Investors are demanding clear standards for "green" bonds to get finance flowing towards projects that benefit the climate.
Some US$38 billion worth of green bonds was issued in 2014. That's more than double the previous year's total, but still a tiny part of the US$100 trillion global bonds market.
Nonetheless, green bonds have "enormous potential" to pump money into clean energy and other low carbon solutions, according to a network of pension funds, insurance companies and asset managers convened by Ceres, a sustainability advocacy group.
Yet in the absence of strict rules on what counts as "green", issuers have used the label to cover projects of questionable sustainability value.
"Strong standards and clear disclosure will be crucial for the further development of the green bond market," said Cecilia Reyes, chief investment officer at Zurich Insurance Group.
There needs to be better defined criteria for labelling projects "green", reporting on the use of bond finance and independent assurance, the investor group argued.
Green bonds are seen as critical to get institutional investors on board with climate finance. As custodians of large sums of money, they look for investments with predictable, fixed returns.
By contrast, clean energy technology has been seen as a more risky prospect, suited to a different class of investors.
In 2014, Ceres launched a Clean Trillion campaign to try and boost clean energy investment globally to US$1 trillion a year.
"Green bonds are a critical financing mechanism for the clean energy solutions we urgently need, and the growth and integrity of the market will be supported through clearer standards that provide further guidance to issuers," said Chris Davis, director of investor programs at Ceres.