  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.
