Banks seem keen to be green. The number backing the Green Bond Principles has doubled in a few months. The principles, drafted by a committee comprising Bank of America Merrill Lynch, Citigroup, Crédit Agricole Corporate and Investment Bank and JP Morgan Chase, were announced in January with the backing of nine other banks. This month another 12 declared their support.

There is one problem, though: the principles do not define greenness, but give guidelines for issuance. Offering guidelines for issuing green bonds without defining what they are begs the question of what exactly is "green" and how rigorously definitions should be policed. That question is only likely to become more important as the market grows and diversifies.