Transportation accounted for 28% of global final energy demand in 2017. The sector released 8.2 Gt [1 Gt = 1 billion tonnes = 1 trillion kg] of CO2 in 2018 alone, which represents roughly 23% of global CO2 emissions across all sectors the same year. Transportation’s CO2 emissions have risen by 41% from 2000-2018. In addition to CO2, burning fuel also releases toxic pollutants.
Individual car needs are often linked to urban expansion and the proportional weakening of public transport. In other words, as individuals move further away from the city center, a car becomes increasingly necessary. Additionally, as a city expands, more amenities get built far from the city center. So urban populations also find themselves wanting cars. Both these factors help explain why individuals were responsible for 63% of total transportation energy use in 2012.
Freight transport [vs. passenger transport] accounted for the remaining 37% of the transportation sector’s energy use in 2012. Freight planes, boats, trains, and trucks supply us with food, raw materials, and anything else we buy far from home. For equal transportation methods, greater distances imply more fuel consumption and emissions. However, we’ll see that some methods of transport are far more efficient than others.
It should be noted that globalization has some wonderful aspects that allow us to share items worldwide and increase quality of life in many areas of the world. However, the environmental cost is significant. Globalization’s impacts on the environment must be reduced by shortening distances travelled and choosing more efficient transportation methods.
Planes are astounding polluters due to the type of fuel used and the long distances travelled. Interestingly, countries don’t take responsibility for the pollutants that international airliners emit. Instead, international flights are bundled together in a separate category that countries don’t really need to worry about – since emissions aren’t counted in their national totals.
In 2018, international and domestic flights combined emitted 0.93 Gt of CO2. That represents roughly 11% of the transportation sector’s total CO2 emissions the same year. To put this in perspective – those emissions have the same weight as 9.3 billion giant pandas [with 1 panda = 100 kg] being released in the atmosphere.
This has not been getting better. Aviation’s global CO2 emissions in 2018 were roughly 38% higher than they were in 2000.
Planes use kerosene and other lightweight jet fuels [that have high energy density] to power the engines without adding too much weight to the aircraft. Unfortunately, these fuels release more CO2 emissions than standard fuel [by volume of fuel consumed] – but only 7–8% more. This doesn’t mean jet fuel isn’t as bad as everyone makes it sound, it means that we don’t realize how polluting gasoline is. One of the most interesting areas of innovation within the aviation sector is lowering GHG emissions by opting for alternative fuels.
That’s much easier said than done. We use the fuels we use because they have high enough energy densities [i.e. energy per unit of mass] to allow aircrafts to fly. And although hydrogen-fueled planes have received support from governments and aviation companies due to hydrogen’s extremely high energy density and possible eco-friendliness, these types of planes currently face numerous challenges that make them an unlikely option in the near future.
The graph below shows the amount of energy required by different passenger transportation modes – per passenger for an equal travelling distance.
The only reason that planes aren’t that much worse than cars – as far as energy required per passenger-km, is that they can hold many people at once and are often full [that’s why private jets are such terrible polluters]. On the other hand, cars are often half-empty. Also, note that a single plane trip emits far more than a single car trip, but that’s really just due to the difference in distances travelled.
Ships and Boats
Boats are pretty big polluters as well. In 2018, shipping emitted 0.86 Gt of CO2. That represents over 10% of the transportation sector’s total CO2 emissions the same year – equivalent to roughly 8.6 billion pandas emitted.
This has not been getting better. Shipping’s global CO2 emissions in 2018 were nearly 38% higher than they were in 2000.
Additionally, oil spills can occur when ships crash or leak. That can be absolutely devastating to local populations and biodiversity. Media coverage has been present for a few of these oil spills in the past – so we know exactly how terrible these spills are for local health, food, and economy.
As far as international freight transport goes, there’s no question that boats emit much less GHGs per unit weight of cargo than aircrafts. While both boats and planes end up emitting roughly the same amounts of CO2 every year, that has to be put in perspective with the massive quantities of goods that ships transport. For example, maritime shipping accounted for roughly 75% of imported/exported goods [by weight] between the EU and the rest of the world in 2019. Aircrafts were mostly used to transport higher-value products.
If the goal is to reduce the transportation sector’s emissions as a whole – then transitioning away from freight aircrafts will be necessary.
Nonetheless, boats still pollute since they burn fuel [often burning highly polluting residual fuel left after oil refining processes]. Large and heavy ones especially, so it’s no surprise that cruise ships are scrutinized extensively for their environmental impacts.
Trucks and Cars
Trucks and cars emit the bulk of the transportation sector’s emissions. In 2018, passenger and freight road vehicles emitted 6.0 Gt of CO2. That represents over 73% of the transportation sector’s total CO2 emissions the same year – equivalent to roughly 60 billion pandas emitted.
This has not been getting better. Road transport’s global CO2 emissions in 2018 were over 41% higher than they were in 2000.
Heavier vehicles like trucks consume more fuel and emit more GHGs per km than lightweight cars. Other factors like speed can also increase individual emissions. Additionally, while gasoline may not be scrutinized as much as jet fuel, it remains a terrible polluter that nearly releases the same amount of GHGs per kg of fuel consumed.
Road vehicles are high polluters because they are individualized. As such, the easiest thing to do to cut our transportation emissions in half is to carpool with one other person. Divide by three? Two people. This is why buses, although heavier, can emit much less CO2 per passenger-km. Improving shared services like public transit is one of the best ways to ensure cleaner mobility in cities and neighboring areas.
Trains have been one of the cleanest transportation modes for a while, ever since most rail systems abandoned coal. In 2018, rail emitted 0.08 Gt of CO2. That represents just 1% of the transportation sector’s total CO2 emissions the same year – equivalent to roughly 0.8 billion pandas emitted.
This is getting better, despite passenger rail usage almost doubling from 1996-2016. Rail’s global CO2 emissions in 2018 were 2.4% lower than they were in 2000. That’s partly due to around 75% of passenger trains worldwide relying on electricity as a power source in 2016– a considerable increase from 60% in 2000.
Trains are without a doubt the most eco-friendly way to travel long distances. Despite accounting for 7% of freight and 8% of passenger transportation worldwide, rail only accounted for 2% of the transportation sector’s energy demand in 2016. Even for short distances, urban rail is proving very energy efficient.
Unfortunately, rail is still an expensive option in many countries. To counter this, governments can improve policies and subsidies to incentivize rail usage. Greater investments in green, convenient, and affordable public transit is required to reduce the transportation sector’s emissions.
Electric vehicles [EVs] can also reduce the transportation sector’s emissions. EVs run on electricity stored chemically in batteries in the car ‘floor’ [other types of EVs exist, but battery EVs are the most common]. EVs therefore require charging instead of refueling.
In many countries, the lack of EV infrastructure and the length of charging times are problematic for EV owners, so manufacturers are always looking to improve the range of their vehicles.
One option would be to load EVs with more batteries, but that isn’t an optimal solution. EV batteries are extremely heavy for the amount of energy stored [low energy density] compared to gasoline, which explains why EVs already weigh much more than similarly sized combustion vehicles [CVs]. As such, increasing the number of batteries would increase EV weight significantly. While that can effectively improve EV storage capacity, it can also worsen power consumption.
The solution for increasing EV range isn’t in the number of batteries, it’s inside the batteries. Researchers worldwide are working on increasing the energy density and lifetime of batteries. In 2018-2019, the battery energy density in average electric vehicles was 20-100% higher than it was in 2012, while battery costs have decreased by over 85% since 2010. Governments and companies are also working on increasing the number of charging stations along popular travel routes to improve EV infrastructure.
Alas, EVs have much more important problems than range anxiety.
EVs are metal-intensive. For example, an estimated 19 kilotonnes of cobalt, 17 kt of lithium, 22 kt of manganese, and 65 kt of nickel were required to make batteries for EVs sold in 2019. That’s over 1 million pandas-worth of metals.
By 2030, lithium demand for EVs alone is required to increase to at least 365.3 kt per year to align with the Sustainable Development Scenario – much higher than the 17 kt required in 2019. Global supply of lithium across all industries was just 77 kt in 2019.
That’s just for lithium and EVs. Renewables also depend on this metal for energy storage, as do high-tech devices. And unfortunately, similar projections exist for other metals that are critical to battery production – like cobalt, manganese, and nickel.
Now this doesn’t mean that governments and companies won’t be able to meet this projected demand by 2030. However, it means mining activity will boom, which will increase the sector’s energy consumption and pollution.
Growing mining activity will also accelerate the depletion of finite resources. There were ‘only’ 7.2 million EVs on the road in 2019, compared to over 1 billion CVs. The massive increase in mining activity described above is only projected to help EVs reach 13.4% of the total car share by 2030 – up from 0.8% in 2019. If EVs are supposed to replace all CVs in the future, we can expect this increasing mining activity to continue for decades.
The following graph shows how important and metal-intensive batteries are for the upcoming green revolution. The scale at which governments decide to adopt EV technology will have massive implications on mining activity and the speed at which we complete our energy transition.
The potential dirtiness of electricity is another concern for EVs. If coal, natural gas, and oil continue to dominate electricity mixes around the world for the foreseeable future, then setting up EV infrastructures wouldn’t make much sense.
It’s important that individuals look up their local electricity emission factors before purchasing EVs. An emission factor essentially measures how dirty an electricity mix is, in grams of CO2 equivalent emitted per kilowatt-hour of electricity consumed, or ‘g CO2e/kWh’. The ‘CO2e’ or ‘CO2 equivalent’ unit accounts for all GHGs and their greenhousiness, not just CO2.
The following graph shows the life-cycle GHG emissions of mid-size battery EVs and CVs.
The black line above the EV scenarios shows how important clean electricity is to reduce emissions. The top of the line represents an electricity emission factor of 800 g CO2e/kWh, while the bottom represents just 50g CO2e/kWh. For reference, the US national average grid electricity emission factor was around 400 g CO2e/kWh in 2021 – while the EU had an emission factor of 275 g CO2e/kWh in 2019.
It’s important to note that lower fuel-consumption for CVs can also help slash the transportation sector’s emissions significantly. With more research and stronger regulations, affordable fuel-efficient vehicles could become the new ‘norm’ for CVs.
EVs can and will help lower the transportation sector’s emissions. However, metal intensity and life-cycle electricity emissions have to be considered when evaluating EV impacts. While emission factors can often improve over time, the finiteness of metals will not.
One way to minimize the technology’s impacts while maximizing benefits is by implementing it at an appropriate scale. As such, electrifying public transit is an effective solution – as long as policies are also incentivizing individuals to use public transit instead of personal vehicles.
Artificial intelligence [AI] also has the potential to reduce the transportation sector’s emissions. AI could allow our cities to become well-oiled machines, where efficiency rules. This would reduce fuel waste, encourage the use of public transport, and redefine vehicle sharing apps.
Unfortunately, we’re still decades of investments, research, and trials away from driverless vehicles becoming available to the public. Additionally, even if the technology were proven and integrated in our societies, a driverless vehicle wouldn’t magically stop consuming energy. Although fuel consumption and ride-sharing could be optimized with efficient AI software and car-to-car communication, that would barely make a dent in the sector’s total emissions. Especially considering that this type of AI technology would have environmental impacts of its own [e.g. through data storage].