not be phased out before 2040 and may show increasing concentration before 2040. Others do
not deplete ozone and are therefore not governed by the Montreal Protocol. Concentrations of some
of them are increasing rapidly.
Projections of a changing climate have added a new dimension to the issue of the stratospheric
ozone layer and its recovery. New data and models show the interconnections between these two
global environmental concerns, with varied impacts of stratospheric temperature change on ozone
distributions. Ozone-depleting chemicals and ozone itself provide positive forcing of climate. The
reduction of ozone-depleting substances not only helped the ozone layer but also lessened climate
forcing. Because of the close interaction between climate and stratospheric processes there is a
continuing need to monitor atmospheric composition throughout the troposphere and stratosphere.
Being able to distinguish changes arising from a decrease in ozone-depleting substances from those
due to other sources of climate forcing is essential for establishing climate policy.
Changes in tropospheric composition have an impact on air quality as well as climate change. Several
tropospheric trace gases and aerosols play key roles in both domains. Tropospheric ozone and
aerosols are both radiatively active and air pollutants. Other trace gases, such as NO 2 , SO 2 , CO, and
HCHO, are not directly active radiatively but are precursors for tropospheric ozone and secondary
aerosols (i.e., aerosols that are formed in the atmosphere). Methane is a precursor for tropospheric
ozone and stratospheric water vapour, as well as a GHG. Observations of precursors are needed for
an emission-based view on the radiative forcing (due to both anthropogenic and natural sources) by
tropospheric ozone and secondary aerosols.
Estimates of the emissions of the precursors are still very uncertain and are mostly based on
sometimes quite dated inventories compiled using socio-economic statistics. More accurate and more
up-to-date knowledge of the emission sources is urgently needed as input to climate and air quality
models, which are used both for climate monitoring via data assimilation and for climate prediction.
High spatial and temporal resolution is needed for accurate emission estimates, especially for NO 2
and SO 2 .
Recent studies, including some providing input to the IPCC process, have considered the coupling of
air quality and climate change, not only by the direct link via tropospheric composition but also
because climate change will lead to different air pollution levels, which in turn will lead to different
regional forcings (e.g., by recurrent dust or biomass burning events) and impact on global change.
Future implementations of air quality legislation will reduce emission sources of pollutants that are
either directly radiatively active (e.g., aerosols) or precursors to tropospheric ozone and secondary
aerosols.
Atmospheric aerosols are minor constituents of the atmosphere by mass, but a critical component in
terms of impacts on climate and especially climate change. Aerosols influence the global radiation
balance directly by scattering and absorbing radiation and indirectly through influencing cloud
72 WMO (2009): The State of Greenhouse Gases in the Atmosphere Using Global Observations through 2008. WMO
Greenhouse Gas Bulletin No. 5 (25 November 2009). http://www.wmo.int/pages/prog/arep/gaw/ghg/GHGbulletin.html
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