The once-soaring global market for green bonds – issued to support investments in renewable energy, sustainable agriculture and other environmental causes – is forecasted to reach about $50 billion in 2016. After a rapid expansion over the past three years, growth is expected to slow to 4 percent.

As new and exciting players jump in and the market widens, the question now is whether a gradual diversification of the market will give this market new energy – and to what extent the recent climate talks in Paris will give green bonds a boost.

Clearly, a lot more investment is needed to close the gap between where we are today and the money we need to transition the world to a low-carbon economy. Can green bonds be the catalyst to attracting the capital we need?

Apple, N.Y. subway, Georgia Power: We’re in

The appearance of some unconventional players demonstrates that green bonds are a tool with broad applicability.

Apple issued $1.5 billion in bonds earlier this year dedicated to financing clean energy projects at its facilities worldwide. It was the largest green bond ever issued by an American company.

Apple’s move came on the heels of New York Metropolitan Transportation Authority issuing $500 million in green bonds, the first time the busy transit agency offered a debt security to help slow climate change.

In another first, Georgia Power issued $325 million to support investment in renewable energy sources in the Southern state, the first of its kind offered by a United States utility.

Green bonds also got a global boost when, at the COP21 climate talks in December, they were held up as a vehicle to help raise at least $100 billion annually. The money, from public and private sources, will be used to help developing nations mitigate and adapt to climate change by 2020.

As the momentum for green bonds continues to grow, however, the debate about their integrity is intensifying.

So what’s keeping investors out?

Among the missing pieces before we get to $100 billion and beyond are consistent and universal rules that assure investors that the environmental projects they support actually help expand renewable energy or address deforestation and other challenges.

Groups such as Green Bond Principles and the Climate Bonds Initiative are focusing on giving investors that transparency, and their efforts are gaining traction.

Setting strong standards offers an opportunity to continue the momentum for these bonds while driving innovation and the chartering of new territory in sustainable finance. And they will help a growing sector of investors connect with sound financial opportunities that also generate material environmental benefit.

This is the big opportunity, and challenge, for an investment tool so powerful it may help protect our planet and our global livelihood.

Stay tuned for the next big thing: Blue bonds.

