Cows on a misty morning near a tree

International Panel of Scientists Puts GWP* to Test

Frank Mitloehner joins Agustin del Prado and John Lynch on paper evaluating GWP* with case studies

Quick Facts:
  • A new paper published in Animal: The International Journal of Animal Biosciences puts GWP* to the test in real-life scenarios. The authors, all internationally known and well-regarded climate scientists, physicists, animal scientist and air quality specialists, use six case studies to compare GWP* to GWP100 and demonstrate the effect various scenarios may have on global warming using each matrix.
  • Although no single metric can fully address the complexity and nuances of global warming, GWP* gives us insight not offered by GWP100 when measuring methane’s impact on warming.
  • Each CO2 emission can be thought of as raising temperatures in a straightforward, additive manner, and the warming contribution of a CO2 emitter can be determined by simply summing all of their past CO2 emissions to date. For methane, emissions do not indefinitely impact temperature. Instead, their contribution to temperature increases wanes over time after emission, as it is broken down naturally.
  • Judging gases solely based on the GWP100 would leave us unable to anticipate the different outcomes of reducing emissions of either gas, with methane-induced warming rapidly reversed once emissions start to decline but CO2-induced warming persisting for the very long-term even when emissions have ceased.
  • GWP* is still a simplifying tool that misses some elements revealed in more complex climate modelling approaches, and we should remember that for some uses climate modelling remains a superior option.
  • It is still better for the climate for us not to emit any greenhouse gases, where they can be avoided without significant trade-offs. The animal research community should continue their valuable efforts to reduce emissions from the sector.

A new article published in Animal: The International Journal of Animal Biosciences (Cambridge University Press) takes the metric GWP* and applies it in six case studies related to animal agriculture and methane emissions.

Along with First Author Agustin del Prado (Basque Center for Climate Change), five internationally known and highly regarded climate scientists, air quality specialists, physicists and/or animal scientists – including Frank Mitloehner of University of California, Davis, and John Lynch of University of Oxford, – make a case for using GWP* in place of GWP100 to assess global warming from methane, a major greenhouse gas that is emitted by livestock production.

The authors go beyond a strictly academic discussion of the merits of GWP* versus GWP100, applying the former to six case studies based on real-world scenarios in order to evaluate its effectiveness in measuring warming from methane. The exercise has important planetary implications; as the world attempts to rein in global warming and halt climate change, it’s essential to know what path to take.

“This is a study that will better inform the discussion on measuring methane,” Mitloehner said. “I was privileged to be among colleagues from throughout the world as we put our heads together in an effort to better understand that measuring greenhouse gases independently is critical to understanding how each impacts global temperatures. GWP* doesn’t make a methane emitter look better or worse, but shows that if you increase emissions, you add significant warming and if you decrease emissions you can seriously limit your warming.”

The case studies are as follows:

  1. A 2019 comparison of the warming dynamics of methane, carbon dioxide and nitrous oxide as they relate to lab-grown meat versus beef from cattle.
  2. A study of European sheep in order to determine the change in temperature contribution from ruminant production systems subject to decreasing production.
  3. The change in temperature contribution from ruminant production systems with increasing emissions, a study that deals with Brazilian cattle from 1981 to 2018. 
  4. The effect on warming of manure management designed to mitigate methane emissions in the California dairy sector.
  5. Life-cycle greenhouse gas emissions of a methane-mitigation initiative whereby Australian cattle were fed dietary supplements to decrease methane emissions.
  6. Differences in global temperature change contribution for different scenarios of future trends in meat production, emissions intensity, and gas composition. This study involves Sub-Saharan Africa.

Each case study is discussed in detail, as are the results of applying GWP* to each scenario. The authors neither proclaim the metric the be-all and end-all nor do they reject it. Instead, they honestly point out some limitations and challenges, suggesting it is a “simplifying tool that misses some elements revealed in more complex climate modeling approaches.”

However, when climate modeling is impractical or out of reach, GWP* should be a go-to metric for assessing the warming impact of methane. It is more accurate than GWP100 when considering methane, accounting for the process whereby methane from animal agriculture warms in the atmosphere for approximately 10 years before it is naturally broken down.

“When today’s emissions equal what they were about 10 years ago, a balance is reached and warming caused by that emissions source is nearly stabilized,” Mitloehner said. “But we can’t demonstrate that crucial point with GWP100. It’s simply not a good tool for the measurement of methane’s climate impact.”

The paper notes that if we assess greenhouse gases only with GWP100 we wouldn’t be able to fully understand the very different outcomes of reducing emissions of methane and CO2, with methane-induced warming rapidly reversed once emissions start to decline but CO2-induced warming persisting even when emissions have stopped.

As such, the collective goal of stabilizing our climate – reaching climate neutrality – implies that we should have different targets for different gases. A methane emitter may be able to reach climate neutrality or reduce emissions enough so that temperatures drop below current levels caused by past emissions, while still emitting methane and not having to reach GWP100-defined net-zero. GWP* can reveal these dynamics in a way that GWP100 cannot.

GWP* allows us to see how methane emissions impact temperature, whether they are rising or falling,” Mitloehner said. “Regardless of what matrix we use though, we must keep in mind that it is important to reduce emissions. That is how we stabilize our climate.”

To read the article, visit Animal: The International Journal of Animal Biosciences.

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