Estimating Global CO2 Emissions from Fossil Fuels

Hello! Welcome to the third lesson in our module on the Global Carbon Cycle.

Introduction

In our last lesson, we quantified the major carbon fluxes and established that fossil fuel combustion is the largest single driver of the increase in atmospheric CO₂, contributing approximately 9.7 GtC per year. This is the number that appears in global carbon budgets and headlines.

Today, we will demystify that number. Our goal is to understand how such a figure is derived by performing our own 'back-of-the-envelope' calculation. This exercise is essentially a large-scale accounting problem: tracking energy consumption and applying conversion factors to estimate emissions. Given your background in economics and statistical analysis, you'll recognize the process of aggregating disparate data sources and dealing with underlying assumptions to arrive at a top-line figure.

By the end of this lesson, you will be able to: Perform a 'back-of-the-envelope' calculation of global CO₂ emissions from fossil fuel combustion using energy consumption data.

1. The Basic Emission Calculation

At its core, calculating CO₂ emissions from burning fuel is a straightforward process based on a simple formula:

Emissions = Quantity of Fuel Burned × Emission Factor

To make this practical for a global calculation, we can refine it slightly:

1. Energy Consumption: We need to know the total amount of energy derived from each major fossil fuel type (coal, oil, and natural gas) globally in a given year.
2. Carbon Content: We need an emission factor that tells us how much carbon (or CO₂) is released per unit of energy for each fuel type. This factor is a fundamental property of the fuel's chemistry.
3. Unit Conversion: We need to ensure all our units are consistent and convert the final result into a standard unit like Gigatons of Carbon (GtC) to compare with our previous lesson.

The fundamental chemical conversion is from the mass of carbon (C) in the fuel to the mass of carbon dioxide (CO₂) produced. Since the atomic weight of carbon is approximately 12 and that of oxygen is 16, the molecular weight of CO₂ is 12 + 2*16 = 44. Therefore, to find the mass of CO₂ produced from a given mass of carbon, we multiply by the ratio 44/12.

Let's formalize this.

As the resource explains, the key is to connect the amount of fuel burned to the amount of CO₂ emitted. For our calculation, we will use a slightly different but equivalent approach:

$\text{Emissions (GtC)}={\sum }_{\text{fuels}}\left(\text{Energy Consumed (EJ)}\times \text{Emission Factor}\left(\frac{{\text{kg CO}}_2}{\text{GJ}}\right)\right)\times \text{Conversions}$

We will perform this calculation for coal, oil, and gas, and then sum the results.

2. A 'Back-of-the-Envelope' Calculation

Let's perform the calculation for a recent year, using approximate data.

Step 1: Gather Energy Consumption Data

The image below shows global primary energy consumption by source. We will use it to estimate the energy derived from coal, oil, and natural gas.

Examine the left-hand graph, "Global primary energy consumption". For a recent year like 2020, please estimate the energy consumed from Coal, Oil, and Gas. The units are in ExaJoules (EJ), where 1 EJ = 10¹⁸ Joules.

Step 2: Find Emission Factors

Next, we need the emission factors for each fuel. These factors vary depending on the specific type of coal or oil, but we can use widely accepted average values for our calculation. These factors represent the kilograms of CO₂ emitted per Gigajoule (GJ) of energy produced.

• Coal: 95 kg CO₂ / GJ
• Oil: 73 kg CO₂ / GJ
• Natural Gas: 56 kg CO₂ / GJ

Notice that for the same amount of energy, coal emits the most CO₂, and natural gas emits the least. This is due to the different hydrogen-to-carbon ratios in their chemical makeup.

Step 3: Calculate Total Emissions

Now, let's combine the data. We need to be careful with our units.

• Energy data is in ExaJoules (EJ). 1 EJ = 10⁹ GJ.
• Emission factors are in kg CO₂ / GJ.
• Our final target is Gigatons of Carbon (GtC). 1 Gt = 10¹² kg.
• The conversion from CO₂ to C is a multiplication by (12 / 44).

Let's calculate the emissions for coal first:

Coal Emissions (GtC):
$160\text{ EJ}\times \frac{10^9\text{ GJ}}{1\text{ EJ}}\times \frac{95{\text{ kg CO}}_2}{1\text{ GJ}}\times \frac{1\text{ Gt}}{10^{12}\text{ kg}}\times \frac{12\text{ GtC}}{44{\text{ Gt CO}}_2}$

$\text{Emissions}=160\times 95\times \frac{10^9}{10^{12}}\times \frac{12}{44}\text{ GtC}$
$\text{Emissions}\approx 15.2{\text{ Gt CO}}_2\times 0.273\approx 4.15\text{ GtC}$

Now, please perform the same calculation for oil and natural gas using the energy figures from Step 1.

Step 4: Sum the Results

Finally, let's sum the emissions from all three sources:

Total Emissions = 4.15 GtC (Coal) + 3.79 GtC (Oil) + 2.29 GtC (Gas) = 10.23 GtC

This result of ~10.2 GtC for the year 2020 is remarkably close to the official figures from the Global Carbon Project, which reported fossil fuel emissions of 9.9 ± 0.5 GtC for that year. This demonstrates that even with simple data, the fundamental relationship between energy consumption and emissions allows for a robust estimate.

3. From Simple Models to Official Inventories

Our calculation was a simplification. Official inventories, like those compiled by the EPA or used by the IPCC, are far more detailed. Your experience with econometrics and statistical modeling will give you an appreciation for the layers of complexity involved in creating a robust, verifiable emissions inventory.

Let's explore some of these complexities.

This reading highlights several factors that refine our simple model:

• Detailed Fuel and Sector Data: Instead of using one number for "coal," inventories use consumption data for different types of coal (e.g., anthracite, bituminous), each with a specific emission factor. Data is also broken down by economic sector (residential, commercial, industrial, transportation, electricity generation), as different sectors use different fuel mixes and technologies.
• Variable Carbon Content: The carbon content of fuels is not constant. Coal from one country can have a different energy and carbon content than coal from another. Crude oil is refined into many products (gasoline, diesel, jet fuel), each with a distinct emission factor. The table in the Engineering Toolbox resource shows this variation.
• Combustion Efficiency: Our calculation assumed 100% of the carbon in the fuel is oxidized into CO₂. In reality, a small fraction might be emitted as carbon monoxide (CO), soot, or other products of incomplete combustion. Official inventories account for this.
• Non-Energy Use: A significant portion of fossil fuels is not burned for energy but used as feedstock for materials like plastics, asphalt, lubricants, and chemicals. The carbon in these products is "stored" or sequestered, at least for the lifetime of the product, and is not immediately emitted. This must be subtracted from the consumption totals to avoid overestimation.

Building a global emissions inventory is a monumental data aggregation and analysis task, involving reconciling data from national energy statistics, import/export records, and industry reports.

Conclusion

In this lesson, you learned how to connect real-world energy consumption data to the abstract figures of global carbon emissions.

Key Takeaways:

• CO₂ emissions can be estimated by multiplying energy consumption for each fossil fuel by a fuel-specific emission factor.
• A 'back-of-the-envelope' calculation using publicly available data on global energy use yields a result remarkably close to official figures.
• Coal is the most carbon-intensive fossil fuel, followed by oil, and then natural gas.
• Official emission inventories are far more complex, accounting for detailed fuel sub-types, variations in carbon content, combustion efficiencies, and non-energy uses of fossil fuels.

In the previous lesson, we identified two main sources of anthropogenic emissions: fossil fuels (~9.7 GtC/yr) and land-use change (~1.1 GtC/yr). Having just analyzed the fossil fuel component, our next lesson will focus on the second part of the equation: describing the methodologies and uncertainties involved in estimating carbon emissions from land-use change.