Advisors interested in socially responsible investing should not limit themselves to stocks as the only asset class to capitalize on environmental, social and corporate governance (ESG) standards or social impact initiatives such as serving poor communities. They should also consider SRI bonds, most commonly known as green bonds.

That market is surging and poised to double in size this year to $175 billion to $185 billion, according to a new report from Bank of America Merrill Lynch. A Chinese bank, Shanghai Pudong Development Bank Co Ltd, is the single biggest corporate issuer of a green bond this year, having sold $5.33 billion worth of debt. In the U.S. the biggest corporate green bond issuer is Apple, which issued $1.5 billion worth of green bonds.

“Apple has been one of the most ground-breaking [issuers] — being the largest single Green Bond issued by a US corporation and the largest from a non-pureplay corporate,” the authors of the Bank of America Merrill Lynch report wrote. “This could potentially open the floodgates for other major companies to enter the market.”

Apple has said it will use the proceeds from the bond sale to finance clean energy projects throughout its global operations including the use of more renewable energy sources, environmentally friendly materials in its products and processes and energy-efficient processes.

Stephen Liberatore, the managing director of TIAA Global Asset Management, says demand for green bonds “is coming from a very broad swath of investors, from endowments, pension funds, high net worth individuals, family offices” and increasing among millennial investors and female investors.

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Despite the growing demand many investors “are really unaware of what is available to them in this space,” says Liberatore, who manages the $750 million TIAA-CREF Social Choice Bond Fund (TSBHX), which has a five-star Morningstar rating, as well as other socially responsible fixed income portfolios. He suggests that advisors “do more education to let investors know what’s available.”

What is available in the green bond market, in addition to socially responsible mutual funds, are corporate, government and municipal bond issues. Of the top 20 global green bond issuers from 2015 through the end of April 2016, 13 were corporate, primarily investment grade, and the rest were government, including multinational entities like the European Investment Bank and U.S.-based state and local muni issuers, according to the Bank of America Merrill Lynch report. Forty-one percent of the market is denominated in U.S. dollars.

In addition to Apple, U.S. corporate green bond issuers include Starbucks, which issued a $500 million corporate sustainability bond in mid-May to fund programs to insure its coffee is grown and distributed in ways that provide fair pay for workers and help maintain sustainability practices. The proceeds will also be used to support a loan program to help farmers rotate their crops and fund a network of eight farmer-support centers for suppliers in Rwanda, Tanzania, Colombia, China, Costa Rica and other countries.

There are also many municipal bond issuers of green bonds, which include the traditional water and sewer bonds as well as bonds to finance renewable energy programs and affordable housing.,

These bonds are “an opportunity for munis to talk about the good that they do,” says Liberatore. In just six year that market has grown from $500 million 2010 to $3.8 billion as of the end of 2015. They include tax-exempt and taxable bonds.

Green bonds are also an opportunity for institutional investors like TIAA to have a greater impact on corporate behavior. “On the fixed income side we have the purse strings,” says Liberatore.  “We’re able to identify and work at various levels of the capital structure.” For example, said Liberatore, TIAA was able to work with Georgia Power , a subsidiary of Southern Company which therefore doesn’t issue equity, on a green bond issue to install solar plants on unused land of U.S. military bases, producing enough power to take 42,000 cars off the road.

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