Top-down carbon fluxes supporting regional-scale carbon cycle science and national-scale greenhouse gas inventories

Fossil fuel combustion, land use change and other human activities are now adding ~40 billion tons of carbon dioxide (CO 2 ) to the atmosphere each year. These emissions have increased the atmospheric CO 2 concentration by ~50% since the beginning of the industrial age (~277 to > 419 parts per million by volume (ppm)). The CO 2 growth rate would have been much larger if natural sinks in the land biosphere and ocean had not absorbed over half of the anthropogenic CO 2 emissions. Over this same period, anthropogenic emissions have contributed to a ~160% increase in atmospheric methane (CH 4 ) concentrations (~0.72 ppm to > 1.923 ppm). Together, these CO 2 and CH 4 increases account for ~90% of the present-day global warming.

Recognizing the increasing threat of climate change, 197 nations signed the Paris Agreement, which encourages rapid reductions in the emissions of CO 2 , CH 4 and other greenhouse gases (GHGs). To track progress toward this goal, each Party to this agreement must submit Transparency Reports, including a national inventory of anthropogenic GHG emissions by sources and removals by sinks. Parties agreed to compile their inventories following IPCC guidelines, which require emissions to be quantified by sector (e.g., energy, industry, AFOLU, Waste) using bottom-up methods (e.g., direct measurements or estimates based on activity and emission factors). Progress toward the goals of the Agreement are tracked at five year intervals in the Global Stocktakes, the first of which was scheduled for 2023.

CO 2 and CH 4 sources sinks can also be tracked by measuring their concentrations both in situ and with remote sensing at high spatial resolution over the globe and then deriving fluxes from them using atmospheric inverse methods. The utility of this top-down approach has advanced rapidly over the past decade as the quality and density of space-based remote sensing data have improved. To demonstrate these capabilities, the GHG community produced global, national-scale budgets of CO 2 and CH 4 flux to support the first Global Stocktake. Our hope was that this information would both facilitate the bottom- up inventory development process and provide a way to validate the accuracy of the inventories, at least on regional scales.

This exercise provided new insights into the global carbon cycle, but did not work quite as we expected. Nations scrambled to meet their inventory reporting deadlines, employing the mandated, IPPC bottom-up approach. Many acknowledged the value of top-down products, but there is no evidence that any nation used them to compile or validate their inventories. However, all was not lost. These global budgets provide a critical top-down baseline for use in future Stocktakes. They also provide what might be the best independent assessment of collective progress toward the Mitigation goals of the Paris Agreement. The science community now has the opportunity to work with the IPCC to determine how best to use these products for that application. Here, I will summarize the lessons learned from this experience and suggest ways to more forward.