---
title: a loan book is a map
canonical_url: https://ensurance.app/guide/a-loan-book-is-a-map
markdown_url: https://ensurance.app/guide/a-loan-book-is-a-map.md
subtitle: "mortgages, farm loans and town bonds cluster in one footprint and share the ground under it. geographic concentration risk, and the source nobody in the book funds"
category: nature-finance
---

# a loan book is a map

*mortgages, farm loans and town bonds cluster in one footprint and share the ground under it. geographic concentration risk, and the source nobody in the book funds*

**What is concentration risk?** Concentration risk is the chance that one event, one sector or one counterparty causes losses across a large part of a lender's book at once, because too much of the book is exposed to the same thing. Its geographic form is the one most books carry without counting: loans that cluster where people live, farm and build, so the same flood, fire or drought reaches many of them together.

Print a loan tape on a map and it clusters around the towns, valleys and coasts the bank serves. Around each cluster is a footprint, the area one event touches. Under it is the ground that sets how big that event gets: the floodplain, the forest, the aquifer, the marsh. That ground is rarely in a credit file as an input, and it does its work whether or not anyone lends against it or pays for it. [**Ensurance**](/manual/ensurance?from=guide) is how the people who depend on it can pay for its keeping. It is not the ground, and it is not insurance.

## what concentration risk is, and its geographic form

Lenders already count concentration by borrower, industry and product. **Geographic concentration risk** is the same idea set down on a map. A loan book is concentrated in a place when a large share of its collateral and its borrowers' income sit inside one footprint. A mortgage, a farm equipment line, a restaurant's working-capital loan and a county's bond look unrelated on the tape. Inside one river basin or fire-prone valley, they share an event.

That is not a mistake. Community banks lend where their depositors live; that is their job. It does mean the book holds a position in a place, whether it reports one or not.

## four books that cluster

| book | why it clusters | what the collateral shares | what the borrowers' income shares |
|---|---|---|---|
| **Mortgages** | Branches, brokers and buyers are local | The flood reach, the fire path, the surge line | Local jobs, local wages |
| **Farm loans** | Farms sit where the soil and water are | Land whose value rides on water and soil | The same rain, the same aquifer, the same prices |
| **Small-business loans** | Shops serve the town they are in | Storefronts in the same streets | The same customers, roads and tourist seasons |
| **Municipal bonds** | A town borrows against its own tax base | Property values that make up the tax base | Taxes paid by the households and firms above |

The farm row is the plainest case. As of the first quarter of 2026, the St. Louis Fed counted agricultural banks (those with at least 25% of total lending in agricultural loan types) as 20% of all US banks, holding 40% of total agricultural loans. Banks like that lend where the farms are, so their books tend to track the weather and water of the regions they serve.

Bond markets price some of the town row. In a 2020 study, Marcus Painter found that US counties more likely to be affected by climate change paid more in underwriting fees and initial yields on long-term bonds than counties unlikely to be affected. His measure was exposure to sea level rise, so it speaks to coastal places. The gap disappeared on short-term bonds and was driven by lower-rated issuers. In that study the market read the place into the long paper.

## collateral and income share one footprint

A footprint hits a book twice. It damages the collateral, so loss given default rises. And it hits the income that services the loan (the job, the harvest, the shop's sales, the property tax), so the chance of default rises too. Inside one footprint, both arrive in the same season.

Spreading across borrowers does not fix that: the unit is the place, not the borrower. Spreading across regions helps, until the footprints are larger than the regions; [diversification lowers your share, not the loss](/guide/diversification-lowers-your-share-not-the-loss?from=guide) takes that up.

## insurance is the collateral's first guard

Lenders know this, which is why a mortgage requires hazard cover and, in a federally designated flood zone, flood cover. Insurance is the collateral's first guard: it turns a damaged house back into a repaired one and keeps the loan serviceable up to the policy's limits.

The Federal Reserve's pilot climate scenario analysis with six of the largest US banks, published in May 2024, shows how much rests on that guard. Asked to model a severe hurricane in the Northeast, the banks generally picked storms that ran through the areas where their real estate loans were concentrated. Every bank reported gaps in its insurance data (coverage, replacement cost, deductibles), so banks had to assume how much insurance would absorb. The exercise also ran the storm with no insurance at all, which the Fed called an extreme outcome.

The same report notes that insurers reset home policies every year, that some have raised premiums sharply or left markets altogether, and that those changes can reach a wider region than the area a disaster hit. When cover reprices or leaves, the risk stays, and the lender and the bondholder hold more of it. [When the insurer leaves, the risk stays](/guide/when-the-insurer-leaves-the-risk-stays?from=guide) follows where it lands; what keeping a zone writable takes is in [keep the policy before you cut the price](/guide/keep-the-policy-before-you-cut-the-price?from=guide).

:::johnson
**a loan book is a map. the ground under it is rarely funded by the book.** Collateral, income and insurance in one footprint all depend on the same floodplain, forest or aquifer. Rarely do the lenders in the book pay to keep it.

[capital providers: how a position in a place can be held →](/solutions/capital-providers?from=guide&topic=spatial-risk)
:::

## the source is a credit input nobody models

Under every footprint is a **source**: the ground that sets how big the hazard gets there, often off the borrower's parcel and owned by someone else. It shows up in credit whether or not anyone models it.

- **A farm region and its water.** The loans are to many farms. The aquifer or the river they draw on is one. When it runs short, yields, land values and repayment move together. The farmer's crop policy pays after the loss; the soil works on drought and downpour before it ([the soil under the crop policy](/guide/the-soil-under-the-crop-policy?from=guide)).
- **A port city and its canal.** The loans are to shippers, warehouses and the households who work there. A canal whose locks run on a lake that rain fills is one source, and a dry year reaches all of them at once.
- **A ski town and its forest.** The loans are to lodges, shops and second homes. The forest above shapes how fire reaches the town and the season the town sells. One stand of trees sits under the whole book.

Three posts work through pieces of this. [Follow the supply chain and the number climbs](/guide/follow-the-supply-chain-and-the-number-climbs?from=guide) reads the European Central Bank's count of lending to firms that depend on nature. [The correlation outside the matrix](/guide/the-correlation-outside-the-matrix?from=guide) shows names in one book sharing a river that never prints as a coefficient. [The land is the collateral](/guide/the-land-is-the-collateral?from=guide) lends against a living place itself.

Keeping a floodplain, a forest or a wetland can change the size, timing or reach of some hazards in some places. It does not prevent a catastrophe, and a record flood or fire can exceed any buffer. It moves part of the loss curve, not all of it.

## what a lender can do with the map

A lender cannot move its book off the ground it sits on. It can read that ground.

1. **Map the book by footprint, not just by county.** Put collateral and borrowers on one map, draw the footprints that matter (flood, fire, drought, heat) and count exposure per footprint, beside how well insured each one is.
2. **Read the sources.** For each heavy footprint, name the ground that sets the hazard (the floodplain upstream, the forest above the town, the aquifer under the farms) and who owns it.
3. **Find who else is in the footprint.** Other lenders, bondholders, insurers, the utility, the town and the owners all depend on the same source. That list of shared exposure is also the list of who gains when the ground is kept.
4. **Fund the source together.** No one lender should carry the upkeep of a watershed. A shared account at the place, funded by those in the footprint, can.

Lending against a place while the ground that sets its hazard goes unfunded by those who share the footprint is a position with a missing leg. [How to manage spatial risk at the source](/guide/how-to-manage-spatial-risk-at-the-source?from=guide) works the steps through for a whole book.

## what we do and do not do

We do not score borrowers, run credit or catastrophe models, lend, hold risk, or pay claims. No lender or carrier is known to credit ensurance funding in a loan price or a premium today, and ensurance has not changed anyone's cost of credit.

What **ensurance** does today is the part after the map: give a named place an [account](/agents?from=guide), let the people who depend on it fund it through a [certificate](/specific?from=guide), and route the proceeds to the ground first and the people who keep it second. Those pieces are live. Borrowing against these instruments and parametric ensurance are designs, not live products. How capital can hold a position in a living place: [ensurance for capital providers](/solutions/capital-providers?from=guide&topic=spatial-risk).

This is not investment, lending, legal or regulatory advice.

## frequently asked questions

### what is concentration risk?

Concentration risk is the chance that a single event, sector or counterparty causes losses across a large share of a portfolio at once. Its geographic form is the one a map shows and a loan tape often hides.

### what is geographic concentration risk in lending?

It is the share of a loan book whose collateral and borrowers' income sit inside one hazard footprint, such as a floodplain, a fire-prone valley or a drought region. Mortgages, farm loans, small-business loans and municipal bonds in one place can look unrelated and still lose together in one event.

### how does climate risk affect a mortgage portfolio?

Through damage to the homes that secure the loans and strain on the local income that repays them, often in the same season. Insurance absorbs much of the first; where cover reprices or leaves, more stays with the lender. The floodplain, forest or marsh around the homes shapes how large some of those events get.

## sources

[Board of Governors of the Federal Reserve System, *Pilot Climate Scenario Analysis Exercise: Summary of Participants' Risk-Management Practices and Estimates* (May 2024)](https://www.federalreserve.gov/publications/files/csa-exercise-summary-20240509.pdf) — Six large bank holding companies; insurance data gaps (p. 1, p. 21); no-insurance scenario as "an extreme outcome" (p. 11); hazard events chosen where participants "have concentrations of loans" (p. 15); insurers raising premiums or exiting, across "a wider region" (box 1, p. 20)

[Federal Reserve Bank of St. Louis, *Banking Analytics: Agricultural Lending by Community Banks Remained Strong in Q1* (30 July 2026)](https://www.stlouisfed.org/on-the-economy/2026/jul/banking-analytics-agricultural-lending-community-banks-remained-strong-q1) — Agricultural banks (at least 25% of total lending in agricultural loan types) were 20% of US banks and held 40% of agricultural loans, as of Q1 2026

[Marcus Painter, "An inconvenient cost: The effects of climate change on municipal bonds," *Journal of Financial Economics* 135(2), 468–482 (2020)](https://ideas.repec.org/a/eee/jfinec/v135y2020i2p468-482.html) — Abstract: counties more likely to be affected by climate change pay more in underwriting fees and initial yields on long-term bonds; no difference on short-term bonds; driven by lower-rated bonds; exposure measured as projected sea level rise (see the paper's data section)

## the series

1. [risk has an address](/guide/risk-has-an-address?from=guide) — **Read first:** what spatial risk is, the pin, the footprint and the source, and the one move finance does not make
2. [a footprint turns a thousand policies into one event](/guide/a-footprint-turns-a-thousand-policies-into-one-event?from=guide) — Accumulation risk: why insurers count roofs by the footprint
3. [a loan book is a map](/guide/a-loan-book-is-a-map?from=guide) — Concentration risk: what lenders and bondholders share with the ground under their borrowers
4. [diversification lowers your share, not the loss](/guide/diversification-lowers-your-share-not-the-loss?from=guide) — Where geographic diversification works, and where the footprints outgrow it
5. [when the insurer leaves, the risk stays](/guide/when-the-insurer-leaves-the-risk-stays?from=guide) — Uninsurable areas, and who ends up holding the risk
6. [how to manage spatial risk at the source](/guide/how-to-manage-spatial-risk-at-the-source?from=guide) — Map the footprint, find the ground that sets the hazard, fund it together

**Companion series:** [what multi-peril insurance covers](/guide/what-multi-peril-insurance-covers?from=guide) — the several perils one place carries, and the ground as the layer under the policy.