Global warming will largely increase waste treatment CH4 emissions in Chinese megacities: insight from the first city-scale CH4 concentrationobservation network in Hangzhou, China

Atmospheric CH4 is the second-largest anthropogenic contributor to global warming. However, its emissions, components, spatial-temporal variations and projected changes still remain largely uncertain from city to national scales. CH4 emissions from waste treatment (including solid waste landfills, solid waste incineration and sewage) account for > 50% of total anthropogenic CH4 emissions at the city scale, and considering the high temperature sensitivity of CH4 emission factors (EFs) to biological-process-based sources such as waste treatment, large differences will occur when estimating future CH4 emissions under different global warming scenarios. Furthermore, the relationships between temperature and waste treatment CH4 emissions have only been studied in a few site-specific studies and lack representativity for whole cities, which contain various biophysical conditions and show heterogeneous distribution. The above factors cause uncertainty in the evaluation of city-scale CH4 emissions (especially from waste treatments), and projected changes still remain unexplored. Here we conduct the first tower-based CH4 observation network with three sites in Hangzhou, which is located in the developed Yangtze River Delta (YRD) area and ranks as one of the largest megacities in China. We found the a priori total annual anthropogenic CH4 emissions and those from waste treatment were overestimated by 36.0% and 47.1% in Hangzhou, respectively. In contrast, the total emissions in the larger region, i.e., Zhejiang Province or the YRD area, were slightly underestimated by 7.0 %. Emissions from waste treatment showed obvious seasonal patterns following air temperature. By using the linear relationship constructed between monthly waste treatment CH4 emissions and air temperature, we find the waste treatment EFs increase by 38 %-50% with temperature increases of 10 degrees C. Together with projected temperature changes from four climate change scenarios, the global-warming-induced EFs in Hangzhou will increase at the rates of 2.2 %, 1.2 %, 0.7% and 0.5% per decade for IPCC AR5 (Intergovernmental Panel on Climate Change, the Fifth Assessment Report) RCP (Representative Concentration Pathway) 8.5, RCP6.0, RCP4.5 and RCP2.6 scenarios, respectively. And the EFs will finally increase by 17.6 %, 9.6 %, 5.6% and 4.0% at the end of this century. Additionally, the derived relative changes in China also show high heterogeneity and indicate large uncertainty in projecting future national total CH4 emissions. Hence, we strongly suggest temperature-dependent EFs and the positive feedback between global warming and CH4 emissions should be considered in future CH4 emission projections and climate change models.