Most climate scientists agree that we can only succeed at stopping global temperatures from increasing, and avoiding the worst impacts of climate change, if we halt the increase in carbon emissions. The consensus view is  that it is no longer enough merely to reduce the growth rate of carbon emissions to avoid environmental disaster due to global warming; rather, the total amount of carbon added to the atmosphere each year must be brought to zero.

The next step for policy makers is to set specific targets and determine how to measure progress toward them. The Paris Agreement of 2015 reiterated a goal that was first internationally adopted by the Cancun Agreements of 2010—to keep climate warming to under 2° Celsius from pre-industrial levels. In order to reach that goal, the Paris signatories agreed that global carbon and greenhouse gas emissions should reach net zero by the middle of the 21st century. The phrase “net zero”—also referred to as “net zero carbon,” “carbon neutrality,” and “climate neutrality”—has thus become a central concept for environmentalists and planners. 

What is net zero?

A simple way to think of net zero is as a matter of addition and subtraction. For every ton of carbon or other greenhouse gas (GHG) that is emitted, whether due to the burning of fossil fuels or other activities, a ton needs to be removed from the atmosphere. This process is also known as offsetting. 

After all technologically and economically feasible measures are taken to reduce a company’s or country’s carbon footprint, the emissions that remain are commonly called “residual emissions.” For an entity who engages in offsetting to reach net zero, there cannot be any residual emissions—they must all be offset. 

A common metaphor used to illustrate the concept is a filled bathtub: In order to keep the water in a tub at its current level, it is necessary to drain an amount equal to the water added to it. A bank account also serves as a good analogy. For it to be “net zero,” total deposits have to equal the total withdrawals and debits at the end of each period. 

The path to the net zero emissions goal requires that, eventually, the amount of carbon that every country emits must be matched by steps to remove an equal amount of carbon from the atmosphere—and this effort requires all the households and companies in that country to work towards that goal. The Paris Agreement imagines different timelines for countries to reach net zero, with large developed economies expected to reach the goal faster than developing countries.  

Some can even surpass net zero and remove more carbon from the atmosphere than they add to it. The terms “carbon negative” and “climate positive” are used to describe that ideal.

What is gross zero?

Gross zero, on the other hand, means that an entity (country, business, household, or individual) is not emitting any carbon into the atmosphere at all, without having to factor in any corresponding reductions or offsets. It is a goal that can be achieved by only a few individuals, companies, and governments through steps to limit carbon-producing activities and increased efficiency combined with the adoption of renewable energy sources. Think: driving an electric car or riding a bike for transportation, using wind and solar energy, and eliminating food waste.

While gross zero may seem like a more laudable goal than net zero, it is impossible to achieve in some commercial sectors and some parts of the world without bringing the economy to a halt. For example, air travel, at least in its current state, will always produce carbon emissions, even after every possible step has been taken to make flights as green and energy efficient as possible.  

The role of carbon credits and offsets in net zero

A more practical goal than stopping all air travel is to provide an alternative means for airlines to compensate for their emissions. Planting trees is a commonly thought of carbon offset; however, there are several other carbon removal technologies—such as direct air capture, biochar, and mineralization—that are available as carbon offsets today. If an airline invests in these carbon removal projects to offset their total emissions, they will not achieve gross zero but they would achieve net zero.

Air travel is not the only area where it can be prohibitively expensive, if not technologically impossible, to achieve carbon gross zero. In those cases, the Paris Agreement and other frameworks provide that a company (or city, state, or country) with residual emissions can purchase carbon credits from parties that are in better positions to achieve emissions reductions. Those parties’ decisions to go beyond net zero (or another carbon cap) are rewarded with credits that can be sold in carbon marketplaces.

Carbon offsets, either voluntary or mandated, are another option. These are payments to fund carbon removal projects, like those mentioned above. 

Both the University of Oxford and the Science Based Targets Initiative have provided guidance to organizations on net-zero aligned offsetting, recommending that carbon removals are the best approach to compensate for those hard to abate emissions.

Who has adopted net zero goals?

The exact number of countries that have adopted long-term goals of reaching net zero by 2050 is somewhat disputed as it depends on the criteria used to judge their commitments. In some cases, like France and the United Kingdom, the net-zero goal is legally binding. When the UK and France took this step (both in June 2019), they joined smaller economies like Costa Rica and Sweden that had already legally adopted them. Other countries have proposed but not passed net-zero legislation and some have adopted the goals in non-binding policy statements. 

In the United States, president Joe Biden has announced his intention to pursue a net-zero by 2050 goal and the president of China announced that that country will reach net-zero by 2060. Two nations which both happen to be densely forested, Bhutan and Suriname, already absorb more carbon than they emit, though whether they will maintain that status remains to be seen. Bhutan has announced that it will strive to be carbon neutral as its carbon emissions rise with increasing industrialization.

In addition, city and state (or provincial) governments have often adopted net-zero goals ahead of the countries where they are located. For example, Boston, Denver, and Portland, Oregon are among the US cities that have adopted net-zero pledges while California and New York are among the US states to have taken that same step. These state and local measures cover a broad array of activities, from energy-efficient building requirements to mandates for utilities. Finally, corporations including Amazon, Facebook, and Mastercard have adopted net-zero commitments. Companies that have embraced net-zero pledges have a combined revenue of around $11.4 trillion.