Painter (2020): An Inconvenient Cost

Why this paper is on the syllabus

This paper anchors the municipal-bond portion of the lecture because it establishes that climate risk can raise borrowing costs well before the underlying physical damages arrive. That is a central lesson in climate finance. Forward-looking capital markets do not need to wait for realized flooding to reprice the debt of exposed jurisdictions.

The setting is also pedagogically useful because the identification idea is direct. If sea level rise threatens a county’s future tax base, public infrastructure, and service delivery, then long-maturity municipal bonds issued by that county should command higher yields than short-maturity bonds issued by the same or comparable counties. Painter tests exactly that implication and shows that the maturity-based pricing gradient lines up with the economics.

The question

Do investors demand higher yields and underwriting fees from municipalities whose future economies are more exposed to climate risk, and do these effects concentrate at long horizons?

The paper focuses on physical climate risk arising from sea level rise. The economic logic is that local governments are geographically fixed. If projected inundation is expected to damage infrastructure, lower property values, shrink the property and sales tax base, or force costly adaptation investments, then the probability that long-term debt is serviced on its original terms falls. Investors who price that probability should demand compensation today in the form of higher yields, higher issuance costs, or both.

Why municipal bonds are a useful setting

Municipal bonds are well suited to this question for two reasons.

First, municipalities are immobile. A corporation can relocate a plant away from a coastline. A county cannot relocate its roads, schools, water systems, and property tax base nearly so easily. That immobility means the exposure measure is tied closely to the issuer, not to an adjustable production decision.

Second, municipal debt is issued across a wide maturity spectrum, from short-dated notes to multi-decade general obligation bonds. That variation provides a built-in test of the climate-pricing hypothesis. Sea level rise is a slow-moving risk whose realizations are concentrated in the distant future, so it should matter much more for debt with long duration than for debt that matures within a few years. If the data show an effect concentrated in long-term bonds, that pattern is much more consistent with pricing of long-horizon climate risk than with a generic coastal penalty.

Data and measurement

Painter studies new issues in the U.S. municipal bond market. The main outcome variable is the annualized issuance cost, measured as the sum of two components:

1. the initial yield that primary-market investors require, and
2. the gross spread paid to underwriters.

This outcome is informative because it captures the total cost of raising funds for the issuer, not just the coupon that investors observe. The two components also reflect somewhat different margins: the yield reflects investor compensation for risk and liquidity, while the gross spread reflects the cost of placing the bond through intermediaries.

The climate risk variable is a county-level measure of expected mean annual loss from sea level rise, expressed as a share of county GDP. The construction is forward-looking: the measure is built from projected sea level rise exposure rather than from realized flood history. That distinction is important, because the economic question is about pricing of future risk, not about reactions to past realizations.

What the paper does

The empirical design compares issuance costs of bonds from counties with different levels of projected climate risk, conditional on the bond and issue-level characteristics that ordinarily determine municipal pricing. These controls include maturity, issue size, credit rating, call provisions, and other standard issue-level features, as well as broader market conditions at the time of issue.

The core comparison is between long-term and short-term bonds. That maturity split functions as an internal test. If climate risk is being priced because it represents genuine long-run economic damage, then long-maturity debt from exposed counties should carry a premium and short-maturity debt from the same counties should not. If instead the risk variable were proxying for some time-invariant coastal feature, we would expect to see effects across the maturity spectrum.

The paper also pushes on the result in two directions. First, it examines heterogeneity by credit rating, asking whether climate risk interacts with underlying credit fundamentals. Second, it exploits a shift in investor attention by comparing the period before and after the 2006 Stern Review, which sharply raised the salience of climate economics in policy and financial markets.

How the empirical strategy works

The identification logic is not that climate-exposed counties are identical to unexposed counties on every observable or unobservable dimension. They clearly are not. The logic is that climate exposure should predict issuance costs specifically where theory predicts: at long maturities, for lower-rated issuers, and after investor attention rises.

That layered approach is stronger than a single cross-sectional comparison. If climate risk were capturing an unrelated coastal feature, such as a regional business cycle or a time-invariant amenity, we would not expect the effect to load sharply on long maturities, interact with credit quality in the predicted direction, and strengthen after 2006. The fact that all three patterns appear simultaneously, in the same direction the economics predicts, is what makes the pricing interpretation credible.

The paper also runs a placebo-style check using noncoastal counties adjacent to exposed ones. If the climate-risk variable were simply capturing broad regional characteristics, we would expect similar patterns in those neighboring jurisdictions. That pattern does not appear.

Main findings

The headline estimate is that a one-percentage-point increase in county-level climate risk, as measured by projected mean annual loss from sea level rise as a share of GDP, is associated with a 23.4 basis point increase in the annualized issuance cost of long-term municipal bonds. Scaled to the average county’s debt issuance, this translates into roughly $1.7 million in additional annualized borrowing cost.

A basis point is one one-hundredth of a percentage point, so 23.4 basis points is not large relative to typical bond-yield movements over a business cycle. It is, however, economically meaningful on large outstanding principal balances issued over multi-decade horizons, and it represents a persistent wedge between the financing costs of exposed and less-exposed counties.

The effect does not appear for short-term bonds. This is one of the cleanest results in the paper, because it directly distinguishes long-horizon risk pricing from unrelated coastal penalties. The maturity gradient is exactly what the economic logic predicts.

The paper also decomposes the effect across the two components of issuance cost. Both the initial yield and the gross spread rise with climate risk. In other words, climate risk makes it more expensive both to attract investors and to place the bond through intermediaries.

Heterogeneity by credit rating reinforces the interpretation. The effect is larger for lower-rated bonds. That pattern is consistent with the economics: counties with weaker underlying credit fundamentals have less slack to absorb additional future shocks, so an additional long-horizon risk weighs more heavily on their pricing.

Finally, the paper documents a timing pattern that lines up with investor attention. Before the 2006 Stern Review, there is little evidence that climate-exposed counties paid more than comparable less-exposed counties for long-term debt. After the Stern Review, the gap widens significantly at long maturities. The natural interpretation is that markets did not fully price long-run climate risk until a salient information event raised attention.

Why the paper matters

The broad lesson is that climate change can affect public finance substantially before major physical damages are realized. A county does not need to flood today for climate risk to raise its borrowing costs today. If bond investors expect higher future repayment risk, they demand higher yields now, and this compensation is capitalized into the cost of public capital.

The implications for local governments are direct. Higher borrowing costs make it more expensive to build and maintain public infrastructure, and they tighten the budget constraint exactly in the jurisdictions that will later need to finance adaptation. Climate risk can therefore weaken an exposed municipality on two margins: once through expected future physical damages and again through higher current financing costs for the infrastructure needed to prepare for those damages.

The paper is also conceptually important. It shows how expectations about slow-moving environmental change are capitalized into forward-looking asset prices, in the same spirit as the housing and weather-derivative papers in the same lecture. The object being priced here is public debt rather than land or weather contracts, but the underlying economics is closely related.

What to focus on when you read

On a first read, focus on three pieces.

First, understand why municipal bonds are a useful setting. Local governments are geographically fixed, and the maturity structure of municipal debt maps directly onto the horizon structure of climate risk.

Second, understand the maturity comparison. It is the clearest piece of the identification strategy and the feature that distinguishes long-run climate-risk pricing from unrelated coastal penalties.

Third, pay attention to the interpretation of the coefficient. A 23.4 basis point effect may sound small relative to day-to-day bond-yield movements, but on the large principal balances and long maturities characteristic of municipal debt it translates into real resources and real constraints on local investment.

Terms to know

• Municipal bond: debt issued by a state or local government, typically to finance public infrastructure.
• Yield: the annualized return that investors receive from holding the bond to maturity, conditional on the scheduled cash flows.
• Gross spread: the underwriting compensation paid to the intermediaries who place the bond with investors.
• Basis point: one one-hundredth of a percentage point.
• Term structure: how yields and borrowing costs vary across maturities for otherwise comparable issuers.
• Investor attention: the idea that markets may price a risk more strongly once that risk becomes salient to investors, even if the underlying physical risk has not changed.