What Are Emission Equivalence Metrics and Why Are They Necessary?

Emission equivalence metrics are a way to quantitatively compare the effects of different greenhouse gases relative to each other. Carbon dioxide (CO₂) acts as one important greenhouse gas in the atmosphere: it absorbs heat emitted from the Earth’s surface and reemits it, warming the air around it. This warming is, to some extent, natural and a part of the energy balance that makes Earth’s climate livable. But, as human-driven emissions of greenhouse gases like CO₂ increase, more heat is trapped in the atmosphere, causing broader global warming and subsequent climate change. In addition to CO₂, other greenhouse gases include methane, nitrous oxide, and some refrigerants. Each of these gases has a different chemical structure that impacts how much heat it traps, and how long it’s in the atmosphere. For example, methane is short-lived, remaining in the atmosphere on average for approximately 12 years, but it absorbs much more heat than CO₂.

Rather than separately tracking and regulating the emissions of each different greenhouse gas, emissions for each can be converted to a common measure or unit using an emission metric. Emission metrics compare the physical climate effects of different greenhouse gases in the atmosphere, like the amount of heat absorbed or the change in global mean surface temperature caused by an emitted gas. Most metrics consider greenhouse gas emissions as “pulses,” where the climate effect peaks at emission and then decreases over a specific amount of time (or time horizon). The time horizon of interest, as well as the physical climate effect considered, can vary, creating multiple emission metrics. Further, these metrics can express the total magnitude effect a unit mass of emitted gas has, or they can express the effect of a gas relative to a reference species, most commonly CO₂.

A gas’s carbon dioxide equivalence (CO₂e) metric can be used to determine the amount (in tons) of CO₂ that would have the same effect in the atmosphere as 1 ton of the gas. Total mass of a gas is, therefore, multiplied by the CO₂e value to get the amount in tons of CO₂e. This value can then be applied to regulatory thresholds.

Global Warming Potential (GWP) 100 vs. GWP20

Global warming potential (GWP) is a CO₂e metric that considers the heat absorbed by a greenhouse gas and is commonly used in policy contexts. GWP can be considered over different time horizons, and each greenhouse gas’s CO₂e value varies according to the time horizon used. A shorter time horizon more heavily weights the warming impact of short-lived greenhouse gases, like methane, as these have the largest effect shortly after emission and decline quickly. Alternatively, a longer time horizon more heavily weights long-lived greenhouse gases, like refrigerants. GWP over 100 years (GWP100) is used by parties to the Paris Agreement to report aggregate national emissions (and set emission reduction goals), as well as by the US Environmental Protection Agency to track emissions and sinks of greenhouse gases. New York State, as specified by the Climate Act, uses GWP over 20 years (GWP20).

To understand how these two metrics relate to each other in practice, we can consider a hypothetical example. Imagine a dairy farm in upstate New York, with annual emissions of 3,000 metric tons (mt) of CO₂ and 150 mt of methane. If we use GWP100 metric values to equate these two gases, the total farm emissions are 7,200 mt CO₂e. In comparison, if we use GWP20 to equate these, total farm emissions are much greater—15,600 mt CO₂e—because methane is more heavily weighted. These values, and the GWP conversion factors, are shown in the table below:

Gas	Emissions (metric tons—mt)	GWP100 Value	CO2e Emissions with GWP100 (mt)	GWP20 Value	CO2e emissions with GWP20 (mt)
CO2	3,000	1	3,000	1	3,000
Methane	150	28	4,200	84	12,600
Total			7,200		15,600

NOTE: GWP100 and GWP20 values are taken from the Fifth IPCC Assessment Report (AR5). While AR6 has more up-to-date values, the New York State Climate Act calls for the use of AR5 values. See Statewide Greenhouse Gas Emissions Report—NYSDEC.

What Are the Policy Implications of Using Different Emission Metrics?

The use of these different emission metrics can change which entities meet thresholds to qualify for federal and state regulations. For example, the recently published New York State Mandatory Greenhouse Gas Reporting Program requires entities meeting a certain emissions threshold to report their emissions to the New York State Department of Environmental Conservation; any facility emitting at or above that threshold would be subject to the program and required to report emissions. The typical emissions threshold is 10,000 mt CO₂e, using GWP20. Under this regulation, the hypothetical dairy described above must report its emissions. However, if New York State were to require the use of the GWP100 metric instead, the dairy’s total calculated emissions would be lower, and it would not be subject to the reporting program.

Different emission metrics also change the tangible meaning (resulting climate impacts) of net-zero greenhouse gas emissions. Some emissions of greenhouse gases, like methane, cannot be fully abated, leaving “residual” emissions that must be balanced by the removal of CO₂ from the atmosphere to get to net-zero. Under GWP100, this “drawdown” of CO₂ would cause global temperatures to peak and then decrease. Under GWP20, because methane is more heavily weighted relative to CO₂, far more CO₂ would have to be removed to get to net-zero. While theoretically, this would cause global temperatures to decline faster after they peak, experts have suggested that the sheer magnitude of CO₂ drawdown required may make reaching GWP20 net-zero more challenging to achieve over the same period of time.

The value of using different metrics has been a part of New York State climate policy discussions, especially as concerns the state’s landmark Climate Leadership and Community Protection Act (CLCPA), which was enacted in 2019.

ABOUT THE AUTHOR(S)

Maggie DeLessio is a New York State Science Fellow at the Rockefeller Institute of Government.