GCOS Essential Climate Variables (ECV)
Atmospheric Composition (over Land, Sea and Ice)
Methane (CH₄)

Definition: Methane is a colorless, inflammable gas of formula CH₄; the simplest hydrocarbon. Methane enters the atmosphere as a result of the anaerobic decay of organic matter in, for example, swamps and rice paddies, and is also produced in large quantities by cattle and termites. It is formed along with coal and oil in fossil fuel deposits, and released to the atmosphere on mining. Methane is itself burned as a fuel, being the major constituent of natural gas. The atmospheric mixing ratio of methane is currently about 1.7 parts per million and has been rising gradually since the industrial era began. The atmospheric lifetime of methane is about eight years. As well as influencing the chemistry of the atmosphere, methane is a strong greenhouse gas and an important source of stratospheric water vapor, and it contributes to global warming. (AMS Glossary of Meteorology)

Introduction: Trace gases other than ozone may be divided into three categories: — greenhouse gases affecting climate change; — chemically aggressive gases affecting the environment (including the biosphere); — gases and radicals impacting on the ozone cycle, thereby affecting both climate and environment. The presence of trace gases in the atmosphere can have a significant effect on global change as well as potentially harmful local effects through increased levels of pollution. The chemical composition of the troposphere, in particular, is changing at an unprecedented rate. Meanwhile, the rate at which pollutants from human activities are being emitted into the troposphere is now thought to exceed that from natural sources (such as volcanic eruptions). As explained in Part I of this document, the IPCC noted in 2007 that: — changes in atmospheric concentrations of greenhouse gases and aerosols, land cover and solar radiation alter the energy balance of the climate system; — global greenhouse gas emissions due to human activities have grown since pre-industrial times, with an increase of 70% between 1970 and 2004; — carbon dioxide (CO2) is the most important anthropogenic greenhouse gas. Its annual emissions grew by about 80% between 1970 and 2004. The IPCC concluded that “most of the observed increase in globally averaged temperatures since the mid-20th century is very likely (over 90% probability) due to the observed increase in anthropogenic (man-made) greenhouse gas concentrations”. They consider that reductions in greenhouse gas emissions and the gases that control their concentration would be necessary to stabilise radiative forcing.

Satellite Observations: Measurements from satellite sensors have already made an important contribution to the recognition that human activities are modifying the chemical composition of both the stratosphere and the troposphere, even in remote regions. A variety of instruments provide measurements on the concentration of trace gases. In general, high spectral resolution is required to detect absorption, emission and scattering from individual species. Some instruments offer measurements of column totals, i.e. integrated column measurements, whilst others provide profiles of gas concentration through the atmosphere (usually limited to the upper troposphere and stratosphere, using limb measurements). To date, the instruments on UARS (operated 1991–2005) have provided the most significant source of data on trace gases and have been vital for studies of stratospheric chlorine chemistry, stratospheric tracer-tracer correlation, tropospheric water vapour, the chemistry of the Arctic lower stratosphere in winter, and tropospheric aircraft exhaust studies The last few years have seen the arrival of new and significant capabilities, with advanced instruments on Terra (MOPITT, providing global measurements of carbon monoxide and methane in the troposphere), and Envisat (GOMOS, MIPAS and SCIAMACHY, providing profiles of trace gases through the stratosphere and troposphere). AIRS (on Aqua) and IASI (on MetOp) also contribute to such information and their sounder products can help quantify atmospheric mixing and help determine sources and sinks. On NASA’s Aura mission, HiRDLS, an infrared limb-scanning radiometer, carries out soundings of the upper troposphere, stratosphere and mesosphere to determine concentrations of trace gases, with horizontal and vertical resolutions superior to those previously obtained. On the same mission, MLS measures concentrations of trace gases for their effects on ozone depletion, TES provides a primary input to a database of 3D distribution on global, regional and local scales of gases important to tropospheric chemistry, and OMI continues the TOMS record for atmospheric parameters related to ozone chemistry and climate. JAXA’s GOSAT mission (from 2008) and NASA’s OCO mission (also from 2008) are expected to make significant contributions to observations of trace gases, particularly carbon dioxide and methane. The IGOS IGACO Theme for observations of atmospheric chemistry has considered all relevant chemical species to interpret properly the observations and intends to monitor the research required to improve understanding of Earth processes so that air quality evolution can be predicted. ESA is considering atmospheric composition missions (such as TRAQ, PREMIER and A-SCOPE) to meet these needs. The CEOS Response to the GCOS IP cautions that demonstrations of potential future operational measurements are neither complemented by plans for operational implementation nor any R&D follow-on. CEOS agencies will participate in planning, by 2011, the current chemistry missions and those planned for the next 5 to 7 years. (Satellite Missions) (from the CEOS web site)

Data, Product, Metadata and Information Access

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