Global column-averaged methane mixing ratios from 2003 to 2009 as derived from SCIAMACHY: Trends and variability

Abstract

After a decade of stable or slightly decreasing global methane concentrations,\nground-based in situ data show that CH4 began increasing again in 2007 and that\nthis increase continued through 2009. So far, space-based retrievals sensitive to the\nlower troposphere in the time period under consideration have not been available.\nHere we report a long-term data set of column-averaged methane mixing ratios retrieved\nfrom spectra of the Scanning Imaging Absorption Spectrometer for Atmospheric\nCartography (SCIAMACHY) instrument onboard Envisat. The retrieval quality after\n2005 was severely affected by degrading detector pixels within the methane 2v3 absorption\nband. We identified the most crucial problems in SCIAMACHY detector degradation\nand overcame the problem by applying a strict pixel mask as well as a new dark current\ncharacterization. Even though retrieval precision after the end of 2005 is invariably\ndegraded, consistent methane retrievals from 2003 through 2009 are now possible.\nRegional time series in the Sahara, Australia, tropical Africa, South America, and Asia\nshow the methane increase in 2007¿2009, but we cannot yet draw a firm conclusion\nconcerning the origin of the increase. Tropical Africa even seems to exhibit a negative\nanomaly in 2006, but an impact from changes in SCIAMACHY detector degradation\ncannot be excluded yet. Over Assakrem, Algeria, we observed strong similarities between\nSCIAMACHY measurements and ground-based data in deseasonalized time series.\nWe further show long-term SCIAMACHY xCH4 averages at high spatial resolution\nthat provide further insight into methane variations on regional scales. The Red Basin in\nChina exhibits, on average, the highest methane abundance worldwide, while other\nlocalized features such as the Sudd wetlands in southern Sudan can also be identified\nin SCIAMACHY xCH4 averages.