Methane and Nitrous Oxide Budgets for Australasia: A Regional Assessment of Natural and Anthropogenic Sources and Sinks

We present the CH₄ and N₂O budgets for anthropogenic and natural sources and sinks of Australasia (Australia and New Zealand) from 2010 to 2019 using bottom-up and top-down methods, in line with the RECCAP-2 initiative, with extensions to 2022. We show that the bottom-up CH₄ budget for Australasia (2010–2019) was a net source of 14.1 ± 5.5 Tg CH₄ yr⁻¹, with Australia and New Zealand contributing 84% and 16%, respectively. Anthropogenic sources contributed 55% of all CH₄ emissions, the rest coming from natural sources, primarily wetlands. The bottom-up N₂O budget was a net source of 0.5 ± 0.3 Tg N₂O yr⁻¹, with Australia contributing the majority (92%), mainly from natural sources (82%). Australasia top-down CH₄ (10.4 ± 0.5 Tg CH₄ yr⁻¹) and N₂O budgets (0.8 ± 0.5 Tg N₂O yr⁻¹) differ in magnitude from the bottom-up budgets but remain consistent within their uncertainties. Similar consistency is observed for Australia, while New Zealand shows significant discrepancies, particularly for N₂O, where the bottom-up estimate is 71% higher than the top-down estimate. In terms of trends, bottom-up natural wetland CH₄ emissions increased in both countries between 2010 and 2019. CH₄ emissions from enteric fermentation slightly declined in Australia but increased in New Zealand. Soil N₂O emissions from nitrogen additions increased in both countries, with a significant rise in New Zealand driving the overall positive trend in anthropogenic emissions. These findings highlight critical sectors with large mitigation potential and the significance of monitoring natural sources for possible biogeochemical-climate feedback.