This year Verizon, the American telecoms group, embarked on a $1bn experiment: in February it became the first telecoms company in the US to issue a "green" bond, or a security that raises funds for sustainable business.

You might have thought this exercise would incur some cost for Verizon's treasurers, given that green finance has traditionally been a cumbersome endeavour. Not so. Verizon's $1bn green bond attracted such frenzied investor demand that it was eight times oversubscribed, making it the most popular security Verizon has ever sold. Indeed, funding costs were slightly "cheaper than a brown bond [or normal