Carbon is a solid, non-metal element. It is often described as the "king of the elements" due to its versatility and importance. Essential to life, it is the fourth most abundant element in the universe (after hydrogen, helium and oxygen).
Carbon makes up around half of the dry mass of all trees and plants. It is the second most abundant element in the human body after oxygen, making up about 18.5 per cent of human body mass and constitutes a similar percentage of all living creatures.
There are an estimated 1.85 billion, billion tonnes of carbon on earth. Ninety-nine per cent of it is stored deep in the earth's core and mantle. Just 0.2 per cent of it (an estimated 43,500 trillion tonnes) is in visible carbon sinks such as the air, soil, oceans, plants and animals.
Carbon is an extremely versatile element with atoms that can bond together in many ways to build pure-carbon physical forms, or allotropes, including diamond, graphite, charcoal (top image) and graphene.
Carbon atoms can also bond with atoms of other elements to form an astonishing range of chemical compounds such as carbon dioxide, calcium carbonate, carbohydrates (including sugars: table sugar is 40 per cent carbon) and hydrocarbons.
Over ten million carbon compounds have been identified so far and there are thought to be many millions more that have yet to be discovered.
While the total amount of carbon on earth does not change, the element is endlessly circulated between land, sea and sky. This carbon cycle, along with similar cycles of water and nitrogen, is a key part of the reason why life is sustained on earth.
Plants and animals absorb or eat carbon, releasing it again when they die. Sometimes this carbon makes its way into the atmosphere and sometimes it accumulates on earth (in sedimentary rocks or fossil deposits, for example) or is dissolved in the ocean.
From there it is eventually again recycled via erosion, sedimentation, lifeforms and – particularly since the industrial revolution – human activity, notably via the burning of fossil fuels, which transfers carbon that was once contained in living creatures and plants into the atmosphere. This activity is disrupting the natural carbon cycle.
Carbon sinks are large deposits of carbon on earth that sequester more atmospheric carbon than they emit. The key active carbon sinks are organisms (both living and dead), soil, the oceans and the atmosphere. Together these are able to absorb around half of all human-induced carbon emissions.
Past carbon sinks that no longer actively sequester carbon include fossil-fuel reserves and sedimentary calcium carbonate deposits such as limestone and chalk.
Carbon sources are the opposite of carbon sinks, producing a net increase in atmospheric carbon dioxide. These include industry, the built environment (which accounts for around 40 per cent of all emissions) agriculture, wildfires and volcanoes.
When a single carbon atom bonds with two oxygen atoms, molecules of carbon dioxide gas are formed. This is a natural chemical reaction that happens, for example, when hydrocarbons are burned, when cells metabolise carbohydrates into energy and when living organisms decompose.
Carbon dioxide plays an important role in regulating the earth's climate. Like other greenhouse gases, CO2 molecules absorb radiation from the sun and then release this energy by vibrating. These vibrations generate heat, forming an insulating blanket around the earth that makes life possible since, without CO2 and other greenhouse gases, the planet would be too cold to sustain life.
However, the more carbon dioxide there is in the atmosphere, the greater the warming effect.
Airborne carbon dioxide molecules eventually return to earth as part of the carbon cycle but this is a gradual process that takes a very long time. For a tonne of CO2 emitted today, 40 per cent of it will still be in the atmosphere in 100 years time and 10 per cent of it will still be there 10,000 years from now.
Carbon dioxide is the second-most abundant greenhouse gas after water vapour which, along with the clouds it forms, has four times the total impact on the climate of CO2.
However, carbon dioxide is universally regarded as the single biggest cause of global warming, partly because it remains in the atmosphere for so long (up to 1,000 years compared to nine days for water vapour) and partly because there is so much of it.
Carbon dioxide equivalent
Carbon dioxide equivalent (CO2e) is a scale that allows the various greenhouse gases to be compared in terms of their warming effect on the earth.
A tonne of the carbon dioxide equivalent of a given gas has the same warming effect as a tonne of CO2. This is calculated by studying the global warming potential (GWP) of each gas, taking into account the length of time it remains in the atmosphere, and comparing this to the warming potential of CO2, which has a GWP of 1.
Methane (another carbon-based gas) has a GWP of 25, meaning that a tonne of methane emissions have the same climate impact as 25 tonnes of carbon dioxide. Nitrous oxide has a GWP of 298. The hydrofluorocarbon HFC-23, which is used as a refrigerant and a fire suppressant, has a GWP of 14,800. Sulphur hexafluoride, used as an electrical insulator, has a GWP of 22,800, the highest of any greenhouse gas.
When people talk about carbon emissions or carbon dioxide emissions, they are often using this as shorthand for carbon dioxide equivalent emissions (see below).
Carbon emissions are human-induced releases of greenhouse gases into the atmosphere. The term provides a way of measuring the amount of greenhouse gases being added to the atmosphere and, by extension, a way of calculating the amount that needs to be removed to stabilise the climate.
The term is misleading since it is generally used as a synonym of carbon dioxide equivalent emissions (see above), which covers all greenhouse gas emissions. But since carbon dioxide is by far the most problematic greenhouse gas, and since heavy carbon atoms make up the main bulk of CO2 emissions (oxygen atoms are much lighter), carbon has become a byword for all greenhouse emissions.
Carbon emissions are measured in metric tonnes (equivalent to 1,000 kilogrammes). A metric tonne of CO2 would fill a balloon with a diameter of 10 metres. In 2021, humans will add around 40 billion tonnes of emissions to the atmosphere, according to the International Energy Agency. If 40 billion balloons each containing a tonne of CO2 were stacked in a cube, the cube would measure 342 kilometres on each side.
The Greenhouse Gas Protocol, which draws up widely adopted standards to measure and manage emissions, divides carbon emissions into three types under the Scope 3 Standard. Scope 1 emissions are direct emissions from sources the emitter has direct control over. Scope 2 emissions are indirect emissions from services the emitter consumes such as power and heating. Scope 3 emissions are all other indirect emissions caused by an emitter.
Atmospheric carbon dioxide
Atmospheric carbon dioxide is the term used to describe the total amount of CO2 in the atmosphere, adding together human-induced emissions and natural concentrations.
The concentration of atmospheric carbon dioxide has been rising steadily since the industrial revolution and has accelerated over recent decades: half of all emissions over the last 300 years have happened since 1980. A quarter have happened since 2000.
As a result, the number of carbon parts per million (PPM) in the atmosphere has risen 50 per cent from 280 in pre-industrial times to 420 as of 4 June this year, according to the CO2.Earth daily calculator.
The Intergovernmental Panel on Climate Change (IPPC) estimates that we will need to remove between 100 and 1,000 gigatonnes of emissions from the atmosphere by the end of the century in order to keep global warming below the 1.5 degrees set out in the 2015 Paris Agreement.
A carbon footprint is the total amount of carbon dioxide equivalent emitted directly and indirectly by an individual, organisation, product, building or activity. Working out your carbon footprint (by using a carbon footprint calculator or conducting a carbon audit, see below) allows you to take steps towards reducing, eliminating or even negating your climate impact.
The concept is controversial as it pushes the responsibility for emissions away from large-scale emitters such as fossil-fuel companies onto end-users of emissions-generating products and services.
The term "carbon footprint" was popularised by oil giant British Petroleum in a 2004 advertising campaign that included slogans such as "Reduce your carbon footprint. But first, find out what it is." The ads included a link to a carbon footprint calculator on BP's website.
The USA has the highest carbon footprint per capita, with each citizen responsible for an estimated 16.5 tonnes of CO2. In the UK, it is 6.5 tonnes.
Carbon calculators and carbon auditing
A growing number of companies and websites offer simple calculators that allow organisations and individuals to estimate the carbon footprint of their activities or individual aspects such as a building or a product. These are often crude but help give a basic understanding of the climate impact of your activities.
Carbon auditing, which is another growing field, offers a more complete and complex analysis in order to calculate a carbon footprint more accurately.
The aim of both is to help organisations and individuals to erase their carbon footprints for example by reducing their energy use, switching to renewable power sources and offsetting their emissions (see below).
Carbon offsetting is a concept designed to allow individuals and organisations to neutralise their greenhouse-gas emissions by signing up to schemes that compensate for those emissions.
There are many offsetting schemes available. Many of these are voluntary, allowing companies and individuals to offset some or all of their emissions via a range of methods including afforestation, avoided deforestation, renewable energy development and the capture of atmospheric carbon and other greenhouse gases.
However, offsetting schemes vary in their effectiveness. Rather than reversing emissions, many of them simply defer them or displace them. For example, offsetting by investing in a new renewable energy scheme may lead to a reduction in fossil-fuel emissions in the future, but will not do anything to negate emissions made today, since that carbon is already in the atmosphere, contributing to global warming now and for the next few hundred years (see above).
Carbon offsetting is measured in carbon dioxide equivalent (see above).
The Oxford Principles for Net Zero Aligned Carbon Offsetting provides a useful guide to offsetting approaches that align with the concept of net-zero emissions (see below).
Carbon trading is a system that allows polluters to buy and sell credits for their emissions. It turns carbon emissions into a commodity and allows countries, companies and organisations that do not use all their permitted emissions to sell them to others who have exceeded their allocations.
Such schemes rely on their being an agreed, finite amount of permittable emissions in a given country or region. The concept was established as part of the 1997 Kyoto Protocol, assigning countries that signed up to the climate treaty emissions quotas that could be traded.
Carbon trading does not reduce atmospheric carbon dioxide. Instead, it attempts to regulate the amount of new carbon emissions.
The largest and most sophisticated emissions trading scheme is the European Union's Emissions Trading System. This is a legally binding framework that compels companies to limit their emissions as part of the EU's policy to combat climate change. The idea is that the total permissable emissions will fall over time with the aim of achieving carbon neutrality across the EU by 2050.
Under the scheme, polluting companies can buy rights to continue producing emissions via a "cap and trade" system. The scheme currently covers 40 per cent of EU emissions and may soon be extended to cover buildings and road transport, which are a major source of emissions that are currently exempt from the EU's cap and trade system.
Carbon neutrality is achieved when no additional carbon dioxide equivalent is added to the atmosphere by an entity such as an individual, a company, a building or a country. This can either involve eliminating emissions in the first place, negating emissions through offsetting, or a combination of both.
Carbon neutrality is defined by the internationally recognised PAS 2060 standard.
This Dezeen tag contains examples of carbon-neutral architecture and design.
Net-zero is considered the benchmark standard for decarbonisation and is different from carbon neutrality (above), although there is no internationally recognised standard for net-zero.
According to the Carbon Trust, net-zero targets must be aligned to the Paris Agreement goal of limiting global warming to 1.5°C and be achieved within a timescale compatible with the agreement's 2050 target.
All emissions, including indirect emissions in the value chain as well as direct emissions, must be taken into consideration. This involves eliminating Scope 3 emissions, which include emissions generated by purchased goods and services, third-party distributors and "use of sold products", which means the emissions generated when customers use a company's products.
To become net-zero, a company must eliminate these emissions on top of its Scope 1 emissions, which are emissions it is directly responsible for, and Scope 2 emissions, which are emissions generated by "purchased electricity, heat and steam".
Any hard-to-remove emissions that cannot be eliminated should be compensated for using certified greenhouse gas removals (GGR) that permanently remove carbon from the atmosphere. Offsets that avoid emissions cannot be used.
Achieving net-zero is a key step towards tackling climate change but it does not address the greenhouse gases that are already in the atmosphere.
Organisations can learn about and commit to net-zero targets via the United Nations' Race to Zero initiative.
In its lexicon, the Race to Zero initiative considers net-zero to have been achieved when "an actor reduces its emissions following science-based pathways, with any remaining GHG (greenhouse gas) emissions attributable to that actor being fully neutralized by like-for-like removals (eg permanent removals for fossil carbon emissions) exclusively claimed by that actor, either within the value chain or through purchase of valid offset credits".
Carbon negativity and carbon positivity
Confusingly, these mean pretty much the same thing. The UN's Race to Zero initiative prefers the term "carbon positive" but the architecture sector has largely adapted the term "carbon negative", which is also the Dezeen currently uses.
Carbon negativity is when an entity such as a company, individual or building removes more carbon dioxide equivalent from the atmosphere than it emits. This will be essential if the targets of the Paris Agreement are to be met since, as well as achieving net-zero new emissions, large amounts of greenhouse gases will need to be removed from the atmosphere via carbon capture (see below).
To achieve true carbon negativity, the lifetime carbon footprint needs to be taken into account and exceeded by the amount of carbon captured. For a building or product, this means taking into account both embodied carbon and operational carbon (see below).
This Dezeen tag contains examples of carbon-negative architecture and design.
Embodied carbon refers to the total amount of carbon dioxide equivalent generated to produce any kind of physical asset such as a chair or a building. This includes all the emissions generated by the extraction and processing of raw materials plus the manufacturing, transportation and construction or assembly of the asset, as well as the deconstruction and disposal of the building's components at the end of its life.
Operational carbon refers to the total amount of carbon dioxide equivalent emitted by the operation of a building or other asset over its lifetime. This includes emissions generated to provide power to the building plus those generated by the heating system as well as any other emissions generated to run the building.
Carbon capture, also known as carbon removal, involves removing carbon dioxide from the atmosphere. This can be done by both natural and industrial processes.
The best-known method is afforestation and tree-planting trees. Carbon can also be captured in soil by adopting appropriate agricultural and land management techniques.
Industrial carbon-capture methods include direct air capture (DAC), which involves extracting CO2 from the atmosphere with machines. The carbon can then either be sequestered (see below) as part of a combined process known as carbon capture and storage (CCS). Or it can be used as a raw material via a process known as carbon capture and utilisation (CCU).
Carbon capture will need to play a key role in reducing atmospheric carbon since even if new emissions were to cease immediately, there is enough carbon already in the sky to ensure temperatures on earth would continue to rise for 40 years, ice caps will continue to melt and seas will continue to rise. Left to its own accord, it would take thousands of years for the climate to return to pre-industrial conditions.
CO2 can also be captured from fossil fuels, preventing much of the gas from entering the atmosphere when hydrocarbons are burned. Pre-combustion capture involves chemically removing carbon dioxide from fossil fuels before they are burned. Post-combustion capture involves "scrubbing" the gas from flues at power stations and industrial plants that burn fossil fuels.
However, neither pre-combustion capture nor post-combustion capture is perfect and neither reduces the amount of CO2 that is already in the atmosphere.
Carbon sequestration refers to the long-term storage of carbon dioxide that has been captured from the atmosphere. This happens naturally, for example when trees and other plants absorb CO2. The earth's biomass holds an estimated 560 billion tonnes of carbon.
The oceans and the creatures in them also sequester vast amounts of carbon. This is known as blue carbon.
Carbon can be sequestered by mineralisation processes that turn carbon dioxide into solid material. This happens naturally during weathering processes such as when CO2 in rainwater reacts with silicate rocks, trapping the carbon in new carbonate material.
Carbon can be mineralised synthetically by mimicking weathering processes or by pumping carbon dioxide underground where it reacts with bedrock to create a new type of rock. This is known as subsurface mineralisation.
Captured carbon can be put to a wide variety of uses. It can be combined with hydrogen to create synthetic hydrocarbons, which can be used to produce the same variety of fuels, plastics and chemicals as fossil fuels.
It can be turned into construction materials such as cement and aggregates via synthetic mineralisation processes.
It can be fed to algae to produce biomass that in turn can be made into fertiliser, food and a range of chemicals and supplements.
This article is part of Dezeen's carbon revolution series, which explores how this miracle material could be removed from the atmosphere and put to use on earth. Read all the content at: www.dezeen.com/carbon.