GHG-CCI (Greenhouse Gas-Climate Change Initiative)
The GHG-CCI project is one of several projects of ESA's (European Space Agency) CCI (Climate Change Initiative). The goal of the CCI is to generate and deliver data sets of various satellite-derived ECVs (Essential Climate Variables) in line with GCOS (Global Climate Observing System) requirements. The ECV Greenhouse Gases is the global distribution of important climate relevant gases – namely atmospheric CO2 and CH4 - with a quality sufficient to obtain information on regional CO2 and CH4 sources and sinks. The main goal of GHG-CCI is to generate long-term highly accurate and precise time series of global near-surface-sensitive satellite observations of CO2 and CH4, i.e., XCO2 (Mole Fraction Carbon Dioxide) and XCH4, starting with the launch of ESA’s ENVISAT satellite. 1) 2)
These products are currently retrieved from SCIAMACHY/ENVISAT (2002-2012) and TANSO-FTS/GOSAT (2009-today) nadir mode observations in the near-infrared/shortwave-infrared spectral region. In addition, other sensors (e.g., IASI and MIPAS) are also considered and in the future also data from other satellites. The GHG-CCI data products and related documentation are freely available via the GHG-CCI website (http://www.esa-ghg-cci.org/). Here, an overview is presented about the latest data set CRDP#2 (Climate Research Data Package No. 2) focusing on the GHG-CCI core products and present a short overview about GHG-CCI-related achievements in terms of scientific publications.
Carbon dioxide (CO2) is the most important anthropogenic greenhouse gas responsible for global warming [IPCC (Intergovernmental Panel on Climate Change), 2013]. 3) - Despite its importance, our knowledge of the CO2 sources and sinks is inadequate and does not meet the needs for attribution, mitigation and the accurate prediction of future change. Despite efforts to reduce CO2 emissions, atmospheric CO2 continues to increase with approximately 2 ppm/year (Figure ). Appropriate knowledge about the CO2 sources and sinks is needed for reliable prediction of the future climate of our planet (IPCC, 2013). This is also true for methane CH4. Global near-surface-sensitive satellite observations of CO2 and CH4 combined with inverse modeling yields information on the regional sources and sinks of these gases.
Figure 1: Northern Hemisphere XCO2 , i.e., the column-averaged CO2 dry air mole fraction (in ppm), as retrieved from SCIAMACHY/ENVISAT and TANSO-FTS/ GOSAT using four different GHG-CCI retrieval algorithms (image credit: IPCC 2013)
Legend to Figure 1: Clearly visible is the CO2 seasonal cycle - primarily caused by uptake and release of CO2 by the terrestrial biosphere - and the atmospheric CO2 increase with time, which is primarily caused by burning of fossil fuels (fraction not taken up by the terrestrial biosphere or the oceans). Also visible is the good agreement of the different GHG-CCI CRDP#2 XCO2 data products. Corresponding XCO2 and XCH4 maps are shown in Figure 2.
Figure 2: GHG-CCI CRDP#2 XCO2 (top) and XCH4 (bottom) maps for July-September 2003 (left) and 2013 (right), image credit: IPCC 2013
Multi-year measurements from two satellite instruments can be used to retrieve information on CO2 and CH4 with sufficient near-surface-sensitivity: SCIAMACHY on ENVISAT (2002 - April 2012) and TANSO-FTS on-board GOSAT (launched in 2009). Both instruments perform (or have performed) nadir observations in the NIR/SWIR (Near Infrared/Shortwave Infrared) spectral region covering the relevant absorption bands of CO2, CH4 and O2 [needed to obtain the “dry-air column” used to compute the GHG column-averaged dry-air mole fractions, i.e., XCO2 (in ppm) and XCH4 (in ppb)]. These two instruments are therefore currently the two main sensors used within GHG-CCI. The corresponding retrieval algorithms are referred to as ECAs (ECV Core Algorithms) within GHG-CCI.
In addition, a number of other sensors are also used within GHG-CCI (e.g., MIPAS/ENVISAT and IASI/MetOp-A) as they provide additional constraints for atmospheric layers above the planetary boundary layer. The corresponding retrieval algorithms are referred to as ACAs (Additional Constraints Algorithms) within GHG-CCI.
Even moderate to strong CO2 and CH4 sources and sinks only result in quite small changes of the column-averaged mole fractions relative to their background concentration. High relative accuracy of the satellite retrievals is required because even very small (regional) biases can lead to significant errors of the inferred surface fluxes. One of the first activities within GHG-CCI was to establish the user requirements, e.g., in terms of required accuracy and precision of the different data products. The result of this activity was the initial version of the GHG-CCI URD (User Requirements Document). Note that the GHG-CCI URD requirements are more detailed and often also more demanding compared to the GCOS requirements.
An overview about the various satellite-derived data products stored in the CRDP#2 data base is shown in Table 1, providing a general overview for ECA and ACA products, and Table 2, providing details on ECA products.
Table 1: Overview CRDP#2. For details on the core algorithms see Table 2.
Table 2: Overview GHG-CCI core (“ECA”) data products. (*) The latest version, EMMAv2.0, covers 4 years and is also available on the GHG-CCI website. (§) Improved v5.2 products also available on GHG-CCI website.
Ongoing Activities and Future Plans
Currently (May 2015) GHG-CCI team members are working on further improving the retrieval algorithms to be used for re-processing of the satellite data. The next version of the GHG-CCI Climate Research Data Package, CRDP#3, will be released in April 2016 (including documentation on validation PVIR (Product Validation and Intercomparison Report) and initial user assessments (CAR (Climate Assessment Report).
GHG-CCI retrieval experts are also members of the OCO-2 (Orbiting CarbonObservatory-2) Science Team and involved in the development of retrieval algorithms for Sentinel-5P and the data products of these sensors will also be considered by GHG-CCI. For OCO-2 of NASA, it is initially planned to perform detailed comparisons to determine the consistency of the XCO2 data products and to perform initial retrievals.
GHG-CCI team members are also involved in the specification of future GHG satellites, in particular CarbonSat, a proposed mission of ESA. CarbonSat, if selected for ESA’s Earth Explorer 8 satellite, will continue the time series of greenhouse gas observations from space presented in this manuscript, but will also address many important new aspects which cannot (or only with severe limitations) be addressed with other existing or planned satellites in particular the detection of localized CO2 and CH4 sources and the quantification of their emissions. Like SCIAMACHY, GOSAT and OCO-2, sun induced chlorophyll fluorescence, SIF, will be a secondary data product from CarbonSat, which can be linked to GPP (Gross Primary Production) and for investigating the impact of stress on vegetation and the CO2 uptake at the few km2 spatial resolution scale of CarbonSat.
The main goal of CarbonSat is to advance our knowledge on the natural and man-made sources and sinks of the two most important anthropogenic greenhouse gases carbon dioxide and methane from the global via the subcontinental to the local scale. CarbonSat will be the first satellite mission to image small scale emission hot spots of CO2 (e.g., cities, volcanoes, industrial areas) and CH4 (e.g., fossil fuel production, landfills, seeps) and to quantify their emissions and discriminate them from surrounding biospheric fluxes.
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The information compiled and edited in this article was provided by Herbert J. Kramer from his documentation of: ”Observation of the Earth and Its Environment: Survey of Missions and Sensors” (Springer Verlag) as well as many other sources after the publication of the 4th edition in 2002. - Comments and corrections to this article are always welcome for further updates (firstname.lastname@example.org).