Attributing 2013–2024 drivers of global ammonia emissions using IASI observations: implications under future climate change

Emissions of ammonia (NH 3 ) from agricultural intensification have increased over the past fifty years, degrading air quality. Although controlling NH 3 emissions has been identified as cost-effective means of improving air quality, policy assessment is hampered by large uncertainties in emission estimates. Here, we perform a daily 4°×5° spatial resolution ensemble Kalman filter inversion of 2013 – 2024 IASI NH 3 observations to estimate NH 3 emissions and trends. We find global NH 3 emissions total 85 Tg a -1 on average over the study period, similar to levels estimated by emissions inventories but reallocated regionally. We then use a machine-learning approach to diagnose drivers of NH 3 emissions trends. We find a significant 1.09 Tg a -2 NH 3 increasing trend in global emissions over the 2014–23 period. While peatland fires, correlated with El Niño events, drive global interannual variability, the increasing NH 3 trend is largely associated with agricultural activity, especially livestock farming. We find an observable climate-driven increasing trend of 0.13 Tg a -2 , which in China is enough to offset 48% of NH 3 reductions due to chemical fertilizer controls. We project the climate penalty to 2100 using Coupled Model Intercomparison Project Phase 6 (CMIP6) output and estimate a global climate NH 3 emissions penalty of between 3.5 Tg a -1 (under the 4.5 W m -2 forcing scenario) and 6.6 Tg a -1 (under 7.0 W m -2 ) by 2100, in line with process-based models. Our quantification of human activities associated with increasing NH 3 emissions can inform control strategy, even as air quality control is constrained by increasing climate penalties.