CO2 Emissions for Energy and Transportation

It's hard to visualize the impact of CO2 emissions because you can't even see it. But it's there, it's happening, and you're contributing too.

It’s hard to visualize the impact of CO2 emissions because you can’t even see it. But it’s there, it’s happening, and you’re contributing too.

Executive Summary: As far as carbon emissions go, driving solo in your car is about as bad as you can get, and all forms of public transit (airline, bus, train) beat it handily. But if you carpool with a total of four people in your car, you are competitive with the bus and airliner (the train is still more efficient). And muscle-powered transit (walking or bike riding) trumps them all, with zero net emissions.

The table below shows typical quantities of carbon dioxide that is emitted by common energy sources and various modes of transportation. These numbers are approximate and cannot reflect specific circumstances. For example, electricity is produced many ways and your provider most likely uses a combination of nuclear, coal, oil, gas, solar, wind, and possibly even tidal energy sources. The proportion amongst these sources likely varies depending on several factors including load, time, availability and cost.

Even if these numbers are not completely accurate, they can give you an idea of the CO2 emissions of some common activities. You may be able to use this information to help reduce your carbon footprint.. However, as with any statistics, these numbers must the interpreted correctly to understand them.

For example, suppose that you can travel 10 kilometers to work by bicycle, car, bus, or train. Which choice has the lowest carbon impact?

According to the table, riding your bicycle will result in 210 grams CO2 emission. But the carbon you exhale comes from carbon that was taken out of the air by plants through photosynthesis – it’s a closed-loop system with no net addition of carbon. So by riding your bicycle, you are only returning to the atmosphere the carbon that was originally there – the net increase is zero.

Note the distinct difference: burning fossil fuel releases carbon to the atmosphere that was formerly trapped in the fuel. Release of carbon by respiration recycles carbon that was already free. In the natural world, carbon is continuously cycled between plants and animals, using the atmosphere as a transfer medium.

What about taking the bus or train? Doesn’t it matter how full the bus (or train) is? What if you are the only passenger? Does the entire train still emit only 15-60 g/km? Of course not: an average passenger load is assumed, and the bus (or train, or airline) emissions are computed on a per capita basis for that average load.

Again, what’s important is the marginal increase in CO2 emitted for each of your choices. For example, if you rode your bike 10 km, even though you exhale 210 grams of CO2 there is no net increase. If you drove your car, you would release about 3400 grams CO2 (for a car that achieves 30 MPG). Since your car releases carbon that was formerly trapped in fossil fuels, the 3400 grams is a net increase.

But what if you rode the bus or train? Presumably, these are regularly-scheduled services that run whether you ride them or not. So the marginal increase of CO2 emitted if you take the bus or train is zero – the bus and train will emit the same amount of CO2 independent of your ridership. In effect, it is the same as riding your bike.

There is no question that the worst choice is to drive your car, and that the best way to reduce your carbon emissions would be to not drive your car and either ride either your bike, the bus, or the train.

Here’s a little factoid: suppose you took a round-trip airline flight from California to the East Coast; about 10000 km. How much CO2 emissions could be attributed to your flight? At the low estimate, you’re responsible for 100g/km, so your 10000 km trip results in 1000 Kg of CO2. On a per-capita basis, one round-trip cross-country flight emits one metric ton of CO2.

CO2 Emissions
No.Use CO2 (Metric) CO2 (English) Comments (Notes are listed below the table)
1Electricity 600 g/KWHr 1.3lb/KWHr Note 1
2Electricity 0.166 g/KJ 0.006 oz/KJ" 1 Joule is the work needed to lift 100g 1 meter; Note 3
3Natural Gas 55 g/SCF 2 oz/SCF 1 SCF=1 Standard Cubic Foot (industry standard); Note 3
4Gasoline 0.07 g/KJ 0.0025 oz/KJ Energy equiv of gas = 44 MJ/kg 1.2e5 KJ/gal; Notes 3,7,9
5Airline 100-175 g/km/person 5.6-10 oz/mi/person Emissions per person; Note 4
6Train 15-60 g/km/person 0.84-3.4 oz/mi/person Emissions per person; Note 4
7Car (30 MPG) 340 g/km 1.2 lb/mi One person in car; Note 5
8Car 271 g/km 15.3 oz/mi From Note 8; the car's MPG is not given, but result is similar to 340 g/km for 30 MPG car
9Bus 100 g/km/person 5.6 oz/mi/person Emissions per person; Note 8
10Bicycle 21 g/km 1.2 oz/mi Note that respiration does not add CO2 to the atmosphere since it only recycles what was originally there; Notes 3, 8
11Hot Water (Electric) 50 g/liter 6.7 oz/gal Note 6
12Hot Water (Gas) 22 g/liter 3.0 oz/gal Notes 6, 7

Notes

  1. “Carbon Dioxide Emissions from the Generation of Electric Power in the US”, DoE and EPA, http://www.eia.gov/cneaf/electricity/page/co2_report/co2report.html
  2. 1 KWHr = 3.6e6 Joules or 3600 KJ
  3. http://cdiac.ornl.gov/pns/faq.html
  4. http://www.guardian.co.uk/environment/2010/may/24/kick-addiction-flying
  5. One gallon of gasoline weighs about 6 pounds or 2.7 kg; it is combined with air in a 1:6 ratio, most of which is emitted as CO2. So each gallon of gasoline produces 36 pounds (16 kg) of CO2. (http://www.fueleconomy.gov/feg/co2.shtml) If a car gets 30 mpg (about 50 km/gal) it produces 1.2 lb (0.55 kg) CO2/mi (340 g CO2/km).
  6. Suppose the ambient water temperature is 20ºC (68ºF) and our hot water thermostat is set to 70ºC (158ºF). Each liter of water takes 50 Kcal (209 KJ) to heat. Therefore, if we assume 100% efficiency in the water heater and referring to the CO2 emitted by electricity production, heating one liter of water produces 0.166 g/KJ * 209 KJoules = 35 g/l. More realistically, assume about 70% efficiency for 50 g/l, or 190 g/gal.
  7. One Standard Cubic Foot (SCF) of natural gas has 1.1e6 Joules. If a gas water heater has an efficiency of 50%. Heating one liter of water from 20ºC to 70ºC takes 50Kcal or 209 KJoules or about 0.4 SCF with 50% efficiency. Burning natural gas produces 55g CO2/SCF, so 0.4 SCF produces 22g CO2.. Each liter of hot water produces 22g CO2 or 83 g/gallon.
  8. Bicycling magazine June 2012 page 48 “How Green Are We?” Source: European Cyclists Union.
  9. http://hypertextbook.com/facts/2003/ArthurGolnik.shtml
  10. energy-numbers-04.doc PDF from www.evworld.com/library/energy_numbers.pdf. A 1000 MW power plant daily produces 2.5e14 Joules = 250 TJ. Depending upon its fuel source, it requires varying amount of fuel as shown in the table below. These numbers differ from the number given by the DoE/EPA report cited in Note 1, but are the same order of magnitude.
1000 MW Power Plant: Daily Fuel Usage and CO2 Emission
No.Plant Type Fuel Used Daily Carbon/TJ (1 TJ=10e12J) CO2/TJ CO2/KWHr
1Coal 9000 tons coal 2.5e7 grams 1.5e8 grams 540 grams
2Oil 40000 barrels oil 2.0e7 grams 1.2e8 grams 430 grams
3Gas 1.4e4 SCF natural gas 1.4e7 grams 8.4e7 grams 300 grams
4Nuclear 3 Kg uranium 0 grams 0 grams 0 grams
5Solar Sunlight (About 1 KW/m2) 0 grams 0 grams 0 grams