Analysis: What does revised methane data mean for the Paris Agreement?

§ What did the new study find?

Methane is a potent greenhouse gas and the second biggest contributor to human-caused global warming after carbon dioxide.

Livestock produce large amounts of methane as part of their normal digestive process, largely through passing wind. Also, when the animal manure is stored or managed in lagoons or holding tanks, more methane is released into the atmosphere.

The extent to which methane emissions from agriculture could contribute to future global warming has been examined by international scientific bodies including the Intergovernmental Panel on Climate Change (IPCC) and the US Environmental Protection Agency (EPA).

In 2006, the IPCC used mathematical modelling to simulate future global methane emissions from agriculture using information on agricultural practices recorded in the 1980s and 90s.

The new study, published in the journal Carbon Balance and Management, has now generated revised estimates of methane emissions for the years between 1990 and 2011. Its estimates suggest that global methane emissions from livestock in 2011 were around 11% higher than the estimates that were made by the IPCC in 2006.

To come up with new estimates, the research team used the same formulae as the IPCC, but used more recent data on global agricultural practices, explains Dr Ghassem Asrar, a study author and director of the Joint Global Change Research Institute at the Pacific Northwest National Laboratory. He tells Carbon Brief:

“Our revised estimates of global livestock methane emissions are larger than ones made using IPCC 2006 default information, but with significant variation among global regions. Global atmospheric concentrations of CH4 have been increasing steadily since 2007. Our revisions suggest that global methane emissions by livestock explain about one-half to three-fourths of this increase.”

The map below shows the percentage change in global methane emissions from livestock in 2011 when revised estimates are compared to those made by the IPCC in 2006.

The revised methane emissions exceed previous estimates in many parts of the world, including in north and south America.

The discrepancy between the previous and revised estimates is largely due to differences in the data used, Asrar says. While the data used in IPCC estimates was recorded in the 1980s and 1990s, data used in the new study was recorded in the past two decades and so accounts for recent changes in global agricultural practices, Asrar explains:

“In general, livestock body size, growth rates and productivity have increased [in recent decades]. This change tends to be associated with larger feed intake and larger quantities of methane produced.”

On top of this, many countries, including the US and Canada, have changed the way that they manage animal manure in recent decades. Manure is increasingly being stored in centralised lagoons, which has a larger methane footprint than more traditional types of manure management.

§ What does this mean for Paris?

The finding that livestock populations have released more methane into the atmosphere than once estimated could have important implications for tackling future global warming.

Media coverage of the research has focussed on what it could mean for countries striving to limit future global warming to between 1.5C and 2C, as enshrined in the Paris Agreement. Earlier today, BBC Radio 4’s flagship current affairs breakfast show, the Today Programme, reported on the new study, saying:

News reader: A study has found that Paris climate goals could be harder to achieve because global emissions of methane from agriculture have been underestimated. Methane is released by livestock as part of their digestive process. Our correspondent Claire Marshall reports.

Claire Marshall: Up until now, efforts to limit the increase of the Earth’s temperature have focused on reducing carbon dioxide emissions. However, methane emitted by livestock, such as dairy cows, in their wind and manure is a much more potent greenhouse gas. This research indicates that the targets set in Paris were based on old data. Methane is playing a considerably larger role in climate change than previously thought. However, scientists say there are solutions. For example, improving manure treatment, using different cattle feeds and avoiding food waste.

However, the new study did not strictly evaluate the effect that revised methane emission estimates could have on the Paris targets, says Prof Piers Forster, director of the Priestley International Centre for Climate at the University of Leeds. He was not involved with the new study. He tells Carbon Brief:

“Paris targets do not need to be revised in light of these studies and they do not make the treaty out of date, as the emission goal set out in Article 4 of the Paris Agreement already covers all greenhouse gases including methane. Ambition will need to increase across the board on both CO2 and short-lived pollutants if Paris targets will be met.”

Forster and Chris Smith, a research scientist at the University of Leeds, have analysed for Carbon Brief how the revised methane emissions could affect the remaining carbon budget for limiting warming to 1.5C. The carbon budget is the total amount of CO2 emissions that we can still emit whilst limiting global average warming to 1.5C.

They find that, when the revised methane emissions are included in their analysis, methane plays an increasing role in future global warming and could make the Paris targets marginally more difficult to achieve.

The chart below shows the remaining carbon budget for limiting warming to 1.5C under a range of different scenarios. The blue bar shows the remaining carbon budget in a scenario if all methane emissions were stopped in 2017, red and orange show the remaining carbon budget in scenarios that consider the IPCC’s 2006 estimates of methane emissions, while green shows a scenario considering the revised methane emission estimates.

The results suggest that revised methane emissions estimates will only have a very modest effect on carbon budgets. The revised emissions lead to a decrease in the allowable carbon budget by around 1 to 2% over the next 20 years, Forster and Smith say:

“The revisions roughly approximate to using up to three months allowed emissions of CO2. The new study reinforces that solutions to address methane emissions are more needed than ever and they can also bring considerable co-benefits for society. Yet, we should not lose sight of the need to focus on CO2 to meet Paris targets.”