Gases that retain heat in the atmosphere are called greenhouse gases. They refer to a sum of seven gases that can be classified in 4 groups: carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O) and fluorinated gases. Carbon dioxide accounts for over 80% of greenhouse gases and is therefore the most commonly referred to greenhouse gas (GHG). Greenhouse gases that lead to global warming are mainly caused by burning fossil fuels such as coal, oil and gas in energy-consuming industrial processes or the mobility sector.
Carbon dioxide (CO2)
Carbon dioxide (CO2) is the primary greenhouse gas emitted through human activities. Although it is naturally present in the atmosphere as part of the earth’s carbon cycle, human activities alter the carbon cycle by adding more CO2 to the atmosphere while decreasing the number of natural sinks, like forest or soils, that remove and store CO2.
Carbon dioxide equivalent (CO2e)
A common way to compare greenhouse gases is the carbon dioxide equivalent (CO2e). It is a metric based on each greenhouse gas’ global-warming potential (GWP) that converts amounts of other gases to the equivalent amount of carbon dioxide with the same GWP. For example, methane has a GWP of 25, meaning that its GWP is 25 times as high as that of CO2. In comparison, nitrous oxide has a GWP of 298 and sulfur oxides even of 14,800 to 22,800.
Brander & Davis (2012)
Transport emissions refer to all pollutants (greenhouse gases and particulate matter) that are emitted in conjunction with freight and passenger transportation. Transport accounts for more than one quarter of all greenhouse gas emissions in the European Union. Road transport makes up the greatest share with 62% of freight transport emissions, followed by maritime (27%) and aviation (6%). In general, maritime and rail transportation are seen as greenest transport modes, while aviation and road transport have higher relative greenhouse gas emissions per distance traveled.
Scope 1, 2, 3
Emissions are divided into three parts by the Greenhouse Gas Protocol depending on their source.
Scope 1 includes all direct greenhouse gas emissions from sources owned or controlled by the organization.
Scope 2 comprises the indirect GHG emissions that result from electricity, steam and heat purchased and used by the organization.
Scope 3 covers all other indirect GHG emissions resulting from upstream and downstream business activities such as transportation that are not directly owned or controlled by the reporting entity. Scope 3 emissions usually make up the greatest share of the organization’s carbon footprint.
Especially scope 3 emissions are difficult to calculate and allocate correctly due to global supply chain networks with many different stakeholders. Reports from Smart Freight Centre, the Carbon Disclosure Project and Science-based targets initiative give practical guidance on how to account for scope 3 emissions.
Greenhouse Gas Protocol,
Smart Freight Centre (Registration required
The carbon footprint is a measure of the total amount of emissions that is caused by an activity or organization or is accumulated over the life stages of a product. The measurement includes all direct and indirect emissions (Scope I-III) to account for completeness and accuracy. Similarly, the terms corporate footprint, product footprint and transport footprint have established.
Wiedmann, Minx (2007)
Carbon intensity is the level of CO2 emissions per unit of a specific activity or an industrial production process. It is used to compare the environmental impact of different activities or of the same activity in different execution variations. A well-known example of a carbon intensity measure is grams of CO2 per kilowatt per hour of energy produced that is used to compare different means of energy production such as coal, gas and wind. Carbon intensities can also be used to analyze the environmental impact of companies or supply chains. The measurement is often expanded to CO2 equivalents to embed all greenhouse gases into the analysis and referred to the transport volume in tonne-kilometre or twenty-foot-equivalent units (TEU).
Hoffmann, Busch (2008)
Increased efficiencies in the usage of energy, raw material and water enhance sustainability, but also reduce product, process or service costs, which can in turn raise consumption due to lower selling prices. Hence, original savings are partly or even fully cancelled out due to increasing demand. This phenomenon is known as the rebound effect.
Decarbonization means reducing, and ultimately eliminating CO2 from recurring activities that are powered or enabled by burning fossil fuels. The goal is to decrease the carbon intensity to net zero. This requires a switch to fully renewable energy sources or active capturing of the equivalent amount of emissions from the atmosphere.
The term negative emissions is used by climate scientists of the Intergovernmental Panel on Climate Change (IPCC) as practices or technologies that remove CO2 from the atmosphere. Two main types of negative emissions are distinguished: enhancing existing natural processes that remove CO2 from the atmosphere (e.g. afforestation and reforestation or the increase of other ‘carbon sinks’) or using technological solutions that capture CO2 directly from the ambient air. Commonly used synonyms of negative emissions are carbon dioxide removal or greenhouse gas removal. The term removal can be confusing in this context as only the chemical state of carbon-dioxide molecules change.
Black carbon is the most strongly light-absorbing component of particulate matter. It is formed by the incomplete combustion of fossil fuels and biomass and is emitted in form of fine particles. Climatic influences of black carbon are its strong absorption of light, both directly and indirectly through the reduction of snow and ice reflectivity, leading to increased temperatures and accelerated ice and snow melt. After carbon dioxide, black carbon has the second biggest impact on climate forcing in the atmosphere. Black carbon typically remains in the atmosphere only for days to weeks, until it returns to the earth’s surface through rain.
An emission factor is a representative value intended to estimate the amount of pollutant released to the atmosphere based on an underlying activity. Emission factors are usually expressed as the weight of pollutant divided by a unit weight, distance, volume, or duration of the related activity (e.g. kg of CO2 emitted per kWh of natural gas). In most cases , emission factors are averages of all available data of sufficient quality and assumed to represent long-term averages for all facilities in the source category. Widely used transport emission factors are released by the
GLEC Framework and
Clean Cargo Working Group.
Net zero emissions
The term net zero emissions refers to an achievement of a balance between CO2 sources and sinks, meaning that the amount of CO2 released into the atmosphere must equal the amount that is removed. The underlying implication of this ‘carbon neutrality’ is that the concentration of CO2 in the atmosphere would slowly decline until CO2 emissions from human-related activities can be redistributed and absorbed by the land biosphere and the oceans. This would result in a near-constant global temperature over many centuries and hence an end to global warming.
A CO2 price is a price that companies have to pay for the emission of CO2, either in form of a carbon tax or tradeable emission certificates. Carbon pricing is seen as the most efficient way for nations to reduce greenhouse gas emissions and is applied European-wide for several industries through the EU emissions trading system (ETS). Germany and several other countries launched additional national pricing mechanisms for the transportation sector with a fixed price that gradually increases over time.
Methane (CH4) accounts for around 10% of all greenhouse gas emissions from human activity. Human activities emitting methane include the raising of livestock, leaks from natural gas systems or waste management in form of landfills. Its comparative impact on global warming is 25 times bigger than CO2.
Sulfur oxides (SOx)
Sulfur oxides compound of sulfur and oxygen molecules and are predominantly found in form of sulfur dioxide (SO2). It is mostly produced from combustion of fossil fuels. SO2 concentrations are especially relevant in the maritime shipping industry.
Since 1 January 2020, sulphur emissions from fuel oil used by maritime vessels is regulated to not surpass 0.50% m/m (mass by mass) globally by the International Maritime Organization (IMO). Ships sailing in a Sulphur Emission Control Area (SECA) cannot use fuels with more than 0.1% of sulphur.
Nitrogen oxides (NOx)
Nitrogen oxides refer to a binary compound of nitrogen and oxygen, or a mixture of such compounds. In the context of environmental damage, nitrous oxide (N2O) is known for an having a global-warming potential that is almost 300 times greater than that of CO2. It makes up about 6% of all greenhouse gas emissions from human activities, mostly emitted through agricultural soil management activities. The greatest problem with N2O is that its molecules stay in the atmosphere for an average of 114 years before being destroyed through chemical reactions or removed by a sink.
For the maritime shipping industry, the International Maritime Organization (IMO) introduced NOx Emission Control Areas (NECAs) in the North Sea and Baltic Sea. These will apply to vessels built after the start of 2021 and reduce the amount of NOx emitted by operations of the vessels to thresholds determined by the engine's speed
Fluorinated Gases (HFC/HFO)
Hydrofluorocarbons (HFCs), perfluorocarbons (PFCs), sulfur hexafluoride (SF6), and nitrogen trifluoride (HFCs) are synthetic, powerful greenhouse gases that are emitted from a variety of industrial processes. Unlike most other greenhouse gases, fluorinated gases have no natural source, but exclusively come from human-related activities.
The vast majority is emitted through their use as refrigerants, for example in air conditioning systems in vehicles and buildings. Fluorinated gases have very high global warming potentials of up to 22,800 and are the most potent and longest lasting type of greenhouse gases emitted by human activities. Their reduction especially in cold chain transports (food, pharmaceuticals etc.) is very important.
Logistics site emissions
Logistic sites play a connecting role within transport chains. The term ‘logistic site’ refers to all sites combining different transport legs such as warehouses, cross-docking sites or terminals. Although often being forgotten when referring to an ecological assessment of logistic activities, logistic site emissions equal around one quarter of the logistics transport emissions.
Carbon offsetting and compensation
Carbon offsetting (or compensation) means paying a third party to cut or absorb an equivalent amount of emissions from a dedicated activity (e.g., transport) that set emissions free. It is important to precisely measure or calculate the amount of emissions that is compensated for.
Typical carbon offsetting projects are investments in reforestation or renewable energies and are often located in developing countries. Carbon offsetting is an important part of many organizations’ sustainability programs, but is also becoming increasingly popular for individuals, e.g. to compensate for personal activities such as flight travels.
Well-to-wheel vs. well-to-tank vs. tank-to-wheel
Also referred as direct and indirect emissions, differentiates between the origin of the energy used for propulsion in transport activities.
Well-to-tank (WTT): Emissions caused by the provisioning of the primary propulsion energy. Emissions caused by transforming primary energy (sunlight, biomass, oil, coal, nuclear etc.) to consumable energy for vessels or vehicles (diesel, kerosine, hydrogen).
Tank-to-wheel (TTW): Emissions caused by converting the vehicle or vessel fuel to propulsion, e.g., burning diesel.
Well-to-wheel (WTW): holistic approach, which considers both, well-to-tank as well as tank-to-wheel emissions, and therefore increases the comparability of fossil and renewable fuels, e.g., considering the origin of electricity for battery-electric-vehicles to reflect the share of coal or fossil gas on the local electricity-grid.