Green bonds — debt instruments designed to raise capital for specific, environmentally friendly projects — look like much more than a fad. But this emergent market still needs to overcome some big challenges.

Since the first green bonds were issued by the World Bank in the late 2000s, the market has grown exponentially. According to the Climate Bonds Initiative, total issuance in 2019 is likely to hit $250bn, up from $36.6bn in 2014.