Reply on RC2

Abstract

Atmospheric CH4 is the second largest anthropogenic contributor to global warming, however its emissions, components, spatiotemporal variations, and projected changes present large uncertainties from city to national scales. CH4 emissions from waste treatment account for >50 % of total anthropogenic CH4 emissions at the city scale, and considering the high sensitivity of CH4 emission factors (EFs) to temperature for biological process-based sources, such as waste treatment, large bias 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 determinized in a few site-specific studies, and these findings lack representativeness for the whole city scale, which contains various biophysical conditions and shows heterogeneous distribution. These factors increase the difficulty of evaluating city-scale CH4 emissions (especially from waste treatments), and the projected changes remain unexplored. Here, we conduct the first tower-based CH4 observation network with three sites in Hangzhou city, which is located in the developed Yangtze River Delta (YRD) area and ranks as one of the largest megacities in China. We found that the a priori total annual anthropogenic CH4 emissions and waste treatment emissions were overestimated by 36.0 % and 47.1 % in Hangzhou city, respectively. However, the total emissions in the larger region of Zhejiang Province or the YRD area was only slightly underestimated by 7.0 %. Emissions from waste treatment showed obvious seasonal patterns according to the air temperature. By using the constructed linear relationship between monthly waste treatment CH4 emissions and air temperature, we found that the waste treatment EFs increased by 38 %~50 % as the temperature increased by 10 °C. Together with the projected temperature changes from four climate change scenarios, the global warming-induced EFs in Hangzhou city will increase at rates of 2.2 %, 1.2 %, 0.7 % and 0.5 % per decade for RCP8.5, RCP6.0, RCP4.5 and RCP2.6 scenarios, respectively, and by 17.6 %, 9.6 %, 5.6 %, and 4.0 % at the end of this century, respectively. Additionally, the relative changes derived for the whole of China also showed high heterogeneity and indicated large uncertainty in projecting future national total CH4 emissions. Hence, we strongly suggest the temperature-dependent EFs and positive feedback between global warming and CH4 emissions should be considered in future CH4 emission projections and climate change models.