The 2026 NCGG-10 Symposium on non-CO2 greenhouse gases targets the greenhouse gases methane (CH4), nitrous oxide (N2O), fluorocarbons, and halogenated species (HFC/CFC/HCFC/SF6, etc.). Since the combined contribution of these gases to climate warming since the pre-industrial period is comparable to the warming caused by CO2 alone (46% by non-CO2; 54% by CO2; ref. IPCC AR6), we refer to them collectively as "the other half of the problem." Especially CH4 is becoming increasingly important. Attention should also be given to the impact of hydrogen (H2) and aerosols as short-lived climate forcers agents.
The importance of this ‘other half’ is growing with each new NCGG Symposium. The rapid improvements in global air quality since the turn of the century have provided significant health benefits and act as a multiplier for non-CO2 global warming. Globally decreasing emissions of of Sulphur dioxide (SO2) , Ammonia (NH3), nitrogen oxides (NOX) and non-methane volatile organic compounds (NMVOCs) contribute indirectly to climate change through ozone and aerosol formation in the atmosphere and effects on the lifetime of methane. The relative new kind on the energy block, is molecular H2. Leakages of H2 could prolong the chemical lifetime and thus climate warming of methane. The potential climate impacts of a global transition to a H2 economy is yet to be studied in detail, together with many other societal transitions that are underway in response to climate warming. The balance of warming and cooling agents in the nitrogen cycle, such as N2O, NOx and NH3, can shift. Processes include O3 and aerosol formation as well as changes in CH4 lifetime. Increased NH3 emissions related to its new role in energy transport and production may cause reactive reactive-N leakage, leading to indirect climate impacts, including an increase in N2O emissions and disruptions to the N cycle itself.
Public awareness of the urgent need for climate action has grown rapidly, and it is widely understood that swift CO2 mitigation is necessary. However, for non- CO2 greenhouse gases, additional efforts are required to raise public awareness and widespread societal action. Fortunately, reducing non- CO2 greenhouse gas emissions often aligns with reducing CO2 emissions. Again, CH4 plays a significant role here, due to its relatively short atmospheric lifetime. A swift policy response to reduce CH4 is crucial as one of the steps to keep temperature change as small as possible. Given the balance in warming and cooling through N2O, NH3 and NO2 emissions, a reduction of N-emissions might only contribute to cool the climate on the longer timescales.
