Sustainability-Linked Bonds (SLBs)

A sustainability-linked bond (SLB) is a bond whose financial or structural terms depend on whether the issuer achieves stated sustainability goals by a set date. The most common feature is a coupon step-up: if the issuer misses its target, the coupon rises, typically by 25 basis points, until maturity. Unlike a green bond, the money raised is not ring-fenced; it can fund general corporate purposes. What is being promised is a company-wide performance outcome, measured by key performance indicators (KPIs) such as greenhouse gas emissions or the share of renewable energy.

SLBs are therefore "forward-looking, performance-based" instruments, in contrast to use-of-proceeds bonds such as green bonds, social bonds and sustainability bonds. They appeal to issuers that lack enough discrete green projects to fill a green bond, including companies in heavy industry. The market reference is the Sustainability-Linked Bond Principles (SLBP) of the International Capital Market Association (ICMA), which sit alongside the Green Bond Principles. A similar product exists in bank lending as the sustainability-linked loan.

Think of a teenager who borrows money from a parent for a car and agrees a deal: if they keep their grades above a certain level by the end of next year, the interest stays low; if not, it goes up a little. The money can be spent on the car, insurance or petrol, the parent does not mind. What matters is the grades.

An SLB works the same way. The company borrows for whatever it needs and publicly commits to a target, such as cutting its emissions by a set amount by a set year. An independent checker confirms the result. If the target is met, nothing changes. If it is missed, the company pays investors a higher interest rate from then on, a penalty that makes the promise cost something.

ICMA published the SLBP in June 2020; the latest edition is dated June 2024, and ICMA keeps an illustrative KPI registry, updated in June 2025. The Principles set out five core components:

1. Selection of KPIs. KPIs should be relevant and material to the issuer's core business, measurable on a consistent basis and benchmarkable.
2. Calibration of sustainability performance targets (SPTs). Targets should be ambitious, beyond business as usual, ideally consistent with science-based pathways such as those under the Paris Agreement, and set to a defined observation date.
3. Bond characteristics. The documentation must state how the terms change, for example a coupon step-up or a premium paid at redemption, and what triggers it.
4. Reporting. The issuer reports at least annually on KPI performance.
5. Verification. An external reviewer verifies performance against each target at least once a year and at the trigger date.

The extra interest an investor receives if a target is missed is:

• $F$ is the face value of the bond held.
• $s$ is the step-up, written as a decimal (25 basis points = 0.0025).
• $n$ is the number of years of coupons remaining after the step-up applies.

Suppose a company issues a $500 million, ten-year SLB with a 4.00% coupon. Its target is to cut emissions by 30% by the end of year five. It misses, so the coupon rises to 4.25% for years six to ten. The extra interest paid to all investors is $500{,}000{,}000 \times 0.0025 \times 5 = 6{,}250{,}000$, or $6.25 million in total, $1.25 million a year. For an investor holding $1 million of the bonds, the step-up adds $2,500 a year. The higher coupon also lifts the bond's yield to maturity at the original price, so the expected value of a step-up feeds into the price at issue.

The Italian utility Enel priced the first widely recognised SLB in September 2019: a $1.5 billion five-year bond whose coupon would step up by 25 basis points if renewable sources made up less than 55% of its installed capacity by 31 December 2021. Issuance grew quickly after the SLBP appeared in 2020, especially among European and emerging-market corporate bond issuers. A further boost came from the European Central Bank, among the most influential central banks in the bond market,, which from 1 January 2021 accepted as collateral bonds whose coupons are linked to EU taxonomy environmental objectives or the UN Sustainable Development Goals, and later bought eligible SLBs in its corporate purchase programmes. Chile issued the first sovereign SLB in 2022.

SLBs have drawn more criticism than use-of-proceeds bonds, and issuance slowed after its 2021 peak. Common concerns are:

• Weak targets. Some SPTs restate goals the issuer was already on track to hit, so the bond changes nothing.
• Small penalties. A 25-basis-point step-up is cheap relative to the cost of real change, and can even reward investors for the issuer's failure.
• Late observation dates and call options. If the issuer can redeem the bond before the step-up starts, the penalty may never bite.
• Narrow KPIs. Many targets cover only direct operations and leave out the larger supply-chain footprint, which is why investor groups such as the Net-Zero Asset Owner Alliance press for fuller coverage.

Supporters reply that SLBs bring company-wide targets into legally binding debt documents, with public reporting and independent verification, and that the 2024 SLBP update tightened guidance on KPIs and target calibration. For investors, the bond's main risk remains the issuer's credit risk and its credit rating, with the step-up an additional, usually small, source of yield.

• ICMA: Sustainability-Linked Bond Principles