The Environmental Bias of Corporate Income Taxation

This paper documents and quantifies an “environmental bias” embedded in the U.S. corporate income tax code: CO2-intensive (“dirty”) firms systematically face lower effective tax rates than clean firms, constituting an implicit subsidy on pollution. The authors — Iovino, Martin, and Sauvagnat — establish this cross-sectional fact, trace it to a specific mechanism, provide causal evidence using the 2017 Tax Cuts and Jobs Act (TCJA), and quantify aggregate emissions implications using a calibrated multi-sector general-equilibrium model.

Data and sample. The empirical analysis combines firm-level CO2 emissions from Trucost (scope 1 greenhouse gases) with financial data from Compustat North America for U.S. publicly listed firms, 2003–2021, yielding 11,223 firm-year observations with positive pretax and gross capital income. Effective tax rates are measured as income taxes paid divided by gross capital income (sales minus COGS minus SGA expenses, adding back R&D).

Cross-sectional finding. A one-standard-deviation increase in CO2 intensity is associated with a decrease in the effective tax rate equal to approximately 9% of its standard deviation (coefficient −0.021 to −0.022, significant at 1%). The negative relationship is entirely explained by the lower taxable fraction of gross capital income for dirty firms — that is, by larger interest expense deductions — rather than by differences in the statutory tax rate applied to pretax income.

Mechanism. The chain of causation runs: CO2-intensive production requires tangible capital (primarily machinery and equipment) → tangible capital serves as collateral → higher collateral supports higher debt → higher debt generates larger interest deductions (the “tax shield of debt”) → lower effective tax rates. Once PPE-to-capital-income is controlled for, the coefficient on CO2 intensity in leverage, pretax income, and tax regressions becomes small and statistically insignificant. The relationship holds both across and within industries, including within the energy sector, though the dominant variation is cross-industry.

Causal evidence: TCJA 2017. The paper exploits the federal corporate tax rate cut from 35% to 21% (effective January 2018) in a difference-in-differences design, comparing firms in the top quartile of 2017 CO2 intensity (“dirty”) to cleaner firms. Dirty firms experienced a relative increase in their federal effective tax rate of 2.4 percentage points post-reform. Correspondingly, dirty firms’ total assets grew approximately 11% less than clean firms post-reform. This translates to a semi-elasticity of firm total assets to a one-percentage-point increase in the effective tax rate of approximately −4.8. Parallel pre-trends are confirmed visually and via Rambachan-Roth (2023) sensitivity analysis; a placebo using non-federal taxes shows no differential effect. Results survive controls for other TCJA provisions (interest deductibility limits, international tax changes, net operating loss restrictions), exposure to import tariffs and carbon taxes, leave-one-industry-out specifications, and a triple-difference using foreign firms.

General-equilibrium model and counterfactuals. A 375-sector model with input-output networks (both intermediate and investment networks), financial frictions linking equipment to debt capacity, and endogenous CO2 emissions through fossil fuel usage is calibrated to 2017 BEA and Compustat data. In the Cobb-Douglas benchmark, the 2017 tax cut raises output by 5.9% and emissions by only 4.5% — a less-than-proportional emissions response because clean sectors expand relatively more. A counterfactual eliminating the tax shield of debt while simultaneously cutting the tax rate from 35% to 30% (to hold GDP constant) reduces aggregate emissions by 1.3% with output declining only 0.1%. When equipment and fuel are treated as complements (elasticity of substitution below 1), the emissions reduction under the same policy rises to over 3.7%, implying an absolute reduction of 80–240 million metric tons of CO2 from 2017’s total of 6,457 million metric tons. Monetized at the social cost of carbon, this ranges from USD 8–24 billion (conservative, ~USD 100/ton) to USD 112–336 billion (USD 1,400/ton per Bilal and Kanzig 2024).

Q: What is the central empirical finding of the paper? A: CO2-intensive firms in the U.S. face systematically lower effective corporate income tax rates than clean firms. A one-standard-deviation increase in CO2 intensity is associated with a roughly 9% of a standard deviation decrease in the ratio of taxes paid to gross capital income. This negative relationship is robust to alternative emissions measures (EPA data, scope 2 and 3 emissions), alternative tax scalings (taxes over sales or assets), log CO2 emissions, and leave-one-industry-out specifications.

Q: What is the mechanism linking CO2 intensity to lower effective tax rates? A: Dirty firms rely on tangible capital — specifically machinery and equipment — to produce. Tangible capital is pledgeable as collateral, enabling higher debt. Higher debt generates larger interest expense deductions under the tax code (the “debt tax shield”), which reduces taxable income relative to gross capital income. Once PPE-to-capital-income is included as a control, the coefficient on CO2 intensity in regressions of leverage, pretax income, and taxes paid all become small and statistically insignificant, confirming that PPE fully mediates the relationship.

Q: Which component of tangible capital drives the result? A: Machinery and equipment, not buildings, leases, land, natural resources, or construction in progress, explains virtually the entire positive relationship between PPE and CO2 intensity. This finding is based on the Compustat breakdown of PPE components available for roughly 70% of sample firms.

Q: Does the mechanism operate within industries or only across them? A: Both. Decomposing firm CO2 intensity into an implied industry component (sales-weighted from pure-play firms) and a firm residual, both components are significantly associated with higher tangible capital, leverage, lower taxable fraction of capital income, and lower taxes paid at the 1% level. However, the largest share of the total effect stems from cross-industry variation. Within the energy sector specifically, firms with greater fossil fuel production capacity (from EPA/EIA data) also have more tangible capital, higher debt, and lower effective tax rates.

Q: How does the 2017 TCJA cut affect clean versus dirty firms differently? A: Because dirty firms already shield a large fraction of their capital income from taxation via interest deductions, a uniform cut in the statutory rate benefits them less in proportional terms. The difference-in-differences estimates show that dirty firms (top quartile of 2017 CO2 intensity) experienced a relative increase in their federal effective tax rate of 2.4 percentage points post-reform compared to clean firms, and their total assets grew approximately 11% less than clean firms post-reform. The semi-elasticity of firm assets to a one-percentage-point increase in effective tax rate is approximately −4.8.

Q: How is the parallel trends assumption supported? A: Event-study graphs show no pre-2018 divergence in federal effective tax rates or asset growth between dirty and clean firms. A placebo test using non-federal income taxes (which should be unaffected by the federal statutory rate change) shows no differential post-reform effect. The Rambachan-Roth (2023) sensitivity analysis confirms that the null of no differential effect can be rejected at the 1% level allowing for pre-trend deviations up to M = 0.5, and at the 10% level up to M = 1.

Q: What robustness checks address other provisions of the TCJA and concurrent shocks? A: The authors exclude or control for firms affected by the TCJA’s interest deductibility limitation, multinational firms (more than 20% foreign sales), firms with large loss carryforwards, and manufacturing firms — results are unchanged. They also control for firm-level exposure to import tariff changes and carbon taxes (using the World Carbon Pricing Database), with coefficients of interest remaining virtually unchanged. Leave-one-industry-out specifications and a triple-difference using foreign firms (comparing U.S. dirty vs. clean firms pre/post-2018, against foreign equivalents in countries with stable tax rates) yield a semi-elasticity of −5.8, if anything larger than the baseline.

Q: What does the general-equilibrium model add that the difference-in-differences cannot? A: The DiD design identifies relative effects of the tax cut on dirty versus clean firms but cannot recover the absolute effect on aggregate output and emissions. The GE model, calibrated to 2017 data and validated against the untargeted DiD estimates, quantifies aggregate impacts: the 2017 tax cut raises steady-state output by 5.9% while emissions rise by only 4.5% — a less-than-proportional increase due to compositional reallocation toward clean sectors.

Q: What does the counterfactual removing the debt tax shield find? A: Eliminating the tax shield of debt while simultaneously lowering the corporate tax rate from 35% to 30% (to keep GDP constant) reduces aggregate emissions by 1.3% (Cobb-Douglas benchmark) while total output falls only 0.1% and GDP remains constant by design. The emissions reduction arises because clean sectors, which rely more on less-pledgeable capital, are made relatively cheaper once the tax advantage of debt is removed, redirecting demand away from CO2-intensive sectors.

Q: How does the complementarity assumption between equipment and fuel affect the results? A: When equipment and fuel are modeled as complements (elasticity of substitution below 1) rather than Cobb-Douglas substitutes, both policy counterfactuals yield larger emissions effects. For the tax shield removal policy, the predicted emissions reduction rises from 1.3% to over 3.7% as complementarity strengthens. This is because policies that raise the cost of equipment also induce firms to cut fuel consumption, amplifying the direct compositional effect.

Q: What is the quantified absolute emissions impact of removing the tax shield? A: Given 2017 U.S. total emissions of 6,457 million metric tons, the model predicts an absolute reduction of 80–240 million metric tons of CO2, depending on the assumed complementarity between equipment and fuel. Monetized at conservative estimates (~USD 100/ton), the policy saves USD 8–24 billion; at USD 1,400/ton (Bilal and Kanzig 2024), the value rises to USD 112–336 billion. The authors note that the physical quantity measure is more reliable than the monetized figure given uncertainty in the social cost of carbon.

Q: How does this paper relate to the ECB bond purchasing literature? A: Piazzesi et al. (2022) document that the ECB’s market-neutral bond purchases implicitly favor dirty firms because those firms issue more bonds due to higher tangible capital holdings. This paper identifies the same underlying mechanism — tangible capital → debt capacity — but on the tax side, showing that the corporate income tax code independently provides an implicit subsidy to dirty firms through the debt tax shield.

Q: What is the policy implication for the debt tax shield specifically? A: The debt tax shield — the deductibility of interest payments but not dividends — has no clear economic rationale (both are returns to capital) and, per several policy proposals (CBO 1997, IMF 2016), is a candidate for elimination. This paper adds a new dimension: the tax shield indirectly subsidizes CO2 emissions by differentially benefiting capital-intensive, CO2-intensive sectors. A revenue-neutral reform eliminating the shield can reduce emissions without sacrificing GDP.

Effective tax rate (paper’s definition): The ratio of corporate income taxes paid to gross capital income, where gross capital income equals sales minus cost of goods sold minus SGA expenses plus R&D spending. This differs from the tax-to-pretax-income ratio because it captures how much of total capital earnings — before any deductions — is remitted as tax.

Debt tax shield (tax advantage of debt): The reduction in corporate tax liability arising from the deductibility of interest payments on corporate debt. Because dividends are not deductible, debt-financed capital faces a lower after-tax cost than equity-financed capital. The shield’s value is estimated at approximately 10% of firm value in prior literature.

CO2 intensity: Metric tons of CO2 equivalent per USD 1,000 of output (tCO2/k$). The sample average is 0.1 tCO2/k$, with a heavily right-skewed distribution (median 0.02, 99th percentile 1.5).

Environmental bias of corporate taxation: The paper’s central concept — the systematic difference in effective tax rates between dirty and clean firms that arises not from explicit environmental policy but from the interaction of the debt tax shield with the capital structure of CO2-intensive industries. This constitutes an implicit subsidy on pollution embedded in the corporate income tax.

Asset pledgeability (psi): The fraction of a firm’s assets recoverable by creditors in the event of default. In the model, equipment has higher pledgeability than other capital (estimated b_psi = 0.23 additional pledgeability for equipment, a_psi = 0.35 base). Higher pledgeability allows firms to sustain more debt and thus benefit more from the tax shield.

User cost of capital: The total cost to a firm of using one unit of capital, combining depreciation, tax allowances from accelerated depreciation, and the financing cost advantage of debt over equity. The model formalizes how both the equity-financed component and the debt advantage component respond to tax rate changes, with the debt advantage term being larger for firms with more pledgeable (tangible) capital.

Investment network: An input-output structure capturing which sectors’ outputs are used to produce each type of capital good. The paper extends vom Lehn and Winberry (2021) by constructing separate equipment and non-equipment investment networks across 375 non-fuel BEA sectors, enabling emissions accounting that includes capital production alongside direct production inputs.