More than $1tr a year could be raised through green bonds by 2020, but how should investors navigate this emerging market?
As well as using the green bond label, Zurich assesses each green bond on its own merits. With the market still in its early stages, standards are ill-defined and investment is very much done on a case-by-case basis, says Lewin.
As much as possible, Zurich will engage in "active dialogue" with the issuer from an early stage to understand exactly how the bond is designed to work. This is a crucial part of the process, and could also allow Zurich to influence the deal.
"Transparency is the most important thing in this market," Lewin says. "It's not realistic for there to be a single and absolute standard that will define what is green and what is not. So in the absence of that, it's really important that as investors, we have transparency to make up our own minds."
Are green bonds investing where they should be?
Back in 2011, when the green bond market was tiny, development banks such as the World Bank were the only issuers on the scene so it was relatively easy for investors to assess the credibility of the bond.
But as the market has grown, so too has the range of issuers. This year the biggest proportion of issuers so far has been corporates, while banks and cities have also taken a slice of the market share.
So transparency remains a key concern. Investors should look for a clear and measurable mandate from the issuer on how they plan to invest their money, and then find out how often the issuer plans to report back.
Damian Regan, a director in risk assurance services at consultancy PwC, says investors should be looking for commitment from the issuer to provide assurance on a yearly basis.
"When people typically buy a bond all they're interested in is the coupon and they're not really worried where that money goes," he says. "But as soon as you put a green wrap around it then you've got a reason for investing in it. Therefore, whether you hit that mandate becomes critical to the investor. There's no financial loss to the investor, but they would feel hard done by if they invested in something that purported to be green but wasn't."
What's the impact?
As well as ensuring the money went where the issuer promised, you should also seek to find out if the bond had the desired climate impact. In short, did it deliver on the greenhouse gas reductions that it promised?
Again, those looking to purchase green bonds should be looking for a clear goal from the issuer of what they are hoping to achieve and a timetable for reporting back on progress, says Regan.
Zurich's Lewin says to ensure they are getting what they paid for, his team regularly check in with the issuer - the conversation doesn't just end once the investment has been made.
With the market still in early stages, it's difficult to know how many green bond issuers have failed to deliever on their pledges. But this is probably because some investors are not providing a clear enough mandate at the start, allowing them to then claim they have delivered on promises that were too vague to begin with.
The emergence of the green bond market is opening up a raft of opportunities for companies to combine their investment and environmental expertise to tackle one of the world's biggest challenges. Those companies that lead the emergence of this important new market could find themselves benefiting in the long term both on a financial and reputational level, but only if they embrace the safeguards necessary to ensure green bonds are truly green.