Dual Term Greenhouse Gas Accounting FAQ

Brief Overview

Both short- and long-term global warming impacts should be considered when making decisions about climate policy as it is imperative to address warming both during our lifetimes and for future generations. The Put Climate On Pause (PCOP) coalition propose the adoption of a very simple two-valued reporting standard that maintains the familiarity of the widely-used (and resistant to replacement) global warming potential (GWP), but includes both 20- and 100-year timescales to provide a more complete picture of the climate impacts of specific decisions. On the other hand, the status quo of reporting only one timescale has led to confusion and to unproductive debates within the policy community. For further details we would recommend reading the primer on our policy proposal here.


Frequently Asked Questions

Does this affect me?

If your work involves greenhouse gas or aerosol emissions, no matter the source or country, then this approach is relevant.


Why not an alternative metric like GTP?

GWP is the most widely used metric in climate policy, and decades of efforts to replace this metric with alternatives has failed. Because we need to act on climate immediately, we don’t have time to campaign for a different metric and hope that it catches on – we need to work with what we’ve got. In addition, there is no such thing as a perfect metric, and especially a way to combine all timescales into one number. Therefore, our best chance at improving climate policy decisions is to stick with GWP, but employ two time horizons to capture near- and long-term impacts. This can also be viewed as a ‘bare minimum’ approach, and users can still select other metrics or models if they desire. In fact, for GTP, two time horizons become even more important, because the values become even more stark for different timescales.


Why 20 and 100 years?

Regardless of whether 20 and 100 years are the most appropriate time horizons, they have evolved as defaults within the climate policy community, just as GWP is the default metric. Attempts to shift the community toward different time horizons would encounter the same resistance as switching metrics. Because 20- and 100-year time scales capture near- and long-term climate effects and because of the urgency of climate action, we believe that benefits of advocating different time horizons do not compensate for delays of implementation that continued wrangling could incur.


Isn’t two numbers too complicated?

The two-valued metric, GWP20/100, is similar in construct to many everyday reporting systems: systolic-diastolic blood pressure, city-highway gas mileage, low-high temperatures, verbal-math SAT scores, and more. While each of the pair is important, it is incomplete without the other.


What does this mean for the 1.5/2 C targets?

The implications of this new standard for the 1.5/2C goals are that it becomes more achievable because you’ll get further towards your goal with more actions reducing emissions of short-lived climate pollutants (as long as they are sustained over time).


What does this mean for emissions trading?

One has to make sure the trade works in both timeframes; the trade has to be beneficial for one timeframe and neutral for the other, or beneficial for both, but cannot go through if it makes the climate worse in one of the timeframes.


Does this require legal change for carbon markets?



What does this mean for methane?

It means that methane impacts will be communicated in both near- and long-term timeframes, providing a more complete picture of climate impacts of methane-related activities. The current standard of GWP with a 100-year time horizon masks methane’s important role in near-term warming.


Why is this strategy so important and will it change decisions?

Because we care about climate change on ALL timescales, we need a metric reporting standard that addresses both near- and long-term timescales so that decision makers are fully informed when making decisions. The status quo of selecting one time horizon has led to uninformed decision making, confusion, and even debates. There are several examples of why a two-valued GWP approach is important and how it can impact decisions:


  • Achieving greenhouse gas targets: we need to make sure that we calculate targets for both timescales in order to make sure that we are benefiting both the near and long-term with our emissions actions. The transparency rulebook for the Paris Agreement is set to be published in late 2018, is being negotiated at COP23 APA/SBSTA sessions, and it is critical that they require countries to report emissions in both time horizons.


  • Prioritizing important greenhouse gases and sectors to mitigate: the status quo for comparing emissions of different greenhouse gases is to use carbon dioxide equivalence with a 100-year time horizon. The has led to overlooked near-term impacts of various actions and sectors. Examples include:


  • The U.S. EPA website, a go-to for policymakers, states that methane accounts for 10% of U.S. greenhouse gas emissions. A recent article used this source to downplay the importance of methane by stating that methane is ‘just’ 10% of overall emissions. However, this is misleading because it spreads methane’s powerful 20-year warming over a 100-year period. If the two-valued GWP approach is adopted, it would provide a more holistic representation of methane’s importance by showing that methane emissions account for 25% of American total greenhouse gas emissions when considering impacts on near-term warming.


  • The Iowa Climate Change Advisory Council Report (2009) ranks greenhouse gas reduction strategies in order of most to least climate benefits using GWP100. If GWP20 is used to determine relative benefits in the near-term, the order is considerably altered. Overall, all of the Clean and Renewable Energy sector mitigation actions show more climate benefits in 2020 than the actions within the Agriculture, Forestry, and Waste Management sector. On the other hand, if GWP20 is used, they have the same perceived benefits. For specific actions, an example of reordered benefits is that Large-Scale Manure/Methane Management Capture Utilization action, which previously yielded less climate benefits from 2009-2020 than Financial Mechanisms for Energy Efficiency; Support for Grid-Based Renewable Energy & Development (MGA Target of 20% of retail sales by 2020); Pricing Strategies To Promote Renewable Energy and/or CHP; Fuel Strategies (20% Low Carbon Fuel Standard); Expanded Use of Agriculture and Forestry Biomass Feedstocks for Electricity, Heat, or Steam Production; and Cellulosic Biofuel, now yields more benefits than all of these actions. Because the climate benefits are reordered, the cost-benefit analysis is considerably affected as well.


  • A recent paper showed that the impact of Australian agriculture is underreported with the standard GWP100. Livestock accounts for 15-30% of Australian greenhouse gas emissions with a GWP100, but once short-lived climate pollutants are accurately accounted for and a GWP20 framing is employed, this value nearly doubles to 54%. This shows how significant Australian agriculture is for near-term climate warming, an important insight for climate change mitigation that is overlooked with the status quo.


  • On the other hand, the documentary Cowspiracy only communicates the effects of the livestock industry in the near-term, and does not adequately define this. Therefore, there are a lot of inconsistencies between their statistics and that found on other websites, that can be easily cleared up if a dual timeframe approach is employed.


  • Comparing different technologies: Time horizon plays a major role when comparing the climate impacts of different technologies, and in some cases has led to conflicting conclusions or overlooked consequences. For example:


  • Natural gas is widely considered a cleaner fuel than coal because it emits half as much CO2 on average. However, when accounting for large methane leaks throughout the supply chain, the warming impacts of natural gas can actually outweigh that of coal in the near-term. If only GWP100 is used, one wouldn’t realize that the real benefits of switching from coal to natural gas may not take place until decades later. Further, when different studies use different time horizons, they get different results which has confused policymakers in the past.


  • With the closing of nuclear plants in California, New York, and Vermont, decisions are currently being made on how to replace energy generated via nuclear. Options include natural gas and hydropower, and both of these options yield climate impacts in the near- and long-term because they emit both methane and carbon dioxide. New York, in particular, has ambitious greenhouse gas targets that may be impossible to reach in the near-term because of increased methane emissions from hydropower and natural gas. When only a 100-year time horizon is employed, the warming impacts of these technologies in the near-term is overlooked.


What if there is pushback from NGOs that focus mainly on CO2?

This dual-approach prevents one time horizon from skewing results in either direction. For example, those that only focus on long-term impacts will have to include near-term impacts as well, and vice versa. This strategy therefore ensures consistency across advocacy efforts and the over-inflation of certain actions by neglecting the other time horizon.


How do I use this approach?

  • If referring to the GWP of a gas (i.e. warming impact of a gas compared to CO2), you would say GWP20/GWP100 such that “methane GWP is 84/28.”
  • If referring to carbon dioxide equivalence (i.e. weighting emissions by GWP), you would say “the CNG car emits 115/80 kg CO2e per unit energy, and the gasoline car emits 95/89 kg CO2e per unit energy.”

As Seen On