In September, the governing board of Santa Cruz Metro made a big bet on the future of green public transit when it approved the purchase of 57 buses fueled by hydrogen — the largest order of hydrogen-fueled buses made so far in the U.S. The brand-new vehicles, which could arrive as soon as the end of 2024, have water as their only tailpipe emissions. They will replace most of the authority’s aging buses, which run on compressed natural gas. There’s just one problem: Most of the hydrogen that is currently available is produced from fossil fuels, and that causes more of the greenhouse gas emissi…