What is Carbon Neutral?

The City of Boston has pledged to be carbon neutral by 2050. Here “carbon” refers to carbon dioxide (CO2), the greenhouse gas (GHG) released by the combustion of fossil fuels, land use change, and other human activity. But other gases such as methane (CH4) and nitrous oxide (N20) released by human activity also contribute to the greenhouse effect. The term carbon dioxide equivalent (CO2-e or CO2-eq) captures the combined effect of all anthropogenic GHG emissions in a single metric based on each gas’s global warming potential. The City’s greenhouse gas inventory accounts for all GHGs, so the term “carbon neutral” should be interpreted as “CO2-e neutral.”

“Neutral” refers to the City’s commitment to reduce the net effect on climate from human activity in the City to zero by 2050. Here’s what that looks look from an accounting perspective (all in units of CO2-e):

Anthropogenic GHG emissions
– uptake of CO2 via biological processes
+ release of CO2 via biological processes
= 0

Boston’s Goals in a Global Context

As applied to a city, this definition is consistent with the principles and accounting frameworks of the Intergovernmental Panel on Climate Change (IPCC) and the Paris Agreement of The United Nations Framework Convention on Climate Change (UNFCCC)

In a city like Boston, the GHG emissions from energy use are very, very large compared the biological fluxes, so mitigation efforts focus energy using activities such as transportation, electricity generation, and fuel use in heating and cooling. Of course, vegetation provides many benefits such as cooling and reduction in energy use, improved air quality, decreased stormwater runoff, and improved health and quality of life

Anthropogenic GHG emissions include Scope 1 and Scope 2 emissions, as defined in the Global Protocol for Community-Scale Greenhouse Gas Emission Inventories (Figure 1). This is the emissions inventory methodology used by members of the Global Covenant of Mayors for Climate and Energy, of which the City of Boston is a member.  This emissions scope does not include Scope 3 emissions, often referred to as “consumption-based” emissions.

What Emissions are the City Responsible For?

All major cities such as Boston face a major challenge: they assume responsibility for emissions in the electricity they consume (Scope 2), but most of that electricity is supplied by a large regional grid. In the case of Boston, that is ISO New England which serves more than 7 million people in six states. The city of Boston does not control what energy sources are tapped to provide power to the grid, and thus it does not control the GHG intensity (CO2-e/kWh) of the electricity it purchases. In principle, the City could eliminate all GHG emissions within the City boundaries, but still not reach its mitigation target because somewhere in the ISO New England region fossil fuels are used to supply electricity to the grid that the City is literally plugged into.

The limited ability to control Scope 2 emissions has led to many cities, businesses, universities, and other organizations to purchase offsets and renewable energy certificates (RECs) to meet their mitigation goals. An offset project is a specific activity intended to reduce GHG emissions, increase the storage of carbon, or enhance GHG removals from the atmosphere. Examples of offset projects include investment in energy efficiency or forest management such as reforestation and forest preservation. These projects do not have to be within the city boundary to qualify as an offset.

RECs are the legal instruments used in renewable electricity markets to account for renewable electricity. The purchase or use of renewable energy, verified with RECs, is a decision an organization makes to ensure its electricity is provided from renewable sources that produce low- or zero-GHG emissions, thereby reducing the organization’s Scope 2 emissions. A city that purchases a RECV claims its attributes whether that renewable electricity is generated within the city, or purchased from elsewhere.

Here is the accounting definition for “carbon neutrality” that includes offsets and RECs (all in units of CO2-e):

Anthropogenic GHG emissions
– uptake of CO2 via biological processes
+ release of COvia biological processes
– quantity in offsets and/or RECs
= 0