Abstract
Abstract. Carbon dioxide (CO2) and methane (CH4) are the two most important anthropogenic greenhouse gases contributing to global climate change. SCIAMACHY onboard ENVISAT (launch 2002) was the first and is now with TANSO onboard GOSAT (launch 2009) one of only two satellite instruments currently in space whose measurements are sensitive to CO2 and CH4 concentration changes in the lowest atmospheric layers where the variability due to sources and sinks is largest. We present long-term SCIAMACHY retrievals (2003–2009) of column-averaged dry air mole fractions of both gases (denoted XCO2 and XCH4) derived from absorption bands in the near-infrared/shortwave-infrared (NIR/SWIR) spectral region focusing on large-scale features. The results are obtained using an upgraded version (v2) of the retrieval algorithm WFM-DOAS including several improvements, while simultaneously maintaining its high processing speed. The retrieved mole fractions are compared to global model simulations (CarbonTracker XCO2 and TM5 XCH4) being optimised by assimilating highly accurate surface measurements from the NOAA/ESRL network and taking the SCIAMACHY averaging kernels into account. The comparisons address seasonal variations and long-term characteristics. The steady increase of atmospheric carbon dioxide primarily caused by the burning of fossil fuels can be clearly observed with SCIAMACHY globally. The retrieved global annual mean XCO2 increase agrees with CarbonTracker within the error bars (1.80±0.13 ppm yr−1 compared to 1.81±0.09 ppm yr−1). The amplitude of the XCO2 seasonal cycle as retrieved by SCIAMACHY, which is 4.3±0.2 ppm for the Northern Hemisphere and 1.4±0.2 ppm for the Southern Hemisphere, is on average about 1 ppm larger than for CarbonTracker. An investigation of the boreal forest carbon uptake during the growing season via the analysis of longitudinal gradients shows good agreement between SCIAMACHY and CarbonTracker concerning the overall magnitude of the gradients and their annual variations. The analysis includes a discussion of the relative uptake strengths of the Russian and North American boreal forest regions. The retrieved XCH4 results show that after years of stability, atmospheric methane has started to rise again in recent years which is consistent with surface measurements. The largest increase is observed for the tropics and northern mid- and high-latitudes amounting to about 7.5±1.5 ppb yr−1 since 2007. Due care has been exercised to minimise the influence of detector degradation on the quantitative estimate of this anomaly.
Highlights
The atmospheric concentrations of the two most important anthropogenic greenhouse gases carbon dioxide (CO2) and methane (CH4) have increased significantly since the start of the Industrial Revolution and are about 40% and 150%, respectively, higher compared to the pre-industrial levels (Solomon et al, 2007)
The retrieved global annual mean XCO2 increase agrees with CarbonTracker within the error bars (1.80±0.13 ppm yr−1 compared to 1.81±0.09 ppm yr−1)
The analysis of the seasonal variations shows that the mean amplitudes of the seasonal cycles of the SCIAMACHY data and the TM5 model typically differ by less than 4 ppb for the analysed latitude bands. This manuscript presented and discussed a long-term (2003– 2009) global data set of atmospheric carbon dioxide and methane column-averaged dry air mole fractions retrieved from the spectral near-infrared/shortwave-infrared nadir observations of the SCIAMACHY instrument onboard the European environmental satellite ENVISAT using the improved version 2.0/2.1 of the scientific retrieval algorithm WFMDOAS
Summary
The atmospheric concentrations of the two most important anthropogenic greenhouse gases carbon dioxide (CO2) and methane (CH4) have increased significantly since the start of the Industrial Revolution and are about 40% and 150%, respectively, higher compared to the pre-industrial levels (Solomon et al, 2007). While carbon dioxide concentrations have risen steadily during the last decades, atmospheric methane levels were rather stable from 1999 to 2006 (Bousquet et al, 2006) before a renewed growth was observed from surface measurements since 2007 (Rigby et al, 2008; Dlugokencky et al, 2009). The analysis constitutes seven years (2003–2009) of greenhouse gas information derived from European Earth observation data improving and extending pre-existing WFM-DOAS retrievals (Schneising et al, 2008, 2009). Another SCIAMACHY multi-year methane data set has been generated independently (Frankenberg et al, 2011).
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