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The Global Methane Budget Report – Expert Reaction

The Global Carbon Project (GCP) has today released its latest analysis of the worldwide balance of methane emissions and sinks.

The report is an update from a previous analysis published in 2016. The new results show that global methane emissions increased from 2008 to 2017, and that human-made sources of emissions seem to be responsible for the majority of the increase. The findings of the report have also been published in two academic journals: Earth System Science Data and Environmental Research Letters.

The SMC asked experts to comment on these findings.

Dr Jocelyn Turnbull, Radiocarbon Science Leader, GNS Science, comments:

“While carbon dioxide is the biggest human-produced driver of global warming, methane runs a close second, and methane emissions are much less well understood. There’s nothing controversial in the Global Carbon Project’s (GCP) latest global methane budget. It synthesises what is known about methane emissions and how they have changed through time, combining knowledge from virtually every greenhouse gas research team around the world.

“The methane budget is understood by reconciling what we know at the process level (how much methane does an individual cow produce) with what we can observe in the atmosphere (what’s the total amount of methane that is emitted). This combination of two very different techniques allows us to know the total emission rate, as well as the levers that we could pull to reduce emissions.

“It is particularly pertinent to New Zealand, the only developed nation where methane plays an equal role to carbon dioxide in our emissions profile. Yet this latest effort shows that although there is now a good general understanding of methane emissions, the devil is in the details, and there is still a lot left to understand, particularly at the national and regional scales.

“CarbonWatch-NZ and the new MethaneSAT research programme, funded through MBIE’s Endeavour and Catalyst research funds, are helping to answer some of the unknowns that are specific to New Zealand’s methane budget. It’ll be exciting to see how the Climate Change Commission is able to incorporate these measurements into New Zealand’s ongoing climate policy.

“It’s a pity that this study hasn’t been able to incorporate much of the isotopic work that helps to differentiate the sources that drive changes in global methane emission rates, much of which was pioneered in New Zealand. We’ll likely see that information added in the next iteration of the global methane budget.”

Conflict of interest statement: I am a lead scientist on the CarbonWatch-NZ project, although I have not been directly involved in the methane component of the project. Both the global and NZ greenhouse gas research communities are small, and I have worked with many of the authors on projects over the years.

Dr Mike Harvey, Principal Scientist – Atmosphere, NIWA, comments:

“This new analysis of the global methane budget shows that methane in the atmosphere is currently increasing at a faster rate than any time in the past 20 years. There has been a 9% global increase in annual emissions, or 50 million tonnes per year, between the beginning of the 21st century (when methane in the atmosphere was stable) and 2017.

“This trend is a significant cause for concern in tackling global warming. The analysis indicates that the two main contributors were a human-induced increase split equally from both the agriculture and waste sector and the fossil fuels sector. Methane accounts for a large proportion (43%) of New Zealand’s gross emissions; as a consequence, there is already a strong mitigation research focus in New Zealand that contributes to global effort for agriculture. It is the balance of sources and sinks that determine whether methane is increasing or declining. Because the increases are driven primarily by emission, a focus on methane emission reduction strategies and technologies makes sense.

“NIWA contributed high precision measurements from Baring Head (near Wellington) to this major study by the Global Carbon Project. In line with the finding of a 9% global increase in methane emissions, our measurements show the growth rate of methane in the atmosphere has been 8 – 12 parts per billion (ppb) per year since 2014. A consequence is that methane concentrations are now at the highest levels we have recorded at Baring Head since the start of the 21st century.

“Globally, methane is the second most important greenhouse gas after CO2 and has accounted for 23% of human-induced global warming so far. However, it is also relatively short-lived, remaining in the atmosphere for about nine years (compared with CO2 – which can remain for centuries). This means that efforts taken to reduce emissions can quickly benefit the climate. There are still uncertainties in the methane sources; we see a need for confirmation of geographic emission hot spots and satellite-borne detection will become increasingly important. New Zealand is already contributing to this work, along with research into the atmospheric sink (removal) processes and various feedbacks and responses to global change.”

Conflict of interest statement: I am programme lead for the Understanding Atmospheric Composition and Change programme at NIWA. The programme makes an on-going contribution of observations data and analyses from New Zealand to the global effort on greenhouse gas assessment. I have no conflict of interest relating to this work.

Adjunct Professor Martin Manning, New Zealand Climate Change Research Institute, School of Geography, Environment and Earth Sciences, Victoria University of Wellington, comments:

“Scenarios used in climate models, consistent with staying below 2°C, have atmospheric methane concentrations starting to decrease this year or next year. However, the rate of increase became larger over 2018 – 2019, and it is still not clear why. Consequently, this update on the global methane budget is important. The revised budget has increasing anthropogenic sources of methane as the dominant cause of the observed increase in its atmospheric concentrations. At the same time, it notes the highest research priority should be to resolve questions about emissions from wetlands.

“Revisions to previous source estimates show significant uncertainties still exist. For example, the central ‘bottom-up’ estimate for wetland emissions over 2000-2009 is now outside the full range that was estimated four years earlier. Similarly, its revision to ‘top-down’ ranges for agricultural emissions is inconsistent with the earlier best ‘top-down’ and the current best ‘bottom-up’ estimates.

“An underdetermined methane budget is due to limits in available data, but this is not helped by the new analysis excluding carbon-13 and carbon-14 isotopic data. These have limits, but they allow us to differentiate between hundreds-of-millions-year old fossil fuel sources and modern carbon sources of methane such as agriculture. New Zealand’s development of accelerator mass spectrometry to measure the fossil fuel contribution to methane levels gave estimates of ~30% in the 1990s and, earlier this year, a totally independent analysis of Antarctic firn air gave a very similar result. However, the latest budgets exclude these data and have fossil fuels at less than 20%. Given such differences across methods used to construct a methane budget, citing ‘best estimates’ seems optimistic.

“Removal of methane from the atmosphere by reacting with hydroxyl radicals formed in sunlight is the largest term in a methane budget and important when considering trends or variability. Saunois and colleagues use a recent analysis of results from atmospheric chemistry models on how competing effects have probably increased hydroxyl so that methane is becoming removed a bit more rapidly.This has been backed up recently by several new studies of atmospheric chemistry. Overall, this means that the increase in methane sources is likely to be larger than estimates which are based on no change in removal rate. Evidence that agriculture is contributing to the increase in methane source is very relevant for New Zealand policy.”

No conflict of interest declared.

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