From local governments to Fortune 500 companies, it seems everyone has a climate commitment these days. President Biden has a goal of achieving a net-zero emissions economy by no later than 2050. California Governor Gavin Newsom asked the State’s Air Resources Board and Public Utilities Commission to evaluate pathways to achieve carbon neutrality by 2035 – faster than contemplated in a previously adopted executive order. The City of Irvine adopted a resolution setting a goal of carbon neutrality by 2030. And companies are making similar commitments. Microsoft has pledged to be carbon negative by 2030 and to remove enough carbon dioxide (CO₂) from the atmosphere to cancel out all its historical emissions by 2050. It is not clear that we have come to terms with what these commitments mean – both the language and what it will take to get there.

This post reviews key terms to help make sense of GHG emissions commitments. It will be helpful to anyone developing or interpreting climate commitments, or following the upcoming twenty-sixth session of the Conference of the Parties (COP 26) to the United Nations Framework Convention on Climate Change (UNFCCC) taking place in November 2021 in Glasgow.

Climate Terms

The first step in coming to terms with these climate goals is understanding what they mean.

Unless otherwise stated, the following discussion is based on two sources worth bookmarking: the Intergovernmental Panel on Climate Change’s (IPCC) Glossary of terms for its Special Report: Global Warming of 1.5°C and the glossary of the Carbon Dioxide Removal Primer.

Here are some key terms to help sort this all out. The UNFCCC Article 1 defines an emissions source as “any process or activity which releases a greenhouse gas, an aerosol or a precursor of a greenhouse gas into the atmosphere.” It defines a sink as “any process, activity or mechanism which removes a greenhouse gas, an aerosol or a precursor of a greenhouse gas from the atmosphere.” Sources and sinks can be both natural or human-caused. As examples, emissions from the tailpipe of a truck are human-caused sources while the emissions from a wildfire would be a natural source. Trees, which absorb CO₂ and store carbon, are examples of natural sinks. While engineered processes that remove CO₂ from the atmosphere and store it underground are an example of a human-caused sink.

Another way to say this is that sources are positive emissions and sinks are negative emissions. Positive emissions occur when a source that is created or enhanced by human activity adds greenhouse gases (GHG) to the atmosphere. Negative emissions are the opposite and occur when a sink that is created or enhanced by human activity pulls emissions out of the atmosphere. And, by extension, net negative emissions occur when negative emissions (sinks) are greater than positive emissions (sources).

Negative emissions are sometimes referred to as carbon dioxide removal. See the previously mentioned Carbon Dioxide Removal Primer for more information on a range of removal technologies. Also, Lawrence Livermore National Laboratory released a report in January 2020 that evaluated negative emissions strategies that could help California achieve carbon neutrality.

Factors to Consider

There are many factors to consider when developing or interpreting a climate commitment. Here are a few.

• Emissions Covered – CO₂  is the most prominent GHG but there are others, including CH₄, CO, N₂O. Also, some businesses track emissions according to “Scopes” (Scope 1,2,3). Distinguishing between CO₂ and all GHG emissions and among the scopes is important. For example, California has a target of carbon neutrality that includes all GHG emissions, including those related to natural and working lands (e.g., wildfire, land and agriculture management, etc.) (Figure 1).

Figure 1 California Air Resource Board Carbon Neutrality Slide

• Time Period – Most climate commitments set a specific numeric target to be achieved in a given year. California seeks to achieve carbon neutrality as early as possible but no later than 2045. This means that in 2045, sources of GHGs should be equal to sinks. Microsoft, on the other hand, has committed to removing enough carbon dioxide from the atmosphere to cancel out all historical emissions since its founding by 2050. In this case, net zero emissions means all emissions ever created by Microsoft rather than those in a given future year (Figure 2).

Figure 2 Microsoft Climate Commitment

• Use of Carbon Offset Credits – Whether carbon offset credits can be used to cancel out remaining emissions is another key question. And further, whether those offset credits are from projects that avoid or reduce emissions or whether they are from projects that remove emissions from the atmosphere and store them in stable ways.

The Commitments

In the end, the entity adopting the commitment can define what it means. Nonetheless, it is helpful to have a basic framework to interpret and develop such commitments. Here are generally accepted definitions for some common commitments.

• Carbon Neutrality or Net Zero CO₂ Emissions – At the global level, these mean the same thing and occur when the amount of remaining human-caused CO₂ emissions are balanced globally by the amount removed from the atmosphere by human intervention over a specified period. A variant of this is Net Zero Emissions, which would take into account all GHGs, including methane and other high global warming potential gases. At scales smaller than global, these commitments can mean slightly different things. Some guidelines (here and here) allow carbon offset credits from projects that reduce or avoid emissions to cancel out remaining emissions in the case of carbon neutrality, while others allow only carbon removals from offset credits or otherwise to achieve net zero.

• Zero Carbon Emissions – This occurs when human-caused CO₂ emissions equal zero. A variant of this would be Zero GHG Emissions. In this case, no GHGs would be emitted. As an example, setting a zero carbon goal for 2045 would mean in that year no CO₂ was emitted.

• Net Negative Emissions – This occurs when, through human intervention, more greenhouse gases are removed from the atmosphere than are emitted into it. This is what California’s executive order sets as the long-term commitment.

• Carbon Positive – Using the definition of positive emissions above, it is confusing that some businesses claim to be “carbon positive.” At least one business defines carbon positive as “taking more carbon and GHGs out of the environment than we’re creating.” Based on our definitions here, they would be achieving net negative emissions, but they are using the non-mathematical sense of “positive,” perhaps because “net negative” is not a compelling marketing term.

Future posts will consider other important topics as we come to terms with climate commitments like what is the magnitude of remaining emissions that will have to be canceled out to achieve neutrality or net zero. And, what options exist to cancel out residual emissions.