How a footprint is defined and measured
A footprint measures the resources a population, product or activity consumes and the waste it generates. What a footprint figure means depends on the gases and activities it counts, on whether emissions are reduced or only balanced, and on the method used to calculate it.
What a footprint measures
The ecological footprint measures the area of biologically productive land and water a population needs to produce all the resources it consumes and to absorb the waste it generates, using prevailing technology and resource management.
Greenhouse gas emissions are also simply known as carbon emissions. The primary greenhouse gas is carbon dioxide; the others include methane, nitrous oxide, water vapour and fluorinated gases. A carbon footprint is a measure of the greenhouse gas emissions produced by an organisation, activity, event, product or individual, in units of carbon dioxide. It is usually given as a mass, in kilograms or tonnes, of carbon dioxide, carbon dioxide equivalents or greenhouse gases, and other footprint measures include global hectares.
In 2023 carbon footprinting was not yet fully standardised: its methodologies were still evolving, and its calculations and metrics were not completely harmonised. The Greenhouse Gas Protocol provides recommendations for carbon accounting by companies, organisations, cities and countries.
Carbon neutral, net zero and real zero
Carbon neutrality means a balance between emitting carbon and absorbing it from the atmosphere in carbon sinks, which remove carbon dioxide from the atmosphere and store it. The oceans sequester carbon, but not enough to match emissions, so net zero requires all worldwide greenhouse gas emissions to be counterbalanced by carbon sequestration, ideally more than counterbalanced, to reduce the atmospheric concentration of carbon dioxide.
In 2024 one researcher described the terminology as complicated, often confusing and ambiguous, and thought it still needed a lot of work to refine. By her account, carbon neutral technically refers only to balancing carbon dioxide emissions, not other greenhouse gases, and to balancing them by any means, so that emissions can continue while offsets are bought. By a second definition, carbon neutral targets carbon emissions only and is achieved when they are balanced by carbon removals. By a third, carbon neutrality means balancing an organisation's greenhouse gas emissions against measures to offset them, for example by planting trees.
Net zero covers all greenhouse gas emissions, including carbon dioxide, methane, nitrous oxide and fluorinated gases. It involves cutting greenhouse gas emissions to as close to zero as possible and offsetting only the remaining emissions. It balances the emissions produced against reductions made by more stringent methods, so that emissions are shown to be reduced rather than only offset.
In 2024 the same researcher held that carbon neutral had fallen out of favour and that net zero, though starting to fall out of favour, was still the preferred term. She thought net zero's loss of favour more a matter of public opinion than a formal shift, and whether it would become formal remained to be seen. Real zero, a newer term, called for going beyond balancing emissions to negative emissions. A written account of her November 2024 talk put it differently: carbon neutral and net zero were both falling out of favour in preference for real zero, a more stringent measure that includes all greenhouse gases and cannot be reached through offsetting.
She thought the Intergovernmental Panel on Climate Change probably the best place to keep on top of the terminology, because its way of looking at it is more formalised, and suggested perhaps also the Science Based Targets initiative, because it provides more corporate definitions.
The three scopes of corporate emissions
The Greenhouse Gas Protocol defines three scopes of emission, which can be used to measure an organisation's carbon output. Scopes 1 and 2 look at operational energy: the electricity and fuel a company uses to power its plants, produce its products and power the vehicles it owns.
Scope 1 covers direct emissions from sources a company owns or directly controls on site, such as boilers and manufacturing facilities: usually the core emissions of its core activities. At one company, scope 1 included on-site fuel combustion such as a gas boiler, the company's fleet vehicles, fluorinated gases and some solvents. At another, it was mainly natural gas and gasoline, with some refrigerants.
Scope 2 covers indirect emissions from the generation of energy purchased on the company's behalf. That energy is generated off site and imported, and it is mostly electricity but can also include steam.
Scope 3 covers all other indirect emissions in the value chain, the emissions inherited or purchased through suppliers, including raw materials, distribution, transport and healthcare communications consultancies. It includes emissions from premises and vehicles the company does not own or control; upstream, from the chain of suppliers of goods and services, from raw materials and manufacturing to business travel and waste disposal; and downstream, from patients' use and disposal of the company's products. Scope 3 emissions are often greater than a company's direct emissions. They are generally the hardest to reduce, because companies do not directly control them.
How a corporate target is validated
A company that signs on to net zero can have the Science Based Targets initiative, a third-party group, validate its targets and its methodology. The initiative checks whether the targets are in line with limiting the rise in global temperature to 1.5°C, reviews how the company collects and counts its carbon, and assesses whether its plans can realistically deliver the reductions.
One company's sustainability lead noted that it can be hard to work out which actions are meaningful, and that some of the visible ones are not always the most meaningful. In his view, validation requires a company to think about what is having an impact and set targets that really will reduce its scope 1 and scope 2 emissions.
Embodied energy and measures beyond carbon
A footprint should count not only the energy and resources used during operation, such as the electricity that runs an electric car, but also the electricity and carbon dioxide embodied in the production process. An electric car run on electricity from fossil fuel more or less defeats its purpose. By Tesla's own account, as one speaker relayed it, so much carbon dioxide goes into producing its batteries and cars, with lots of steel and rare metals in the battery, that its cars become better than petrol cars only after something like more than 100,000 kilometres, and only on green electricity.
In 2020 footprint calculation focused on carbon dioxide and energy, well-known metrics linked to a 1.5°C or 2°C rise in the planet's temperature, and these were already a very good guide. Beyond them, there are general life-cycle analyses and ecological and social metrics. Each of the Sustainable Development Goals has a couple of metrics associated with it, against which a company or an individual can measure itself. The amount of non-recyclable waste produced is also a fairly good measure of a footprint, though not of its energy or carbon dioxide.
What a life-cycle assessment involves
A life-cycle assessment is a methodology for assessing the environmental impacts associated with all stages of the life cycle of a product, process or service, a top-to-bottom analysis of a product's environmental impact throughout its entire life. Life-cycle assessments are powerful tools in healthcare sustainability research, assessing the environmental impact of products such as medications and medical devices, and they can take a cradle-to-grave approach that considers every step of manufacture, use and disposal.
A life-cycle assessment is a very complex methodology that accounts for every last bit of carbon: a proper one around a clinical trial looks at how many gloves were used, where they were manufactured and where the tubes were used. One clinical research organisation's carbon calculator, which looks at around 25 components of a trial at most, complements a full assessment rather than competing with or substituting for it, and it can still show how much carbon a change would save. A life-cycle assessment is a really formal process, and its resulting calculation needs to be certified. A calculator could perhaps be used internally to estimate the carbon a change saves without any claim being made outside, so it does not need the same scrutiny as an official assessment that is published or peer reviewed.
According to an AstraZeneca regulatory director, there is a standard methodology for such assessments. AstraZeneca's assessments followed the guidance on appraising the sustainability of the care pathway, which the UK National Health Service delivered in collaboration with parties across the healthcare system, and she did not know whether other companies use a different approach.
Life-cycle assessments in healthcare can compare products: reusable against single-use, and different packaging and sterilisation processes. They need extensive data, which suppliers are often reluctant to share; they rely on assumptions, which sensitivity analysis can remedy; and they rapidly become outdated. A life-cycle assessment is only as robust as its methodology, so medical writers should interpret life-cycle assessments cautiously.
A person's own footprint, and the intuitions about it
Calculators for a personal footprint are available from WWF and other organisations, at different levels of sophistication. Many relate a year's consumption to the earth's carrying capacity, as the number of earth-equivalents consumed, and anything more than one means consuming more than the earth can bear on an ongoing basis. Because of the high intensity of their consumption, Europeans in 2020 were consuming two to three or even four earth-equivalents.
Asked in a 2020 poll which of four human needs, food, clothing, travel and housing, has the largest environmental impact, a webinar audience believed food had the highest impact and the other three much less and about the same as each other. That perception is quite common, especially in the current context of vegetarian and vegan diets and eating less meat. However, transport, especially air travel, is clearly first because of its energy intensity, and housing, especially heating, is typically second. Clothing and food come third and fourth, in an order that depends on how fashionable a person is and what and how much they eat.
Asked in a second poll for the annual carbon-dioxide-equivalent footprint of an average adult European in private life, most participants chose 300 kilograms to 3 tonnes. The poll's bands left a gap between 3 kilograms and 300, which may have pushed some respondents to the higher choice. By one practitioner's estimate, made without measuring it, the average footprint, especially in Europe, is clearly more than 3 tonnes per person, and some individuals exceed 10 tonnes, the main influence being air travel and the number of flights. By the same estimate, footprints came down a bit during the COVID-19 pandemic but remained clearly in tonnes per person per year. In 2019 each resident of Europe generated 6.8 tonnes of carbon dioxide on average.
In June 2023 three medical writers reported their personal carbon footprints for the preceding 12 months, excluding work-related emissions and calculated with a lifestyle calculator: 4.42, 5.35 and 10.09 tonnes of carbon dioxide equivalent, with 60% of the largest from family-related intercontinental travel.
In 2023 another medical writer's carbon footprint was 6.60 tonnes of carbon dioxide equivalent on the UNFCCC Lifestyle Calculator, a quarter of it from family-related intercontinental travel. On the UN carbon footprint calculator, air travel accounted for more than half of the footprint of the writer's family of four.