Green hydrogen is critical for decarbonising hard-to-electrify sectors, but faces high costs and investment risks. Here we define and quantify the green hydrogen ambition and implementation gap, showing that meeting hydrogen expectations will remain challenging despite surging announcements of projects and subsidies. Tracking 137 projects over three years, we identify a wide 2022 implementation gap with only 2% of global capacity announcements finished on schedule. In contrast, the 2030 ambition gap towards 1.5{\deg}C scenarios is gradually closing as the announced project pipeline has nearly tripled to 441 GW within three years. However, we estimate that, without carbon pricing, realising all these projects would require global subsidies of \$1.6 trillion (\$1.2 - 2.6 trillion range), far exceeding announced subsidies. Given past and future implementation gaps, policymakers must prepare for prolonged green hydrogen scarcity. Policy support needs to secure hydrogen investments, but should focus on applications where hydrogen is indispensable.

翻译：绿色氢能对于难以电气化行业的脱碳至关重要，但面临高昂成本和投资风险。本文界定并量化了绿色氢能的雄心与实施差距，表明尽管项目公告和补贴激增，满足氢能预期仍将面临挑战。通过对137个项目进行三年追踪，我们发现2022年的实施差距巨大——全球产能公告中仅有2%按期完成。相比之下，到2030年实现1.5°C情景的雄心差距正逐步缩小，已公布的项目管道在三年内几乎翻了三倍，达到441吉瓦。然而，我们估算，若无碳定价机制，实现所有这些项目需要全球补贴1.6万亿美元（区间为1.2-2.6万亿美元），远超已公布的补贴规模。鉴于过去与未来的实施差距，政策制定者必须为长期绿色氢能短缺做好准备。政策支持需确保氢能投资，但应聚焦于氢能不可或缺的应用领域。