The transport sector is responsible for various types of environmental pollution, mainly from the use and combustion of fossil fuels.
One of those types of pollution is the emission of greenhouse gases like carbon dioxide, which contributes to a changing climate.
In this guide, we outline which forms of transport might pollute the most in terms of CO2 emissions.
(Note – the emission of air pollutants that contribute to outdoor air pollution is a separate issue to the emission of greenhouse gases that contributes to climate change and global warming. For a fuller range of environmental effects of transport, such as air pollution, you can read this guide.)
Summary – Which Form Of Transport Pollutes The Most In Terms Of Greenhouse Gas Emissions?
Along with electricity and energy generation, industrial activity, agriculture and other major sectors, the transport sector is responsible for a significant portion of the world’s greenhouse gas emissions
There’s different types of transport to consider though
On the road, there’s mainly passenger vehicles and trucks.
But, there’s also water (ships), air (planes), rail (trains), and other types of transport such as human transport (such as bikes, and even walking)
There’s even passenger vs freight vehicles to consider
When measuring total global greenhouse gas emissions, road transport comes out as the main emitter in total – specifically, cars and trucks
But there’s other ways to measure transport emissions such as emissions per passenger, per unit of distance travelled.
The picture can change when looking at different measurables
A large train carrying over 150 people might be one of the most carbon efficient forms of transport in terms of emissions per passenger, per unit of distance travelled.
On the other hand, some planes and ships, as well as cars carrying 1 or 2 people might be some of the least efficient
There are also other factors and variables that can change the emissions picture, which we explain below
For example – what type of fuel is the vehicle using (fossil fuel, or electric renewable energy based fuel?), or, how efficient with fuel is the vehicle (can be based on technology, weight, etc.)?
What is not considered in the numbers below is transport such as pedal bikes that don’t use fossil fuels – these modes of transport are assumed to be a much cleaner form of transport for obvious reasons
Total Global Emissions From Different Types Of Transport
In terms of total global emissions, the forms of transport that emit the most GHGs are:
- Cars – 40%
- Trucks – 34%
- Planes – 11%
- Boats – 11%
- Trains – 4%
So, we can see that road transport, and in particular, personal transport vehicles are the main emitters (although there’s freight vehicles to consider too in addition to personal use vehicles).
An extract from a guide on the industries that emit the most GHGs and Carbon Dioxide:
In the United States …
The majority of greenhouse gas emissions from transportation are carbon dioxide (CO2) emissions resulting from the combustion of petroleum-based products, like gasoline, in internal combustion engines.
Over 90 percent of the fuel used for transportation is petroleum based, which includes gasoline and diesel.
The largest sources of transportation-related greenhouse gas emissions include passenger cars and light-duty trucks, including sport utility vehicles, pickup trucks, and minivans.
These sources account for over half of the emissions from the transportation sector.
Forms Of Transport That Emit The Most Greenhouse Gases – Per Passenger, Per Unit Of Distance
Another way to measure greenhouse gas emissions for the different modes of transport is emissions per passenger, per one kilometre carried.
The numbers below are for grams of CO2 emissions, per passenger kilometre.
- Trains (carrying a 156 passenger average) – produces 14 gCO2 (per passenger kilometre)
- Small Car (carrying 4) – 42 gCO2
- Big Car (carrying 4) – 55 gCO2
- Bus (carrying 12.7) – 68 gCO2
- Motorbike (carrying 1.2) – 72 gCO2
- Small Car (carrying 1.5) – 104 gCO2
- Big Car (carrying 1.5) – 158 gCO2
- Plane (carrying 88) – 285 gCO2
- Ship – 245 gCO2
Based on these numbers, what we see is that trains and full small cars have the lowest emission rates, whilst ships, planes, and cars with 1 or 2 people capacities in them have the highest emission rates.
With this method of measurement, it’s clear that the number of people a mode of transport can carry helps lower the emission rates by averaging out total emissions amongst all passengers.
But, these are averages/estimates, and should only be taken as a rough guide.
There are also different factors you have to account for with different types of transport that can change the emission numbers and how emission friendly a certain mode of transport is.
Different Factors & Variables That Can Affect Emissions From Transport
There a both general, and specific factors and variables that can affect the emission totals and rates from different modes of transport.
Some of those include:
Aviation & Air – the length of the flight. Longer flights can be more efficient because the take off energy is averaged out with more distance travelled. But, shorter flights may lead to less total emissions
Cars – variables that lead to different emission rates are the type of fuel used, emission regulations and laws in an area, the brand, model and features of the car, and how fuel efficient the car is (technology and weight of the car might play a role here). The total number of cars on the road also matters – there is little point in making cars more efficient fuel wise if there will be more cars on the road being used in total. The total distance being driven by all drivers also matters in addition to the total number of cars on the road … are people driving longer distances, or shorter?
Buses – inner city buses emit greater rates of GHGs compared to outer city long distance buses. How often a bus has to stop, and traffic jams, detours, and pit-stops can all impact emission rates and efficiency.
Trains & Rail – commuter trains emit at a lower rate than long distance trains. Also, detours, and stop-overs add to total emissions. How many people does the train hold in total?
Shipping & Delivery Vehicles – Delivery vans and trucks tend to have lower emission rates than big rig transport boats and vessels. When measuring by kg of CO2 per Ton-Mile, air cargo is the worst emitter, followed by trucks, trains, and sea freight last.
Other factors that can impact emissions from vehicles of all types might include:
The type of vehicle – conventional internal combustion engine, electric vehicle, hybrid vehicle, hydrogen vehicle
The type of fuel the vehicle runs on
What energy source the electricity is sourced from – coal, natural gas, renewable energy etc.
How fuel efficient the vehicle is
The weight of the vehicle
Whether the vehicle has technology or features fitted or installed that decrease emissions (tailpipe, lighter weight car, regenerative braking etc.)
How often a vehicle is stopping and starting, the distance it covers, and so on
How many passengers the vehicle can hold, and how many it carries on average
Total distance covered by all vehicles
Total number of individual vehicles on roads, in the air, in the water, and so on
Greenhouse Gas Emissions From Different Energy & Fuel Sources
It’s important to note that, apart from the type or mode of transport, the type of energy or fuel source used also contributes to emission rates.
For example, coal, gas, motor gasoline, aviation gasoline, renewables and other energy and fuel sources all have potential to emit GHGs at different rates and with different intensities.
This will become even more important to consider in the future when electric vehicles need an energy source.
Read more about the different energy sources and their emission rates in this guide.
Potential Solutions For Reducing Emissions In The Transport Sector
A few key solutions might include:
- Public transport rails that run parallel with main commuter roads
- More people walking and biking – living car free for one year is estimated by some sources as one of the highest impact actions individuals can take to reduce their own carbon footprint
- Less individual/private use vehicles on the road carrying one or two people, and more use of passenger efficient forms of transport
- Less total vehicles being in use, and less overall miles being travelled by all vehicles
- Using cleaner and/or more efficient fuels for transport instead of carbon emission heavy fossil fuels
Whilst some may point to electric vehicles being the answer, the problem with electric vehicles is that they have their own set of problems, and, if that electricity is coming from a fossil fuel source, there are still greenhouse gas emissions. Hybrid cars could provide some type of answer in the short term.
When it comes to addressing emissions specifically for cities (as opposed to towns and rural areas), various sources note that mass transit, the reduction of single person vehicles, and reducing consumption (as opposed to relying on electric vehicles), are all key solutions to focus on.
Other Environmental Emissions & Issues Arising From Transport
Note that CO2 emissions are just one part of the equation when it comes to transport pollution.
There’s also regular air pollutant emissions from transport via particulates, carbon monoxide, ozone and other air contaminants that decrease air quality and can impact human health.
The potential solutions for reducing air pollutant emissions from vehicles are similar to those suggested for reducing greenhouse gas emissions from vehicles (as both result from the burning of fossil fuels as a main source of the emissions and pollution)