Global warming potential is the metric that is most commonly used to combine the effects of various greenhouse gases (GHGs). Non-CO2 pollutants are evaluated relative to CO2 and their GWP represents the expected global warming impact relative to CO2. Carbon dioxide equivalent (CO2e) is calculated as the sum of each climate pollutant mass multiplied times its GWP.
But GWP has flaws, one of which is the need to select a time horizon for integrating the impacts. Two time-horizons are commonly used: 100 years and 20 years (GWP100 and GWP20). While GWP100 is more common, GWP20 has been adopted for specific applications and by certain jurisdictions. Use of GWP100 for carbon accounting and reporting is well established. The 197 nations to the Paris agreement (including the U.S.) have agreed to use GWP100 to report aggregate emissions and removals of GHGs at the national level.
To address the flaws in GWP, a number of alternative metrics to GWP have been developed and suggested. One of the more interesting is GWP* (the star “*” is added). This metric makes the case that short lived climate pollutants (SLCPs) should be treated differently from long-lived climate pollutants like CO2. The SLCP of most interest is methane, which has a life of 12 to 15 years. Proponents of GWP* argue that what matters with SLCPs is the rate of change in atmospheric concentration. If methane enters the atmosphere at the same rate as it is degraded to water and CO2, there is no global warming effect. An agriculture example is often used. The rancher who first grows his herd from 2 cows to 1,000 cows is responsible for climate change, but the farmer that maintains the 1,000-head herd is not adding to the problem, although if he reduced the size of his herd, the climate would benefit. While GWP* has benefits, it depends on past emissions, and hence raises questions of equity and fairness when applied to anything other than a global scale. Applying GWP* in the national context would benefit countries with high historic emissions and put those with low emissions - mainly developing countries - at a profound disadvantage.
ASHRAE Standard 189.1 (and the IgCC) publishes emission rates for fossil fuels delivered to buildings, electricity used in buildings and thermal energy delivered to buildings from district-level steam, hot water or chilled water plants. A zero-carbon emissions factor (zCEF) is used for evaluation, which represents the ratio of the CO2e emissions of the proposed design to a stable baseline building that does not change over time.  The standard sets zCEF targets based on climate zone and building type. A zCEF of zero would be a zero-emissions building while a zCEF of one would have emissions equal to the baseline building.
But other design decisions are more complex. Once heating loads are reduced through energy efficiency, is it better to use electric heat pumps or gas furnaces for space heating? Which option results in fewer GHG emissions? Heat pumps perform better in warm to moderate climates while furnace efficiency does not vary that much with climate. Furthermore, while emissions from burning natural gas (or other fossil fuels) do not vary much by region, the emissions related to electricity consumption may vary considerably, depending on the mix of electric generators that feed the grid. Click here for more information.
While buildings may last for centuries, space and water heating equipment have a much shorter life, typically in the range of 15 to 30 years. At the end of equipment life, owners can again choose between electric or gas equipment. In these instances, the time frame for evaluating carbon emissions is much shorter than 100 years. The question for building designers becomes, which HVAC or water heating system has the fewest operational GHG emissions over the equipment life time?
The impact of GWP on design decisions, and in particular equipment choices, is the primary reason that the Standard 189.1 project committee decided to use GWP20 for assessing the GHG emissions in buildings. This was by far the most controversial decision by the committee (at least as represented by the number of comments submitted during public review). Using GWP20 places much more emphasis on methane emissions and causes electric equipment to look somewhat better than gas-fired equipment than would be the case with GWP100. Special interests are on high alert anytime a standard encourages one technology over another, which explains some of the public review comments.
With the Standard 189.1 procedure, equipment choices in the baseline building are fixed according to building type and climate zone, so equipment choices in the proposed design make a big difference. Click here for a table that shows HVAC system types for the baseline building. If the baseline building system uses natural gas for heating and the building is located in an eGRID subregion with low electricity emissions, designing the proposed design as an all-electric building would have a significant climate benefit. But the opposite could be true. A gas furnace or boiler might have fewer emissions in a cold climate where electricity emissions are high, e.g., lots of coal-fired generators.
The decision to use GWP20 with Standard 189.1 was not unanimous, but a majority of the committee supported the decision. Also, the decision is consistent with direction from the IPCC. While the IPCC recommends GWP100 to consistently report aggregate emissions and removals at the national level, it recognizes that GWP20 is appropriate for other applications.
 Paul Balcome, et. al. Methane emissions: choosing the right climate metric and time horizon, Royal Society of Chemistry, September 10, 2018. The authors note three issues with GWP: First, is having to choose a time horizon; they note that this is particularly an issue with methane. Second, GWP is designed to address a pulse emission, as opposed to sustained or developing emissions. Third, the physical basis of GWP is radiative forcing and not temperature impact.
 Early versions of the IPCC Assessment Reports included GWP for 500 years, but the 500-year data have been dropped in more recent versions.
 New York State Climate Leadership and Community Protection Act, passed by the legislature and signed by Governor Cuomo mandates that a 20-year time horizon be used for calculating GWP as the state pursues its ambitious goals. See https://legislation.nysenate.gov/pdf/bills/2019/S6599.
See also Methane Emissions and Greenhouse Gas Accounting: A Case Study of a New Approach Pioneered by the State of New York, Robert W. Howarth, Department of Ecology & Evolutionary Biology, Cornell University, Ithaca, NY 14853 United States
The California Air Resources Board uses 20 years in assessing its progress toward state mandated goals and the California Energy Commission used 20 years in the development of its TDV energy metric for 2022.
 Methane, Cows, and Climate Change: California Dairy’s Path to Climate Neutrality, https://clear.ucdavis.edu/sites/g/files/dgvnsk7876/files/inline-files/CLEAR-Center-Methane-Cows-Climate-Change-Sep-2-20_6.pdf
 Climate Analytics, Greenhouse Gas Accounting Metrics under the Paris Agreement A Cautionary Tale of the Implications of Applying Novel Scientific Concepts to an Existing Policy Context, December 2019. See https://climateanalytics.org/media/gwp_star_briefing_final.pdf.
 The baseline building is defined by the performance rating method of Standard 90.1, Appendix G and is roughly equivalent to a building that complies with ASHRAE Standard 90.1-2004.
 Paul Balcome, op. cit., Table 4 indicates that a 20-year time horizon is appropriate for technology assessments.