I’ve always thought of our path to climate neutrality as a journey. And like any sort of travel, there are starts and stops, detours, discoveries – and yes – even a wrong turn or two along the way. So, when I was invited to be part of a team of international scientists evaluating the effectiveness of GWP* in assessing the use of a new metric for methane under various case study conditions, I jumped at the chance to move along in this critically important journey. Our resulting study has just been published in Animal: The International Journal of Animal Biosciences (Cambridge University Press), and I’d like to share some of our findings with you.  

Under the leadership of First Author Agustin del Prado (Basque Center for Climate Change), five internationally known and highly regarded climate scientists, air quality specialists, physicists and/or agriculturalists – including John Lynch of the University of Oxford and yours truly – make a case for using GWP* to better understand warming from methane, a major greenhouse gas attributed to animal agriculture. In order to do so, we applied GWP* to six established case studies related to animal agriculture and methane emissions.

By way of background, when measuring the warming impact of methane, GWP* offers insight on how methane warms our climate that can’t be uncovered with GWP100. The older metric, GWP100, converts each greenhouse gas to a carbon dioxide equivalent, but in so doing, it fails to account for the behavior of the flow gas methane as its impact on temperature wanes over time. Conversely long-lived gas such as carbon dioxide will influence temperature for centuries but the short lived climate pollutant methane does not. As a result, many scientists are of the mind that GWP100 has given us an inaccurate picture of the climate impacts caused by methane, in some cases overestimating the impact of emissions and underestimating in others.

This is important, because measuring greenhouse gases with GWP100 would mean we could not accurately anticipate how reducing emissions of either gas will impact our climate, with methane-induced warming rapidly reversed once emissions start to decline but CO2-induced warming persisting for a long time even when emissions have ceased.

I don’t expect that GWP100 will disappear - It is a standard will continue to be a standard. In fact, GWP* is based on GWP100, and will continue to be an informative metric for society. I also want to note, as the paper does, using either metric does not negate the fact that we’d all be better off by focusing on reducing emissions of both gases.

Methane is the primary greenhouse gas concern for livestock production. I share the belief that we haven’t captured a realistic picture of the current state of methane emissions and more importantly, the effect solution-oriented programs will have on helping the sector reach climate neutrality. It is completely feasible for the livestock sector to reach a point in which they no longer add additional warming to the atmosphere, or climate neutrality, and GWP* can illustrate that. It’s an important mile marker on animal agriculture’s sustainability journey, but not the only one, as the sector can also go further and reduce emissions enough to make up for historical ones.  This would go above and beyond what the fossil fuel sector can achieve.  

I find it invigorating to join with others to discuss, debate and study challenges, and this study was certainly no exception. It was a privilege to be among colleagues from throughout the world as we put our heads together to further move animal agriculture forward on the path to climate neutrality.

As I mentioned, we approached our task armed with six previously published case studies involving animal agriculture and greenhouse gas emissions to determine if the warming scenario under GWP* is different from GWP100. The fourth study is of special interest to me, as it involves manure management strategies in California’s dairy sector, with which I work closely.  

For several years, with support from public sector funded programs, we’ve been building and utilizing anaerobic manure digesters in California – with very promising results. By covering the dairy lagoons, we can prevent methane from entering the atmosphere. At the same time, the resulting biogas can be converted to be an alternative fuel for heavy duty trucks and buses. Research from colleagues and I show that digesters may reduce greenhouse gas emissions by 4.15 million metric tons by 2030, contributing the lion share of the dairy industry’s emission reduction goals.

For our publication, we estimated the methane emissions from manure management in California between 2019 and 2030. We then calculated how these emissions would be measured with GWP*.

From 2010 to 2018, GWP* results showed a decrease in methane after a high in 2012. The increase in warming was due to rising methane emissions from (uncovered) livestock manure, due to the increase in the state’s dairy-cow herd size during the same period.

However, between 2019 and 2030, while GWP100 results show a constant net emission every year, annual GWP* results continually decrease over time and fall below zero in 2022. This substantiates our solid belief that the sector is on the right track with manure management, and in fact, it can achieve climate neutrality and even begin to reverse its warming trend of previous years.

But without GWP*, we can only hypothesize what the warming outcomes would be due to mitigation. Whereas the cumulative GWP100 results show ongoing increases over time, GWP* shows us an entirely different – and infinitely more hopeful – picture, which is the red line in each figure below.

No climate metric is perfect; the complexity and nuances of global warming are far too complex and climate models should be used when possible. However, our work shows that when climate modeling is impractical or out of reach, GWP* should be our go-to metric for assessing the warming impact of methane. It is more descriptive of actual warming impacts than GWP100, accounting for the process whereby methane from animal agriculture warms in the atmosphere for approximately 12 years before it is broken down in the atmosphere.

Simply put, when today’s emissions are nearly equal to what they were 12 years ago, we’ve achieved climate neutrality, the point at which no additional warming is taking place. But we can’t demonstrate that crucial point with GWP100. It’s simply not a good tool for the study of methane.

Without getting ahead of ourselves, after the animal agriculture industry achieves climate neutrality – which I am convinced can be done by California’s dairy sector in less than a decade – it can begin picking up the slack from those that broker in carbon dioxide.

Thanks to GWP*, we can demonstrate that and more importantly, we are on our way to doing that.