Flammer (2020): Green Bonds

Why this paper is on the syllabus

This paper is the central “does it work?” reading in the green-bonds lecture. A green bond is designed to channel financing toward environmentally beneficial investment, but that design claim is only economically interesting if issuing a green bond changes something real about what the firm does. If the label is purely cosmetic, then green bonds are a marketing exercise with no environmental consequences, regardless of how investors feel about them.

Flammer asks exactly that question. Are green bonds a label that firms use for reputation and marketing, or do they affect financial performance, environmental performance, or both? The answer matters for whether green-bond markets should be viewed as a meaningful instrument of environmental policy or as a classification that mainly reorganizes existing investment without changing it.

The question

Do green-bond issuers become better environmental performers after issuance, and does the market interpret these bonds as value-enhancing for the issuing firm?

This is the right question for environmental economics. We do not only care whether investors value the label; we care whether the instrument changes real outcomes, and in particular whether it reduces emissions and improves environmental performance at the issuing firm.

The broader market background

The paper opens with a set of facts about the scale of the global green bond market. Global green-bond issuance rose from approximately $0.8 billion in 2007 to $141.3 billion in 2018. Flammer also documents the rapid growth of the U.S. green municipal bond market, from approximately $0.6 billion in 2010 to $4.3 billion in 2018.

That growth is part of the motivation for the paper. Green bonds were becoming a quantitatively important component of sustainable debt financing at a very rapid pace, but the empirical literature on whether they had any real effects on firm behavior was thin. The paper is designed to fill that gap.

What the paper does

Flammer focuses on green bonds issued by publicly listed firms. That sample restriction is important because public firms are subject to detailed financial disclosure requirements and to established environmental reporting datasets, which makes it possible to track outcomes before and after issuance in a consistent way.

The paper studies three sets of outcomes.

First, it examines the stock market reaction around the announcement of a green bond issue. This is the short-run market test and speaks to whether equity investors view the issue as value-enhancing for the issuing firm.

Second, it examines longer-run financial performance using standard accounting-based measures, including return on assets and return on equity.

Third, it examines environmental performance using firms’ reported carbon emissions and environmental scores from the Thomson Reuters ASSET4 database, which compiles standardized environmental, social, and governance indicators from corporate disclosures.

How the empirical strategy works

For the announcement effect, the paper uses a standard event study. The idea is direct: if equity investors believe a green bond issue is value-relevant news, the issuing firm’s stock price should move around the announcement window, and the sign and magnitude of that movement summarize the market’s assessment.

For the longer-run outcomes, the paper uses a matched difference-in-differences design. Each green-bond issuer is matched to a similar firm that issued a conventional bond in the same year, using pre-period firm characteristics and outcome trends. The matching variables include size, profitability, leverage, and pre-period environmental, social, and governance indicators. The paper then compares the evolution of financial and environmental outcomes for treated firms relative to their matched controls before and after the green bond issuance.

This design matters because firms that issue green bonds are not a random sample of public firms. They may already differ on the dimensions that determine financial and environmental performance. Matching combined with difference-in-differences does not resolve every concern about selection on unobservables, but it makes the comparison substantially more credible than a raw before-after change in the treated group alone.

The role of certification

One of the most important distinctions in the paper is between certified and uncertified green bonds. In principle, any firm can label a bond green. If there is no credible external verification of that label, the designation carries little information content, and investors should reasonably worry about greenwashing.

Flammer treats third-party certification as a governance mechanism. Certification, typically through organizations such as the Climate Bonds Initiative, imposes standards on project eligibility and on the use of proceeds, and it provides ongoing reporting requirements. In economic terms, it raises the cost of misusing the proceeds and therefore makes the green label a more credible commitment device for the issuer.

That distinction turns out to be central to the results. The paper uses certification status as a split of the treated group and shows that the estimated effects load disproportionately on the certified issues.

Main findings

The first headline result is that the stock market reacts positively to green bond announcements. In a two-day event window around the announcement date, the cumulative abnormal return is approximately 0.67 percent, relative to a standard asset-pricing benchmark.

That is an economically meaningful magnitude for a bond issuance event. It suggests that equity investors do not view green-bond issues as wasteful diversions or purely symbolic commitments. They treat them as value-enhancing news about the firm.

The second set of results concerns longer-run financial performance. Relative to their matched controls, green-bond issuers show improvements in post-issuance profitability, with gains appearing in standard accounting measures such as return on assets and return on equity. The magnitudes are consistent with the interpretation that green-bond issues are associated with, or contribute to, financially productive investments rather than purely reputational activity.

The third set of results is the core of the paper for environmental policy. Firms that issue green bonds reduce their carbon emissions and improve their environmental ratings after issuance, relative to their matched controls. Both margins move in the direction the design of the instrument is intended to produce.

Crucially, these effects are concentrated among certified green bonds. The positive stock market reaction is statistically significant primarily for certified issues, and the improvements in financial and environmental performance are likewise concentrated among certified issues. That pattern is strongly consistent with the interpretation that governance and verification matter for whether the label translates into real effects.

What the results mean

The paper’s message is not that every bond labeled green is effective. The message is narrower and more useful: green bonds appear to work when the market has a credible governance mechanism that ties the label to real action on the part of the issuer.

That distinction is important for policy. A label without verification invites greenwashing, because issuers face little cost from using the designation loosely. Certification raises the credibility of the instrument, and in Flammer’s data the certified bonds are precisely the ones for which the evidence of real effects is strongest.

The financial-performance result is also interesting on its own terms. It suggests that green bonds are not purely altruistic activity by firms or investors. They may help firms finance valuable long-run projects, attract a supportive investor base, or strengthen internal commitment to environmental investments that then yield both environmental and financial returns.

Why the paper matters

The paper matters because it connects sustainable finance to real environmental outcomes, rather than stopping at the question of whether investors like green assets. Much of the early green-finance literature focused on pricing and demand. Flammer asks the more demanding question of whether green finance changes what firms do, and it is that question that ultimately determines whether these instruments are policy-relevant.

The paper also carries a clear policy lesson. If governments or regulators want green-bond markets to matter for real environmental outcomes, they cannot rely on issuer self-labeling. Standards, reporting, and third-party verification are likely to be central to whether the market generates measurable environmental gains.

For this course, the paper pairs naturally with Baker et al. Flammer focuses on real effects on firms, while Baker et al. focus on pricing and investor demand. Together, they provide a more complete picture of how green-bond markets operate: one side documents that investors are willing to pay a modest premium for the label, and the other documents that, under credible certification, the label is associated with measurable improvements in firm-level environmental performance.

What to focus on when you read

On a first read, focus on four things.

First, keep the distinction between a label and a real treatment clear. The paper is asking whether green bonds cause meaningful changes in firm outcomes after issuance, not whether they are correctly marketed.

Second, understand the two empirical pieces. The event study speaks to the stock market’s assessment of green-bond issues, and the matched difference-in-differences speaks to longer-run changes in financial and environmental outcomes.

Third, focus on the outcome measures. The paper does not rely on firm promises or disclosures about intent. It uses carbon emissions, standardized environmental ratings, and accounting-based profitability measures to track what actually changes after issuance.

Fourth, keep certification front and center. It is not a secondary result. It is the mechanism that distinguishes cases where the label appears to matter from cases where it does not, and it is the most policy-relevant finding in the paper.

Terms to know

• Green bond: a bond whose proceeds are earmarked for projects with stated environmental benefits.
• Event study: an empirical design that measures the movement in an asset price in a narrow window around a public announcement, typically benchmarked against a standard asset-pricing model.
• Difference-in-differences: a before-after comparison between a treated group and a comparison group, which differences out time-invariant group characteristics and common time shocks.
• Matching: a procedure for constructing a comparison group whose pre-period characteristics are similar to those of the treated group, in order to improve the credibility of the counterfactual.
• Certification: third-party verification that a green bond meets a stated standard for project eligibility and use of proceeds.
• Greenwashing: claiming environmental benefits from an activity when those benefits are weak, misleading, or unverifiable.