A new approach to determine national greenhouse gas budgets

Abstract

We propose a new approach to determine national greenhouse gas (GHG) budgets that are collectively consistent with the 1.5° C goal of the Paris Agreement and compatible with the accounting method of national GHG inventories. Our approach is designed to bridge global mitigation assessments and national GHG inventory accounting, enabling fair and transparent Nationally Determined Contributions (NDCs) aligned with a given temperature goal. It is flexible, allowing the use of different equity principles and timeframes. GHG budgets are based on a fair share of the allowable warming contributions of GHG emissions. The approach translates the CO2-warming-equivalent GHG budget to be compatible with national GHG inventories by applying adjustment terms for non-CO2 GHG emissions and anthropogenic emissions from land use, land-use change, and the forestry (LULUCF) sector. The adjustment term for non-CO2 GHGs equals the bias introduced to estimate CO2-warming-equivalent emissions using Global Warming Potentials. The adjustment term for emissions in the LULUCF sector equals the projected indirect effects on managed lands due to climate change and CO2 fertilization. These adjustments can be made either at the global or national level, leading to different national GHG budgets, depending on the country’s mix of non-CO2 GHG emissions and projected indirect effects. We demonstrate this by showing that Finland’s GHG budget is much larger if the adjustments are made at the global level compared to the national level, resulting from high non-CO2 GHG emissions and indirect effects. Nationally adjusted GHG budgets are consistent with countries’ fair share of warming contributions, but GHG budgets with global-level adjustments are less sensitive to modeling uncertainty in indirect effects and scenario uncertainty of national non-CO2 GHG emissions. Our approach helps improve the Global Stocktake of NDCs and assess the long-term climate strategies of the signatory countries of the Paris Agreement.