Carbon dioxide and methane are the two most important anthropogenic greenhouse gases and contribute to global climate change. The atmospheric concentrations of both gases have increased significantly since the start of the industrial revolution. While carbon dioxide concentrations have risen steadily during the last decades, atmospheric methane levels were rather stable from 1999 to 2006 before a renewed growth was observed from surface measurements since 2007. Further increase of both gases is expected to result in a warmer climate with adverse consequences. However, current knowledge about their variable natural and anthropogenic regional sources and sinks has significant gaps, and this limits the reliability of climate predictions. Fortunately, missing information can be obtained by satellite measurements because of their global coverage. This however requires that the retrievals are accurate and precise enough, that the satellite data have high sensitivity to greenhouse gas variations close to the Earth's surface where the source/sink signals are largest, and that a sufficiently long time series exists.
The main objective of CARBONGASES is to make a contribution to fill the signiï¬cant gaps in the understanding of the global carbon cycle by improving the knowledge about the regional sources and sinks of greenhouse gases. This will be performed by generating the required global multi-year satellite data sets, namely column-averaged dry air mole fractions of carbon dioxide and methane retrieved from SCIAMACHY onboard Envisat. SCIAMACHY was the first of only two satellite instruments currently in space (since 2009 accompanied by TANSO onboard GOSAT) covering important absorption bands of both gases in the near-infrared/shortwave-infrared spectral region. Sensitivity to all altitude levels, including the boundary layer, is achieved by using reflected solar radiation in this spectral range. Therefore, SCIAMACHY plays a pioneering role in the relatively new area of greenhouse gas observations from space. CARBONGASES encompasses seven years (2003-2009) of greenhouse gas information derived from European Earth observation data improving and extending pre-existing retrievals.
The CARBONGASES data sets permit comparisons with the corresponding greenhouse gas data products of the non-European satellite GOSAT. To achieve consistency between SCIAMACHY and GOSAT is an important project objective. CARBONGASES also aims at further improving the accuracy of the SCIAMACHY data products to maximise the quantitative information on greenhouse gases regional surface fluxes that can be inferred from this sensor. This will be done using inverse modelling based on fitting the model emissions to the satellite observations. By making the extended long-term global SCIAMACHY data sets of the relevant greenhouse gases available to the inverse modelling community, CARBONGASES can contribute to a better understanding and quantification of the natural variability and the human-induced changes in the Earth's climate system, most notably the land-atmosphere exchange components of the global carbon cycle.
Within the framework of the project several results have already been derived from the improved data sets. In addition to the pronounced seasonal cycle due to growing and decaying vegetation, the steady increase of atmospheric carbon dioxide primarily caused by the burning of fossil fuels can be clearly observed with SCIAMACHY. The retrieved global annual mean carbon dioxide increase, amounting to about 2 ppm/yr, is consistent with the assimilation system 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.
Major methane source regions like the Sichuan Basin in China, which is famous for rice cultivation, and the interhemispheric gradient are clearly visible in the data. The retrieved methane results also show that, after years of stability, atmospheric methane has started recently to rise again, which is consistent with surface measurements. This renewed methane growth, amounting to about 8 ppb/yr since 2007, is a worldwide phenomenon with the largest increase observed from the SCIAMACHY data at the tropics and northern mid- and high-latitudes. Possible drivers of the rise might be positive anomalies of Arctic temperatures and tropical precipitation.
An initial comparison of SCIAMACHY and GOSAT data indicates that the two data sets are largely consistent. A more detailed comparison with GOSAT and with other independent column-averaged mole fraction data from ground-based Fourier Transform Spectrometers (FTS) is foreseen for the future.