All of the major space agencies have programmes dedicated to the development and stewardship of climate data records from satellite observations. This article highlights the programme of the European Space Agency (ESA).
As a contribution to the response from CEOS to GCOS on the need for ECV products, ESA initiated a programme, the Climate Change Initiative (CCI), to bring together European expertise covering the full range of scientific, technical and development specialisations available within the European EO community and to establish lasting and transparent access for global climate scientific and operational communities to its results. The objective of the CCI programme is to realise the full potential of the long-term global EO archives that ESA together with its Member States have established over the last 30 years, as a significant and timely contribution to the ECV databases required by the UNFCCC.
Since 2010, the CCI programme has performed algorithm development, inter-comparison and validation, and large- scale EO data processing for 13 GCOS ECVs where the potential contribution from ESA was considered unique and/or significant. Importantly, all CCI ECV data products are provided with quantitative uncertainty estimates and have been evaluated by leading members of the climate research community. It has contributed to a rapidly expanding body of scientific knowledge, demonstrating new insights in climate research by maximising the scientific benefits that satellite observations can provide to better inform and respond to needs raised by the IPCC.
To achieve this, substantial community building has taken place to strengthen the relationship between satellite data product developers and climate scientists. As a result of the multidisciplinary approach of the programme, strong links have also been forged spanning the land, ocean, atmosphere and cryosphere science communities. The products are being made available through the CCI Data Portal and the intention is that the processing systems are transferred from CCI to operational programmes, such as Europe’s Copernicus Climate Change Service (C3S).
Responding to the IPCC
The IPCC reports synthesise all the published evidence on climate change and produce an assessment of the current state of knowledge. These assessments come with statements on their confidence as well as reporting on changes since the last report. In the fifth IPCC Report in 2013, CCI made contributions in specific areas with direct impact on summary conclusions as a result of improved ECV products and greater community consensus.
Over the last two decades, the Greenland and Antarctic ice sheets have been losing mass, glaciers have continued to shrink almost worldwide, and Arctic sea ice and Northern Hemisphere spring snow cover have continued to decrease in extent (high confidence) (IPCC 2013).
The Randolph Glacier Inventory (RGI) is the first globally complete inventory of glaciers produced for IPCC AR5, with ESA projects contributing about one-third of all glacier outlines (Figure 1) and consortium members from Glaciers CCI being instrumental in its successful creation.
The RGI has proven vital for global-scale applications, in particular sea level change assessments since the lack of knowledge of the global distribution of glaciers was a significant, but very hard to quantify, source of uncertainty. This has allowed greater agreement on future glacier mass loss.
The rate of sea level rise since the mid-19th century has been larger than the mean rate during the previous two millennia (high confidence). Over the period 1901 to 2010, global mean sea level rose by 0.19 [0.17 to 0.21] m (IPCC 2013).
Improved global mean sea level estimates have been produced through evolution of algorithms and homogeneous reprocessing of satellite data from multiple satellites. These products were developed in the framework of the Ocean Surface Topography Science Team (OSTST) and GLOSS. While they cover the period from 1993 to the present, they tie in well with buoy observations from earlier in the 20th century.
Figure 2: Multi-mission global sea level trend from altimetry. This figure includes all the CCI satellite time series overlaid after being adjusted for biases – 1993 to 2013 (mm/year).
Credit: CNES, ESA, LEGOS, CLS
By combining the CCI products with other ECVs, improved sea level budget studies have been performed at global and regional scales, allowing estimates of unknown or poorly known contributions (e.g. the deep ocean heat uptake and its role in the current hiatus or the land water storage change due to human activities). The provision of these high quality data has encouraged cross-ECV interaction and hence allowed better knowledge of the components necessary for sea level budget closure. Further work on this is currently being pursued by bringing the CCI teams together to focus on sea level budget closure.
Ocean warming dominates the increase in energy stored in the climate system, accounting for more than 90% of the energy accumulated between 1971 and 2010 (high confidence). It is virtually certain that the upper ocean (0−700 m) warmed from 1971 to 2010, and it likely warmed between the 1870s and 1971 (IPCC 2013).
Sea Surface Temperature (SST) datasets have been developed over the period from 1991–2010 using the Along Track Scanning Radiometer (ATSR) and Advanced Very High Resolution Radiometer (AVHRR) satellite data, with Spinning Enhanced Visible and Infrared Imager (SEVIRI) data for diurnal modelling of infrared temperatures. By combining these records with those from passive microwave instruments, SST records and departures from the mean have been produced (Figure 3). The products include both skin SST and SST at standard depth. They are homogeneous and stable throughout the time series, accompanied by context-specific uncertainty estimates and independent from in-situ measurements.
Figure 1: Glacier outlines from the RGI (blue) with contributions from the Glaciers_cci Project Team (Background is MODIS Blue Marble)
To achieve the generation of a complete ECV requires the space agencies and the EO science community to collaborate more closely at a scientific level and to coordinate better to ensure data are accessible to all and that the data are adequately preserved over long periods of time. A part of this substantial community building has taken place within CCI to strengthen the relationship between satellite data product developers, EO scientists and climate scientists. Key examples include the development of the RGI for IPCC, the Ice Sheet Mass Balance Intercomparison Exercise (IMBIE), development of close international collaboration on greenhouse gases and the unification of the global satellite aerosol science community through the AeroSAT initiative as a complement to Aeronet and AeroCom.
The RGI was achieved through a major community effort along with international cooperation between the Global Terrestrial Network for Glaciers (GTN-G), the World Glacier Monitoring Service (WGMS) and Global Land Ice Measurements from Space (GLIMS). This is now evolving into development of a global glacier database (gtn-g.org/ data_browser.html). IMBIE was a collaboration between ESA and NASA to bring together the key scientists in ice sheet mass balance to end a 20-year disagreement. It produced a reconciled estimate of ice sheet mass balance changes in Antarctica and Greenland, as seen from satellites, and their contribution to sea level rise (Figure 4) is being continued through the second IMBIE experiment.
The atmospheric concentrations of carbon dioxide, methane, and nitrous oxide have increased to levels unprecedented in at least the last 800,000 years. Carbon dioxide concentrations have increased by 40% since pre-industrial times, primarily from fossil fuel emissions and secondarily from net land-use change emissions. The ocean has absorbed about 30% of the emitted anthropogenic carbon dioxide, causing ocean acidification (IPCC 2013).
The expertise in greenhouse gas assessment in Europe through the Scanning Imaging Absorption Spectrometer for Atmospheric Chartography (SCHIMACHY) has been coupled with the launches of the Japanese Greenhouse Gases Observing Satellite (GOSAT) and US OCO-2 satellites, which took place in the CCI period.
Figure 4: Reconciled sea level contributions due to mass loss from the Greenland and Antarctic Ice Sheets (Shepherd et al. 2012)
This allowed the groups from Europe to work closely together with those from Japan and the US to test multiple algorithms to improve CO2 and CH4 retrieval accuracies and coverage and in the process generate column average CO2 and CH4 from the SCIAMACHY instrument (2002–2012) and the Japanese Thermal and Near-infrared Sensor for Carbon Observation (TANSO) instrument (2009–2014) and prepare for the Orbiting Carbon Observatory-2 mission (OCO-2).
Total radiative forcing is positive and has led to an uptake of energy by the climate system. The largest contribution to total radiative forcing is caused by the increase in the atmospheric concentration of CO2 since 1750 (IPCC 2013).
One of the greatest uncertainties in climate science is the degree of radiative forcing attributable to aerosols. To help tackle this, the global satellite aerosol science community has been brought together through the AeroSAT initiative as part of the global effort to better understand aerosol behaviour and as a complement to the in-situ Aeronet and AeroCom modelling communities. This has also helped pull together a very disparate community in Europe and thus allowed the development of aerosol property ECV data sets for test years from multiple satellites and an aerosol product from the ATSR-2 and AATSR instruments over the period 1995-2012.
Figure 3: CCI annual mean sea surface temperature (SST) anomaly maps for 1992–2010
Credit: SST_cci Project Team
Cross-domain Exchange and EO and Climate Community Collaboration
In addition to improvements in ECV products and collaboration within the communities that generate them, fundamental needs exist to ensure that consistency exists between different ECVs and also that the products are known by and used by the climate community itself. Major successes of the CCI include transparency in production and documentation, estimation of uncertainty and exchange between teams responsible for given ECVs. In addition, the continuous exchange with the climate research and climate modelling communities has been emphasised through their involvement in individual teams and also through the specific Climate Modelling User Group (CMUG), which represents several of the major modelling groups in Europe. These twin elements have allowed the projects to evaluate their products as they proceed. Regular colocation meetings across the CCI Programme, targeting specific topics, have encouraged
Figure 5: Fire_cci developed spatial distributions of fires at climate model resolution to test the representation of fire in these models.
Credit: Fire_cci Project Team
The IPCC’s AR5 in 2013 highlighted the importance of improvements in both observations and modelling in understanding the Earth as a system and addressing the scientific challenge of monitoring and predicting climate change. Advances in understanding based on better observations and models have been noted and hence there is more confidence in statements made in the report compared with the AR4 released in 2007. These improvements in understanding have been made partly through provision of high quality ECV data products and their availability to the science community. ESA’s CCI has taken a lead role in Europe in organising the community and exploiting the data archives accrued in Europe, but the improvements are also a function of the development of structures and fora to exchange information at the international level as well as greater collaboration between science domains and within the EO community.
Despite the successes demonstrated by the CCI so far, considerably more work needs to be done to meet the growing demand for ECV data products to satisfy the increasingly complex scientific challenges presented by change in the Earth’s climate and the consequent negative impacts on the environment and human prosperity. While understanding of the Earth as a system has been the focus of the first decade of the 21st century, the next decade for science will be to address the urgent societal challenge to support climate adaptation and mitigation. This is already reflected within Europe in terms of the high priority assigned to climate research and climate service development and the implementation of operational climate services through the EU’s Copernicus programme. This new challenge requires even greater effort to go beyond production of high quality observational data sets for climate understanding (i.e. GCOS ECVs) to target information requirements for climate adaptation and mitigation.
Stephen Plummer, Simon Pinnock, Pascal Lecomte (ESA CCI programme management team)
IPCC, 2013: Summary for Policymakers. In: Climate Change 2013: The Physical Science Basis. Contribution of Working Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change [Stocker, T.F., D. Qin, G.-K. Plattner, M. Tignor, S.K. Allen, J. Boschung, A. Nauels, Y. Xia, V. Bex and P.M. Midgley (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA.
Shepherd, A. et al. 2012, A Reconciled Estimate of Ice- Sheet Mass Balance, Science 338, 1183, DOI: 10.1126/ science.1228102