Net-zero emissions

Global net-zero emissions are reached when greenhouse gas emissions and removals due to human activities are in balance. Net-zero emissions is often shortened to net zero. Once global net zero is achieved, further global warming is expected to stop.
Emissions can refer to all greenhouse gases or only to carbon dioxide. Reaching net zero is necessary to stop further global warming. It requires deep cuts in emissions, for example by shifting from fossil fuels to sustainable energy, improving energy efficiency and halting deforestation. A small remaining fraction of emissions can then be offset using carbon dioxide removal.
People often use the terms net-zero emissions, carbon neutrality, and climate neutrality with the same meaning. However, in some cases, these terms have different meanings. For example, some standards for carbon neutral certification allow a lot of carbon offsetting. But net zero standards require reducing emissions to more than 90% and then only offsetting the remaining 10% or less to fall in line with 1.5 °C targets. Organizations often offset their residual emissions by buying carbon credits.
In the early 2020s net zero became the main framework for climate action. Many countries and organizations are setting net zero targets. As of November 2023, around 145 countries had announced or are considering net zero targets, covering close to 90% of global emissions. They include some countries that were resistant to climate action in previous decades. Country-level net zero targets now cover 92% of global GDP, 88% of emissions, and 89% of the world population. 65% of the largest 2,000 publicly traded companies by annual revenue have net zero targets. Among Fortune 500 companies, the percentage is 63%. Company targets can result from both voluntary action and government regulation.
Net zero claims vary enormously in how credible they are, but most have low credibility despite the increasing number of commitments and targets. While 61% of global carbon dioxide emissions are covered by some sort of net zero target, credible targets cover only 7% of emissions. This low credibility reflects a lack of binding regulation. It is also due to the need for continued innovation and investment to make decarbonization possible.
To date, 27 countries have enacted domestic net zero legislation. These are laws that contain net zero targets or equivalent. There is currently no national regulation in place that legally requires companies based in that country to achieve net zero. However several countries, for example Switzerland, are developing such legislation.

History and scientific justification

The idea of net-zero came out of research in the late 2000s into how the atmosphere, oceans and carbon cycle were reacting to CO₂ emissions. This research found that global warming will only stop if CO₂ emissions are reduced to net zero. Net-zero was basic to the goals of the Paris Agreement. This stated that the world must "achieve a balance between anthropogenic emissions by sources and removals by sinks of greenhouse gases in the second half of this century". The term "net zero" gained popularity after the Intergovernmental Panel on Climate Change published its Special Report on Global Warming of 1.5 °C in October of 2018, this report stated that "Reaching and sustaining net zero global anthropogenic CO₂ emissions and declining net non-CO₂ radiative forcing would halt anthropogenic global warming on multi-decadal timescales." An influential and now highly-cited scientific review on "Net-zero emissions energy systems" was also published in June of 2018, which was the first to assess the special challenges of not just reducing energy-related GHG emissions but actually reaching net-zero.
The idea of net-zero emissions is often confused with "stabilization of greenhouse gas concentrations in the atmosphere", a term from the 1992 Rio Convention. The two concepts are not the same. This is because the carbon cycle continuously sequesters or absorbs a small portion of human-caused atmospheric CO₂ into vegetation and the ocean, even after CO₂ emissions are reduced to zero. If CO₂ emissions from human activities are reduced to net zero, the concentration of CO₂ in the atmosphere would decline. This would be at a rate just fast enough to compensate for the slow warming of the deep ocean. The result would be approximately constant global average surface temperatures over decades or centuries. In contrast, stabilising atmospheric CO₂ concentrations would allow for some ongoing emissions, but global temperatures would continue to rise over many centuries due to the ocean's delayed response to warming.

Types of greenhouse gas

It will be quicker to reach net-zero emissions for CO₂ alone rather than CO₂ plus other greenhouse gases like methane, nitrous oxide and fluorinated gases. The net-zero target date for non-CO₂ emissions is later partly because modellers assume that some of these emissions such as methane from farming are harder to phase out. Emissions of short-lived gases such as methane do not accumulate in the climate system in the same way that CO₂ does. Therefore there is no need to reduce them to zero to halt global warming. This is because reductions in emissions of short-lived gases cause an immediate decline in the resulting radiative forcing. Radiative forcing is the change in the Earth's energy balance that they cause. However, these potent but short-lived gases will drive temperatures higher in the short term. This could possibly push the rise in temperature past the 1.5 °C threshold much earlier.A comprehensive net-zero emissions target would include all greenhouse gases.
Some targets aim to reach net-zero emissions only for carbon dioxide. Others aim to reach net-zero emissions of all greenhouse gases. Robust net zero standards state that all greenhouse gases should be covered by a given actor's targets.
Some authors say that carbon neutrality strategies focus only on carbon dioxide, but net zero includes all greenhouse gases. However some publications, such as the national strategy of France, use the term "carbon neutral" to mean net reductions of all greenhouse gases. The United States has pledged to achieve "net zero" emissions by 2050. As of March 2021 it had not specified which greenhouse gases will be included in its target.

Terminology

Countries, local governments, corporations, and financial institutions may all announce pledges for achieving to reach net-zero emissions.
In climate change discussions, the terms net zero, carbon neutrality, and ''climate neutrality'' are often used as if they mean the same thing. In some contexts, however, they have different meanings from each other. The sections below explain this. People often use these terms without rigorous standard definitions.

Approaches

A given actor may plan to achieve net-zero emissions through a combination of approaches. These would include actions to reduce their own emissions, actions to reduce the emissions of others, and actions to directly remove carbon dioxide from the atmosphere.

Reducing emissions

Robust net zero standards require actors to reduce their own emissions as much as possible following science-based pathways. They must then balance their residual emissions using removals and offsets. This typically involves shifting from fossil fuels to sustainable energy sources. Residual emissions are emissions that are not practical to reduce for technological reasons.
Another key measure to reduce emissions is increasing efficiency. Historically, improved energy efficiency has been the most successful measure to reduce emissions. Policies to improve efficiencies include setting fuel efficiency standards for cars and promoting building insulation and public transport.
Experts and net zero frameworks disagree over the exact percentage of residual emissions that may be allowed. Most guidance suggests this should be limited to a small fraction of total emissions. Sector-specific and geographical factors would determine how much. The Science Based Targets initiative says that residual emissions across most sectors should fall to below 10% of an organization's baseline emissions by 2050. It should be even lower for some sectors with competitive alternatives like the power sector. Sectors such as heavy manufacturing where it is harder to mitigate emissions will probably have a higher percentage of residual emissions by 2050.
The ISO and British Standards Institution publish "carbon neutrality" standards that have higher tolerance for residual emissions than "net zero" standards. For example, BSI PAS 2060 is a British standard for measuring carbon neutrality. According to these standards, carbon neutrality is a short-term target, and net zero is a longer-term target.

Carbon removals and offsets

To balance residual emissions, actors may take direct action to remove carbon dioxide from the atmosphere and sequester it. Alternatively or in addition they can buy carbon credits that "offset" emissions. Carbon credits can be used to fund carbon removal projects such as reforestation.
Strong standards such as the ISO and BSI "net zero" standards only allow removal-based offsets that have the same permanence as the greenhouse gases that they balance. The term for this concept is "like for like" removals. Permanence means that removals must store greenhouse gases for the same period as the lifetime of the GHG emissions they balance. For example, methane has a lifetime of around 12 years in the atmosphere. Carbon dioxide lasts between 300 and 1,000 years. Accordingly, removals that balance carbon dioxide must last much longer than removals that balance methane.
Carbon credits can also fund initiatives that aim to avoid emissions. One example would be energy efficiency retrofits or renewable energy projects. Avoided emissions offsets result from actions that reduce emissions relative to a baseline or status quo. But they do not remove emissions from the atmosphere. Weak standards such as ISO and BSI "carbon neutrality" standards allow organizations to use avoided-emissions carbon credits. They do not specify how permanent or durable a credit must be.
Carbon offsetting has been criticized on several fronts. One important concern is that offsets may delay active emissions reductions. In a 2007 report from the Transnational Institute, Kevin Smith likened carbon offsets to medieval indulgences. He said they allowed people to pay "offset companies to absolve them of their carbon sins." He said this permits a "business as usual" attitude that stifles required major changes. Many people have criticized offsets for playing a part in greenwashing. This argument appeared in a 2021 watchdog ruling against Shell.
Loose regulation of claims by carbon offsetting schemes combined with the difficulties in calculating greenhouse gas sequestration and emissions reductions has also given rise to criticism. This argument is that this can result in schemes that do not adequately offset emissions in reality. There have been moves to create better regulation. The United Nations has operated a certification process for carbon offsets since 2001. This is called the Clean Development Mechanism. It aims to stimulate "sustainable development and emission reductions, while giving industrialized countries some flexibility in how they meet their emission reduction limitation targets." The UK Government's Climate Change Committee says reported emissions reductions or removals may have happened anyway or. not last into the future. This is despite an improvement in standards globally and in the UK.
There has also been criticisms of non-native and monocultural forest plantations as carbon offsets. This is because of their "limited—and at times negative—effects on native biodiversity" and other ecosystem services.
Most of the carbon credits on the voluntary market today do not meet UN, UNFCCC, ISO or SBTi standards for permanent carbon dioxide removals. As a result, significant investment in carbon capture and permanent geological storage will most likely be necessary to achieve net-zero targets by mid-century.