Table of Contents
Introduction
Transport is one of the largest and fastest growing sources of energy use and greenhouse gas emissions worldwide. It connects people, goods, and services, but it also relies heavily on fossil fuels. Understanding how and why transport emits so much is essential before exploring renewable options and cleaner mobility solutions.
This chapter introduces the main types of transport emissions, what drives them, and how they vary by mode of transport and region. It sets the stage for later chapters that focus on specific renewable and low carbon alternatives.
Energy Use In Transport
Most modern transport energy comes from oil products such as gasoline, diesel, jet fuel, and marine fuel oil. These are high energy density fuels that are easy to store, transport, and burn in engines, which historically made them very attractive. As a result, the transport sector has become one of the most oil dependent parts of the global energy system.
Transport energy demand covers several modes. Road vehicles include cars, buses, trucks, motorcycles, and vans. Aviation covers domestic and international flights. Shipping covers inland waterways and ocean going vessels. Rail transport uses both electricity and diesel, depending on the country and the level of electrification. There are also smaller segments such as off road vehicles and two and three wheelers in many developing countries.
Because most of this energy currently comes from burning fossil fuels in engines, transport is a major contributor to both climate change and local air pollution.
Greenhouse Gas Emissions From Transport
The main greenhouse gas from transport is carbon dioxide, CO$_2$, released when carbon in fuel is burned. The relationship between fuel consumption and CO$_2$ emissions is straightforward. Each unit of fuel contains a certain amount of carbon, which almost entirely converts to CO$_2$ when combusted.
In simplified form, this relationship can be expressed as:
$$
\text{CO}_2\ \text{emissions} = \text{Fuel use} \times \text{Emission factor}
$$
Fuel use can be measured in liters, kilograms, or energy units such as megajoules or kilowatt hours. The emission factor describes how much CO$_2$ is released per unit of fuel. For example, diesel has a slightly higher carbon content per liter than gasoline, so its emission factor is higher per liter, although per unit of energy they are similar.
For fossil fuels, CO$_2$ emissions are directly proportional to fuel consumption, given by:
$$
E_{\text{CO}_2} = F \times EF
$$
where $E_{\text{CO}_2}$ is CO$_2$ emissions, $F$ is fuel consumed, and $EF$ is the fuel specific emission factor. Burning more fuel always means more CO$_2$.
Transport also emits smaller amounts of other greenhouse gases such as nitrous oxide (N$_2$O) from engines and air conditioning gases from cooling systems, but these are usually much less important than CO$_2$ from the fuel itself.
Direct Versus Indirect Emissions
Transport emissions can be thought of as direct or indirect. Direct emissions are those released from the vehicle itself, for example from the exhaust pipe of a car, truck, or aircraft during operation. For most conventional fossil fuel vehicles, almost all greenhouse gas emissions occur at this direct stage.
Indirect emissions arise from producing and delivering the energy carrier or vehicle. For example, if an electric vehicle is charged with electricity generated in a coal fired power plant, then the emissions occur at the power plant, not from the vehicle. In this case, the car has zero direct exhaust emissions, but driving it still has an associated carbon footprint linked to the electricity mix.
There are also upstream emissions associated with extracting, refining, and transporting oil, gas, or coal that eventually become transport fuels. While these upstream emissions are usually smaller than tailpipe emissions, they can still be significant, especially for fuels from high impact sources.
Modes Of Transport And Their Emissions
Different modes of transport contribute in different ways to total emissions. Road transport is usually the largest source. Cars and light duty vehicles are numerous and used daily, often with low occupancy, for example a single person in a multi seat car. Heavy duty trucks carry freight over long distances and often use fuel intensive diesel engines. Together, passenger and freight road transport typically account for the majority of transport related CO$_2$ emissions globally.
Aviation has a smaller share of total transport trips but a very high energy use per kilometer traveled, especially per passenger on long haul flights in certain seating classes. Aircraft burn large amounts of high energy jet fuel and operate at high altitudes where some non CO$_2$ effects can influence the climate system. As incomes rise, demand for air travel tends to increase quickly, which pushes aviation emissions upward.
Shipping is the backbone of global trade and carries a vast share of international freight by volume. Ships are relatively energy efficient per tonne kilometer, but they travel long distances and use heavy fuels in large engines. As a result, international shipping contributes a substantial and growing share of global emissions.
Rail transport, especially when electrified, can be relatively low carbon per passenger or per tonne kilometer, depending on the electricity mix and loading levels. Diesel rail services can still emit significant CO$_2$, but in many regions rail has lower emissions per unit of transport provided than cars or trucks.
Non motorized modes such as walking and cycling do not directly emit fossil CO$_2$ and are considered zero emission at the point of use, aside from minor indirect emissions from infrastructure and equipment production.
Emissions Intensity And Key Metrics
To compare the climate impact of different transport options, it is not enough to look only at total emissions. It is also useful to consider emissions intensity. This describes how much CO$_2$ is emitted per unit of transport service, for example per passenger kilometer or per tonne kilometer.
Passenger kilometer represents moving one passenger by one kilometer. Tonne kilometer represents moving one tonne of freight by one kilometer. If $E$ is the total CO$_2$ emitted, and $D$ is the total passenger kilometers or tonne kilometers delivered, then emissions intensity $I$ can be written as:
$$
I = \frac{E}{D}
$$
This way of looking at emissions allows comparison between modes even when they carry very different numbers of people or amounts of freight. A crowded bus may emit more CO$_2$ per kilometer than a small car, but if the bus carries many more passengers the emissions per passenger kilometer can be much lower.
Emissions intensity is calculated as:
$$
I = \frac{\text{Total emissions}}{\text{Transport service delivered}}
$$
Lower $I$ means a cleaner mode per passenger kilometer or per tonne kilometer, even if total emissions are high.
Important factors that affect emissions intensity include vehicle efficiency, fuel type, occupancy or load factor, driving behavior, speed, and congestion. Later chapters on efficiency and specific transport technologies explore these aspects in more detail.
Air Pollution And Health Impacts
Transport emissions are not only a climate issue. Combustion in engines produces air pollutants that harm human health, particularly in cities and along major roads. These pollutants include nitrogen oxides, particulate matter, carbon monoxide, and volatile organic compounds.
Many of these pollutants contribute to smog and can trigger or worsen respiratory and cardiovascular diseases. Diesel engines in particular have been a focus of attention because of fine particle emissions close to where people live and work. Even as engines and fuels become cleaner in terms of pollutants, transport growth can still lead to significant local air quality challenges.
Greenhouse gas emissions and air pollutant emissions are related but not identical. Some measures that reduce fuel consumption and CO$_2$ also reduce air pollutants, but others may require specific technologies and regulations focused on air quality. Understanding both climate and health impacts is important when assessing the benefits of changes in the transport sector.
Global Trends In Transport Emissions
Transport emissions have grown steadily with economic development, urbanization, and globalization. As incomes rise, people tend to travel more, own more vehicles, and purchase more goods that travel through global supply chains. While engines and vehicles have generally become more efficient, the growth in demand has often outpaced these efficiency improvements.
There are important regional differences. In many industrialized countries, car ownership is already high and growth in emissions may be slower or even stabilize, especially where policies support more efficient vehicles, public transport, and alternative modes. In many emerging economies, rapid motorization and expansion of road networks can lead to very fast growth in fuel use and emissions.
Aviation and shipping emissions are strongly linked to international trade and tourism. These sectors are more difficult to address within national emission inventories and policy frameworks, because their activities cross borders and international waters. As a result, international cooperation is important in addressing emissions from these modes.
Urban areas concentrate both transport demand and its impacts. Congestion, local air pollution, and exposure to traffic emissions are often highest in cities. At the same time, cities can provide opportunities for lower emission systems, for example through dense public transport networks, walking and cycling infrastructure, and integrated land use planning. These aspects are explored in later chapters on sustainable mobility and urban design.
The Role Of Behavior And Systems
Transport emissions are shaped by more than just vehicle technology and fuels. Patterns of behavior, such as how often people travel, how far they go, and which modes they choose, play a significant role. System level decisions about where homes, workplaces, and services are located, and about how streets and public spaces are designed, also influence demand for travel and the balance between private vehicles and collective or active transport.
For example, a compact city with mixed land uses can shorten daily trips and make walking, cycling, and public transport more attractive. In contrast, a low density city with separated residential and commercial zones can lead to long car dependent commutes. Freight systems are similarly shaped by logistics choices, warehouse locations, and delivery practices.
These structural and behavioral aspects interact with technology and energy choices. Even a very efficient vehicle will generate substantial emissions if used very heavily and powered by fossil fuels. Conversely, demand reduction and modal shifts can significantly lower total emissions even before changing the underlying technology of vehicles.
Why Transport Emissions Are Hard To Address
Transport emissions present several particular challenges. First, the existing vehicle fleet is large and long lived. Cars, trucks, ships, and aircraft can remain in use for many years or even decades. Changes in new vehicle technologies take time to transform the overall fleet.
Second, the energy density of liquid fuels gives them advantages for certain applications, especially long distance aviation and shipping, where alternatives are still developing. Replacing these fuels at scale requires major infrastructure changes and new fuel production systems.
Third, transport involves billions of individual decisions by households and firms. Policy measures need to influence these choices across many actors and contexts, which can be complex and politically sensitive. At the same time, many people depend on affordable transport for basic needs and economic participation, so equity and access must be carefully considered.
Despite these difficulties, transport is central to achieving climate and sustainability goals. Reducing emissions from the sector will require a combination of strategies that affect energy demand, modal choice, vehicle technologies, and fuels. Later chapters on electric vehicles, biofuels, renewable shipping and aviation fuels, and integrated mobility systems explore these strategies in depth.
Summary And Link To Renewable Options
Transport is a major and growing source of global greenhouse gas emissions, driven largely by the combustion of fossil fuels in road vehicles, aircraft, and ships. Emissions can be understood in terms of total quantities, emissions intensity per passenger or tonne kilometer, and the distinction between direct and indirect sources.
Transport emissions also contribute significantly to local air pollution and health impacts, especially in urban areas and along busy corridors. Global trends show rising demand for mobility and freight, with regional variations and particular challenges for international modes like aviation and shipping.
Because transport is so dependent on fossil fuels, it is also one of the key sectors where renewable energy can transform the emissions profile. Electric mobility, sustainable biofuels, renewable based hydrogen, and changes in urban design and transport systems all play important roles. The following chapters explore these renewable and low carbon pathways for different modes of transport and show how they can collectively reshape the sector toward a more sustainable future.