当前，很多城市已做出碳中和承诺，城市水系统也可以为构建碳中和城市贡献力量。本研究采用多城市时序分析法，评估了阿姆斯特丹、墨尔本、纽约和东京等四个城市，在水系统温室气体（GHG）排放管理方 面的进展情况及经验教训。这些城市在水系统温室气体减排目标制定和排放情况报告方面，一直走在世界前列。尽管这些年来多次'反弹'，但相比十多年前，这4个城市水系统的温室气体排放量都有所下降，近三年平均减排量达到13%~32%。温室气体排放量下降归功于多种工程措施，如太阳能和小型水力发电、沼气定价、污泥消化、焚烧及曝气系统优化等。与此同时，这些城市认识到，在实现碳中和目标的道路上仍有众多挑战，包括动态变化的需水量和降雨量、碳密集型防洪和供水战略、满足新的空气和水质标准以及修正温室气体排放核算方法等。本研究也证实，水系统靠自身实现碳中和十分困难，必须扩大常规的系统边界，统筹外部的工程和非工程机会，携手其他行业共同构建碳中和城市。

Many cities have pledged to achieve carbon neutrality. The urban water industry can also contribute its share to a carbon-neutral future. Using a multi-city time-series analysis approach, this study aims to assess the progress and lessons learned from the greenhouse gas (GHG) emissions management of urban water systems in four global cities: Amsterdam, Melbourne, New York City, and Tokyo. These cities are advanced in setting GHG emissions reduction targets and reporting GHG emissions in their water industries. All four cities have reduced the GHG emissions in their water industries, compared with those from more than a decade ago (i.e., the latest three-year moving averages are 13%–32% lower), although the emissions have ″rebounded″ multiple times over the years. The emissions reductions were mainly due to various engineering opportunities such as solar and mini-hydro power generation, biogas valorization, sludge digestion and incineration optimization, and aeration system optimization. These cities have recognized the many challenges in reaching carbon-neutrality goals, which include fluctuating water demand and rainfall, more carbon-intensive flood-prevention and water-supply strategies, meeting new air and water quality standards, and revising GHG emissions accounting methods. This study has also shown that it is difficult for the water industry to achieve carbon neutrality on its own. A collaborative approach with other sectors is needed when aiming toward the city's carbon-neutrality goal. Such an approach involves expanding the usual system boundary of the water industry to externally tap into both engineering and non-engineering opportunities.