Defining the Tax Base: Scope and Coverage
One of the most important decisions in carbon tax design is what to tax. This means defining the tax base: which sectors, which fuels, which gases, and which emitters. Getting this right is crucial for effectiveness, efficiency, and fairness.
What Is the Tax Base?
The tax base is the universe of emissions subject to the carbon tax. It includes:
- Sectors: Power generation, transport, industry, buildings, agriculture
- Fuels: Coal, oil, natural gas, and their derivatives
- Gases: CO2, methane (CH4), nitrous oxide (N2O), fluorinated gases
- Entities: Which companies or facilities must actually pay the tax
A broader tax base means more emissions face the carbon price, leading to more cost-effective reductions. But broader coverage also means more complexity and more entities to monitor.
Starting Point: Cover the Major Fuels
Most carbon taxes start with the major fossil fuels that account for the bulk of emissions:
Coal Highest carbon content per unit of energy. Major source of power sector and industrial emissions.
Petroleum products Gasoline, diesel, jet fuel, heating oil, and other refined products. Main source of transport emissions.
Natural gas Lower carbon than coal or oil, but still a significant emissions source for power, heating, and industry.
Together, these fuels account for about 80% of global CO2 emissions from energy.
Example coverage decision: A country might start by covering:
- Gasoline and diesel (transport)
- Natural gas (heating and power)
- Coal (power and industry)
This captures most emissions while using existing fuel tax infrastructure. Later, coverage can expand to other fuels and emission sources.
Which Sectors to Include?
Different sectors have different characteristics that affect carbon tax design:
| Sector | % of typical emissions | Ease of coverage | Key considerations |
|---|---|---|---|
| Power generation | 25-35% | High | Few large emitters; existing monitoring |
| Transport | 15-25% | High | Many dispersed emitters; fuel taxes exist |
| Industry | 20-30% | Medium | Process emissions harder to measure |
| Buildings | 10-20% | Medium | Many small emitters; affects households |
| Agriculture | 10-15% | Low | Non-CO2 gases; measurement challenges |
Most carbon taxes start with power and transport, then expand to other sectors over time.
Beyond CO2: Other Greenhouse Gases
Carbon dioxide is the primary target, but other gases matter too:
Methane (CH4) 25 times more potent than CO2 over 100 years. Major sources: natural gas leaks, livestock, landfills.
Nitrous oxide (N2O) 298 times more potent than CO2. Major sources: fertilizer use, industrial processes.
Fluorinated gases (HFCs, PFCs, SF6) Hundreds to thousands of times more potent. Major sources: refrigeration, air conditioning, industrial uses.
Think of greenhouse gases like different currencies. Just as you can convert euros, yen, and dollars into a common value, you can convert different greenhouse gases into "CO2 equivalent" (CO2e) based on their warming potential. A carbon tax expressed in $/ton CO2e can then apply to all gases.
Some carbon taxes cover only CO2. Others include multiple gases.
CO2-only approach (simpler):
- Easier to measure (based on fuel carbon content)
- Lower administrative burden
- Misses significant emissions (methane can be 10-20% of national totals)
Multi-gas approach (broader):
- Covers more emissions
- Allows cost-effective reductions across all gases
- Requires measurement of non-CO2 sources
France's carbon tax initially covered only CO2 from fuel combustion. Mexico's carbon tax similarly focuses on CO2. In contrast, some Nordic countries and Singapore cover multiple gases.
The trend is toward broader coverage, but CO2 from fossil fuel combustion is always the starting point.
Exemptions and Reduced Rates
In practice, most carbon taxes include exemptions or reduced rates for certain activities. Common exemptions include:
International aviation and shipping
Often exempt due to international competition concerns and the difficulty of applying national taxes to international trips.
Agriculture
Frequently exempt or taxed at reduced rates due to political sensitivity and measurement challenges.
Export industries
May receive exemptions or rebates to maintain competitiveness with producers in countries without carbon pricing.
Low-income households
Not direct exemptions, but may receive rebates or credits to offset the tax burden.
Every exemption reduces the tax base and overall effectiveness. But some exemptions may be necessary for political feasibility or to address legitimate concerns about fairness and competitiveness.
Setting Thresholds
Another way to limit coverage is through thresholds. Only entities above a certain size pay the tax.
Advantages of thresholds:
- Reduces administrative burden (fewer entities to monitor)
- Protects small businesses from compliance costs
- Focuses on the largest emitters
Disadvantages of thresholds:
- Exempts some emissions from the price signal
- May create incentives to stay below the threshold
- Reduces total coverage
Example: Singapore's carbon tax applies only to facilities emitting more than 25,000 tons of CO2e per year. This covers about 80% of national emissions while including only about 40-50 facilities. Smaller emitters are not subject to the tax.
The Coverage vs Complexity Trade-off
There is a fundamental trade-off between coverage and complexity:
Broader coverage:
- More emissions priced
- More cost-effective reductions
- Greater environmental impact
- BUT: More entities to monitor, more complex measurement, higher administrative costs
Narrower coverage:
- Fewer entities, simpler administration
- Lower compliance costs for business
- BUT: Fewer emissions priced, less effective, potential for leakage between covered and uncovered sectors
It is like a fishing net. A net with smaller holes (broader coverage) catches more fish but is harder to handle. A net with bigger holes (narrower coverage) is easier to use but lets more fish through.
Practical Approach: Start Narrow, Expand Over Time
Many jurisdictions follow a phased approach:
Phase 1: Cover major fuels and large emitters using existing infrastructure.
Phase 2: Expand to additional fuels and sectors as capacity develops.
Phase 3: Add non-CO2 gases and harder-to-measure sources.
Phase 4: Reduce exemptions as competitiveness concerns are addressed.
This allows the system to grow organically while building experience and acceptance.
Key Questions for Defining the Tax Base
When designing a carbon tax, ask:
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What share of national emissions will be covered? Aim for at least 60-70% to ensure meaningful impact.
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Which fuels are included? Start with major fossil fuels; consider biofuels and waste later.
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Which gases are included? CO2 is essential; add others as measurement capacity allows.
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What sectors are covered? Power and transport are easiest; industry and buildings follow.
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What exemptions are necessary? Minimize exemptions but address legitimate competitiveness and fairness concerns.
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What thresholds apply? Balance administrative simplicity against coverage.
Looking Ahead
In the next lesson, we will explore where in the supply chain to apply the carbon tax: upstream at the fuel source, midstream at distribution, or downstream at the point of emission.