What do we mean by “net-zero emissions”?

Achieving net-zero greenhouse gas (GHG) emissions in a few decades is a challenge that is both ambitious and essential in order to keep global warming below 2°C and if possible limit it to 1.5°C. This global objective must now be applied on different scales, in particular at national and regional level. If this is to succeed, it is important to gain a full prior understanding of the concepts, principles and challenges involved.

Stabilising atmospheric concentrations of GHGS

The concept of net-zero emerged about ten years ago. The mechanisms of global warming and its anthropogenic nature (= caused by human activity) were already well established at that time. The increase in temperature at the Earth’s surface (commonly referred to as global warming) is caused by increased concentrations of greenhouse gases (GHGs) in the atmosphere, which in turn results from global emissions of anthropogenic GHGs, especially carbon dioxide (CO₂). The long-term global climate goal entails “stabilising concentrations of greenhouse gases in the atmosphere” in order to halt global warming and achieve a level that “[avoids] dangerous anthropogenic interference with the climate system1”. But how should the level of danger we want to avoid be defined? And precisely what action should be taken? GHG emissions must be cut, but by how much and by what date? Scientific progress in the 2000s has brought more precise answers to these questions. First, a better understanding of the impacts of different levels of warming on climate led experts to consider that an increase of 2 degrees was the danger threshold that must not be crossed (Copenhagen Agreement, 2009), and then to add the 1.5°C goal (Cancun, 2010).

A key concept: the carbon budget

Another key advance for net-zero goals was the discovery of the nearly linear relationship between cumulative CO₂ emissions and changes in surface temperature until 2100 (IPCC, 2014; Rogelj et al., 2015). This showed that for every level of warming there is a corresponding quantity of cumulative CO₂ emissions, in other words a total “carbon budget” that must not be exceeded.
Consequently, limiting global warming means limiting the sum total of cumulative global anthropogenic CO₂ emissions since the preindustrial era, in other words gradually reducing global net CO2 emissions to zero within the boundaries of a total carbon budget (IPCC, 2018). To halt global warming, it is thus not enough to reduce global CO₂ emissions, even drastically; they must be reduced to zero – in fact to “net-zero” (see below for an explanation of the term “net”). The closer the warming threshold is to current warming levels, the smaller the “remaining carbon budget” (i.e. the total CO₂ that can still be emitted), and the more rapidly we must bring emissions down to zero. Accurately estimating the carbon budget that corresponds to a given level of warming is thus crucial in order to plan CO₂ reduction strategies that will bring emissions levels down to zero in time.

Remaining carbon budgets for 1.5°C and 2°C

According to the latest estimates2, the remaining carbon budget at the beginning of 2020 to keep warming down to 2°C with a probability of at least 66% was 985 GtCO2. To limit warming to 1.5°C with probabilities of at least 50% and 66%, the budget was only 395 GtCO2 and 235 GtCO2 respectively (Constrain, 2019). By way of comparison, global CO2 emissions reached 44 GtCO2 in 2019 (UNEP, 2020). This means that the remaining budget required to limit warming to 1.5°C is now so small that, should annual emissions remain at 2019 rates, it would run out in 5 to 9 years (and in 23 years to limit warming to 2°C).
 

Any delay now will be even harder to make up for later

Every year during which CO2 emissions continue reduces the carbon budget by a corresponding amount. Any delay in achieving annual emissions targets must thus be made up for later via even higher rates of reduction in subsequent years, or by using “negative emissions technology” in the coming decades (see below). Global GHG emissions have increased on average by 1.4% year on year over the last decade3. If they had begun to fall in 2010, the annual drop in global GHG emissions required to achieve the 1.5°C target would “only” be 3.3%, compared to 7.6% from 2018 onwards (and a little more today). Our chances of achieving the 1.5°C goal are thus fading with each passing year (UNEP, 2019, 2020).

What are “net CO2 emissions”?

What needs to be done to achieve net-zero emissions globally? And why “net-zero” and not just “zero”? According to the IPCC, “net-zero emissions” refers to a situation where net anthropogenic CO₂ emissions are offset on a global scale by the anthropogenic removal of CO₂ in a given period. This is also referred to as “carbon neutrality”. To fully understand this definition, we must begin by determining where CO₂ emissions come from and what processes can be used to eliminate them.

Anthropogenic sources and sinks of CO2

There are two major sources of global anthropogenic CO₂ emissions.

• Fossil fuels account for about 80% of CO₂ emissions. They make up almost 85% of the global energy mix and are omnipresent in most economic activities and daily life: coal (35% of global CO₂ emissions), especially for power and industrial and domestic heating; petroleum (30%), mainly for transport, heating and petrochemicals; and natural gas (18%), mostly for power, heating and other domestic uses as well as transport. Also generally included is the cement industry, which is responsible for about 4% of global CO₂ emissions (half from the fossil fuels use in firing limestone to make clinker, and half from the limestone itself during the firing process).
• The second major source of CO₂ emissions (about 15% of global emissions) is land use, referred to as “LULUCF” (land use, land use change and forestry) in national UNFCCC greenhouse gas inventories. What characterises the land sector is that it is both an anthropogenic carbon sink (CO₂ removed from the atmosphere by plant photosynthesis and stored in plants and soil) and a source of anthropogenic CO₂ emissions (especially through deforestation and the removal of CO₂ from the soil due to urbanisation and certain farming practices).

On a global scale, emissions from the land sector are higher than CO₂ absorption (a.k.a. “removal”): this sector is thus a net source of CO₂ emissions. This means that, by definition, calculating total anthropogenic CO₂ emissions involves calculating net emissions. The total is the sum of fossil fuel emissions and net emissions from the land sector.
 

From “CO2-only” net-zero emissions…

In the light of the above, two courses of action are required to achieve “net-zero emissions”.

• Reduce CO₂ fossil emissions to (almost) zero in all sectors (energy production, transport, heating, industry, etc.) Carbon-free alternatives to fossil fuels already exist and must be rolled out rapidly on a large scale. This requires accelerating the transition to renewable and carbon-free energy sources, in the most efficient and sustainable way possible from an environmental, social and economic point of view. In parallel, in order to facilitate this transition, we must curtail global energy consumption, not only by improving energy efficiency but also by adopting more frugal modes of consumption, especially in developed countries.
• Reduce net emissions in the land sector to zero, or even achieve negative net emissions (where absorption is higher than emission: the land sector would thus become a net carbon sink). Even in the most ambitious decarbonation scenarios, there will probably still be unavoidable residual emissions in some sectors, such as air transport, which will have to be offset by equivalent absorption levels, for example through afforestation and reforestation. CO₂ capture and storage solutions must also be installed in cement factories and steel works.

To “all GHG” net-zero emissions

Other anthropic GHGs (methane, nitrous oxide, three fluorinated gases, etc.) account for about a quarter of total GHG emissions. Methane (CH₄) accounts for 17% of global GHG emissions and contributes more to short-term warming.
Rapid reduction of methane emissions is thus clearly of value. These emissions mainly come from ruminants (enteric fermentation, slurry and manure), from fossil fuels, from rice farming and from waste. Nitrous oxide (N₂O) mainly comes from nitrogen fertilizer spraying, and fluorinated gases from industrial processes (foams, aerosols, refrigeration fluids, etc.) Rapidly reducing all these emissions will require extensively developing ecofriendly farming practices, reducing the global ruminant population (especially cattle), and reducing the proportion of animal protein compared to plant-based protein in our diet.
It can, however, be difficult to reduce emissions of these gases, depending on their source; this is the case for N₂O emissions from fertilizers and CH₄ emissions from cattle. Emissions of these gases will thus not be reduced to zero, even in the most stringent reduction scenarios (2014 IPCC report). Moreover, unlike CO₂, these gases are difficult, if not impossible, to remove from the atmosphere. Their residual emissions must thus also be offset by an equivalent absorption of CO₂ by anthropogenic carbon sinks.
Achieving net-zero emissions for all anthropic GHGs thus requires going further than net-zero CO₂. It will be vital to achieve negative net CO₂ emissions in order to offset residual emissions of other greenhouse gases.
 

The land sector: a significant challenge

In the space of just a few decades, the global land sector will need to transition from its current status as a “net source” of CO₂ to that of a “net sink” able to absorb enough CO₂ to offset residual emissions of CO₂ and the other GHGs. This will mean significantly reinforcing its CO₂ absorption capacities, in particular by putting an end to net deforestation, increasing carbon storage in agricultural land, and pushing back against land take. In addition and in parallel, it will be necessary to resist the impacts of climate change, especially where forests are concerned (increased risk of fire, drought, dieback and disease), and to ensure that the land sector continues to act as a reservoir of biodiversity.

The dilemma of negative emissions technologies

The estimated carbon budget required to remain under 1.5°C is now so small that even the drastic measures outlined above will probably not be enough to achieve net-zero GHG emissions before it runs out.
In this scenario, it will be necessary to use “negative emissions technologies” to remove about 100 – 1,000 Gt CO₂ by the end of the 21st century. CO₂ removal would be used to offset residual emissions and, in most cases, to achieve negative net emissions and return to a warming level of 1.5°C (IPCC, 2018). Removing more CO₂ from the atmosphere than is emitted would reduce the atmospheric concentration of CO₂ and bring down warming.
According to the IPCC, in scenarios where global warming would be limited to 1.5°C by 2100, even with temporary peaks at 1.6°C or 1.7°C, global CO₂ emissions would have to be zero by around 2050 and negative thereafter. In parallel, non- CO₂ emissions would have to fall sharply, with all GHGs reaching net-zero by around 2067, followed by negative net emissions until the end of the century.
Several “negative emissions technologies” have been devised. Most frequently mentioned is bioenergy with carbon capture and storage (BCCS): growing biomass in large areas, burning it to create bioenergy, capturing CO₂ during combustion and storing it in the soil. However, these technologies are so far practically non-existent, and rolling them out on a massive scale would be subject to numerous feasibility and sustainability constraints from a technical, economic, social and environmental point of view: impacts on land use, biodiversity, food safety, etc. would potentially be huge (IPCC, 2018). This makes it all the more vital to speed up the reduction of emissions now so that we rely as little as possible on the uncertain future of such technologies.
 

Acting faster and more effectively; managing uncertainties

Over the past two years, NZE goals have been adopted not only by countries but also by organisations, cities and regions around the world. If we include the NZE2050 pledge of the future Biden Administration, a total of 127 pays, responsible for 63% of global CO₂ emissions, have pledged in one way or another to achieve NZE goals. This dynamic is heartening but inadequate. Taken together, these pledges, if honoured, would result in estimated global warming of 2.1°C by century’s end—still higher than the goals set out in the Paris Agreement.
We will thus have to up our global climate ambitions over the next decade, especially in developed countries (including France) in compliance with international climate policy equity, a key principle of the Paris Agreement.

Keeping global warming below 1.5°C now seems out of reach, and even 2°C will be difficult to achieve. But “every tenth of a degree matters”. It is thus vital to focus on strategies and investments that aim both at achieving 1.5°C and adapting to at least 2°C. It is also necessary to identify technologies and infrastructure that are likely to lock the economy into a path that is incompatible with NZE goals and to immediately avoid investing in them (HCC, 2020). For example natural gas, which has been promoted as a transition fuel, emits far too much CO₂ and methane to be compatible with NZE objectives. As the IPCC’s 1.5°C report underlines, implementing NZE strategies demands rapid and far-reaching transitions in energy, land management, urban development, infrastructure (including transport and buildings) and industrial systems. These systemic transitions are unprecedented in scope. The pursuance of NZE goals will clearly be no easy task, and it is vital to ensure that it does not exacerbate socio-economic and environmental
inequalities.