Successful implementation of the Paris Agreement of the United Framework Convention on Climate Change (UNFCCC) will require sustained, near-real time monitoring of greenhouse gas (GHGs) fluxes and concentrations to assess the impact and overall effectiveness of mitigation efforts undertaken by the Parties to the agreement. As the Paris Agreement concerns actions that can be implemented by nations, it largely focuses on the evaluation of the part of GHGs that are directly under human control, namely anthropogenic emissions and carbon uptakes. These are estimated using a bottom-up approach or inventory which combines activity data and emission factors. The resulting emission and uptakes estimates are generally considered to be of good quality in industrialized countries for a number of sectors, where good national statistics on economic activities are available as a basis. At the same time, emissions and uptakes estimates for several sectors, including land use change, have large uncertainties. In many developing countries, the underlying data required for bottom-up emission estimations are not available.
The impact of GHGs on climate is driven not only by anthropogenic emissions, but rather by the total net fluxes (the balance between fluxes in and out of the atmosphere) that control atmospheric concentration. Natural sources and sinks of greenhouse gases are typically not included in bottom-up estimation.
This leads to continued uncertainty about some of the processes behind the continued rise in atmospheric concentration of GHGs, about the effectiveness of various kinds of mitigation action, and how the natural sources and sinks of GHGs may respond to the ongoing climate change. In view of these shortcomings and building on the WMO GHG research activities and experience with operational weather and climate monitoring and prediction, the Infrastructure Commission in cooperation with the Research Board and Services Commision decided to develop a concept of GHG operational monitoring in the beginning of 2022. Unlike emission inventory or bottom-up emission estimates, the top-down approach directly uses atmospheric observations in conjunction with modelling and data assimilation for estimating when and where GHGs enter and exit the atmosphere. The models, data assimilation, observing network and data exchange that are required for GHG monitoring have much in common with their parallels in the World Weather Watch (WWW), which has been successfully operated by the WMO Members for 60 years. Furthermore, WMO has long-standing GHG activities – monitoring, research and provision of related services under the auspices of the Global Atmosphere Watch (GAW) established in 1989 and its Integrated Global Greenhouse Gas Information System (IG3IS) – on which GHG monitoring can build.
The role of WMO
The proposed Global Greenhouse Gas Watch (G3W) will provide global fields of the net fluxes of the main GHGs. Parties to the Paris Agreement and other interested stakeholders can use the data to develop products that meet their specific requirements. Analogous to its role in the WWW and GAW, the role of the WMO in G3W will be to establish:
Requirements for an integrated surface-, aircraft- and satellite-based observing system,
A design of a comprehensive surface-based observing system,
Improved and timely exchange of all satellite-, aircraft- and surface-based GHG observations, including the coordinated planning for future satellite observing systems,
Collaboration on common methodologies and practices for GHG modelling and data assimilation,
Common file formats and practices for exchange of model fields,
Common verification and validation methods, and
Common guidance on methods for post-processing and down-stream applications.
WMO will also aim to strengthen research components to continuously support and improve the proposed operational infrastructure. The G3W itself will build on mature research, but additional research will be needed to address several open scientific questions related to surface fluxes and transport uncertainty. Given the need to significantly expand the observational infrastructure, research and development of improved and more cost-effective measurement techniques will also be important.
WMO will also ensure that the implementation of the G3W is accompanied by a comprehensive capacity development and training programme. Staff in various functions – managerial level, operators, data managers, modellers – will require targeted training before, during and after the roll-out.
Configuration and deliverables
The G3W will consist of four main components in its initial configuration:
A comprehensive sustained, global set of surface-based and satellite-based observations of CO2, CH4 and N2O concentrations, total column amounts, partial column amounts, vertical profiles, and fluxes and of supporting meteorological, oceanic, and terrestrial variables, internationally exchanged as rapidly as possible, pending capabilities and agreements with the system operators;
Prior estimates of the greenhouse gas emissions based on activity data and process-based models;
A set of global high-resolution Earth System models representing greenhouse gas cycles; and
Associated with the models, data assimilation systems, optimally combining the observations with model calculations to generate products of higher accuracy.
The individual model systems taking part in the G3W will each deliver at least the following outputs in common standard formats:
Monthly CO2 net fluxes between the Earth surface and the atmosphere with 1° x 1° horizontal resolution delivered with maximum a delay of one month,
Monthly CH4 net fluxes between the Earth surface and the atmosphere with 1° x 1° horizontal resolution delivered with a delay of one month,
3-Dimensional fields of CO2 and CH4 abundance with hourly resolution and data latency to be defined (tentatively on the order of a few days), and
N2O abundances and net fluxes with resolution and latency still to be defined.
For the GGGW, the target resolution of the output would be initially at 1° x 1° (approximately 100 x 100 km at the equator), although some of the participating model systems may operate at higher resolution depending on their individual capacity. The data can then be further processed into a downstream cascade of products to support applications at larger or smaller scales and for individual sectors. While these applications will depend on and build on G3W output, the development of the downstream products themselves is beyond the scope of the initial implementation of G3W.
It will be critical that the input observations meet accuracy and precision standards and that their characteristics be documented per the WMO Integrated Global Observing System (WIGOS) Metadata Standard. As the natural sources and sinks that help determine GHG concentration are often much more distributed in space and have different temporal variability than the anthropogenic sources, the system should also provide adequate spatial coverage and sensitivity to detect changes in natural terrestrial and ocean fluxes, associated with possible carbon-climate feedback.
The detailed requirements for the observations will be further refined using the WMO Rolling Review of Requirements process once the G3W moves to the implementation. These requirements will be largely driven by the desired quality of the model output. A core principle in the G3W is that all participating modelling centres must have access to the same distributed set of input data. However, data selection, pre-processing and data management will be specific to each system/centre due to differences between their individual setups.
Several technical workshops conducted in 2023 stressed the need to sustain existing observational networks and to extend it to the areas where spatial and temporal gaps exist. Another important initial step for the GGGW will be to establish adequate timely international exchange of all already existing GHG observations, both surface- and space-based.
Top-down approach combines observations of atmospheric greenhouse gas concentrations with atmospheric modelling to estimate net fluxes.
Tackling the uncertainties
“The increase in CO2 levels from 2020 to 2021 was higher than the average growth rate over the past decade and methane saw the biggest year-on-year jump since measurements started,” said WMO Secretary-General Professor Petteri Taalas. “But there are still uncertainties, especially regarding the role in the carbon cycle of the ocean, the land biosphere and the permafrost areas. We therefore need to undertake greenhouse gas monitoring within an integrated Earth System framework in order to be able to account for natural sources and sinks, both as they currently operate and as they will change as a result of a changing climate.”
The proposed G3W will provide a wealth of quantitative data to help improve our understanding of GHG cycles. G3W will consolidate existing measurement and analysis capabilities to provide estimates of total net GHG fluxes on a global scale at a relatively high resolution in space and time. The improved understanding of GHG fluxes will allow for better prediction of long-term climate trajectories, with potentially strong implications for the mitigation activities required here and now.