A flattening in “green bond” sales this year is highlighting the uncertainty facing the market for the debt, which is billed as financing projects that would mitigate climate change.

Green bonds have been sold all over the world in recent years and have helped pay for energy-efficient buildings, improvements to water and sewer systems and mass transit. As the market has grown, however, so have concerns about whether projects financed by green bonds are truly beneficial for the environment, and whether the companies, municipalities...