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[1], 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).[2] While GWP₁₀₀ is more common, GWP₂₀ has been adopted for specific applications and by certain jurisdictions.[3] 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.[4] 

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.[5] 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.[6]

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. [7] 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.

The emission rates in Standard 189.1 serve a different purpose from carbon reporting or carbon accounting at the national or corporate level. The purpose for evaluating GHG emissions at the building level is to provide guidance to building designers on which products, systems, and design solutions result in fewer GHG emissions and less impact on global warming and climate change. Some things are obvious. The less energy a building uses (the greater its energy efficiency), the lower its emissions. With the same type of equipment in both the proposed design and the baseline building, the choice of GWP₁₀₀ or GWP₂₀ does not matter in comparing the proposed design to the baseline building, since the same GWP is used for both the numerator and denominator.

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.[8]