Fossil fuels and farming share the blame for rising methane

The Global Carbon Project (GCP) has released its latest analysis of the worldwide balance of methane emissions and sinks, showing emissions increased by 9% – or 50 million tons – from 2007 to 2017.

Human activities are responsible for about 60% of total methane emissions, most of the increase coming from the fossil fuel sector and the agriculture and waste sector.

The researchers say their report highlights the need for stronger mitigation in both areas.

The report updates a previous analysis published in 2016 as part of the GCP’s monitoring of global methane (CH4) sources and sinks to the atmosphere.

The latest study was conducted by an international research team and led by the Laboratoire des Sciences du Climat et de l’Environnement (LSCE, CEA-CNRS-UVSQ) in France, under the umbrella of the GCP which initiated the work. Two articles have been published in Environmental Research Letters and Earth System Science Data.

Methane is the second anthropogenic greenhouse gas after carbon dioxide (CO2). But methane has a warming potential 28 times higher than carbon dioxide (on a 100-year time horizon).

Since 1750, atmospheric methane concentration has more than doubled due to emissions from human activities. After a period of stabilisations in the early 2000s, methane concentrations are rising again since 2007.

The increase in methane concentrations follows trends of future scenarios that do not comply with the objectives of Paris Agreement.

Marielle Saunois, researcher at LSCE-UVSQ and study co-ordinator, specifies that “this budget covers the state of the art of our current knowledge on each source of methane, from the largest (wetlands) to the smallest (hydrates); and more than 90 collaborators contributed”.

The methane budget remains uncertain but about 60% of methane emissions are anthropogenic

Human activities contribute about 60% of total methane emissions. Natural sources are multiple and diverse:  wetland, lakes, reservoirs, termites, geological sources, hydrates etc. The uncertainties on the estimates for each of these sources remain high (more than 100% for inland water systems).

Emissions from agriculture and waste activities contribute 60% of anthropogenic emissions. Anthropogenic emissions are shared as follows between the different main sources of methane:

  • – 30% from enteric fermentation and manure management
  • – 22% from oil and gas production and use
  • – 18% from handling solid and liquid wastes
  • – 11% from coal extraction
  • – 8% from rice cultivation
  • – 8% from biomass and biofuel burning
  • – The rest is attributed to transport (e.g. road transport) and industry

64% of global methane emissions originate from the Tropics (<30°N), 32 % from the Northern mid-latitudes (30°N-60°N) and only 4% from the Northern high latitudes (> 60°N).

Anthropogenic emissions continue to rise, with an equal share between fossil fuel sector and agriculture and waste sector

Between the reference period 2000-2006 when atmospheric methane was stable, and the last year of the presented budget (2017), global methane emissions increased by 9 %, 50 million tons of CH4. 60 % of this increase is attributed to tropical regions and the rest to the northern mid-latitudes. Methane emissions from boreal regions did not increased significantly. This means that the high climate sensitivity of boreal regions does not (yet) translate in large increase in methane emissions.

This increase in methane emissions is mainly attributed to anthropogenic emissions: 60% related to agriculture and waste, and 40% to fossil fuel sources. A small decrease (few millions of tons) in biomass burning sources is derived.

The three main regions contributing to this methane emission increase are likely to be Africa, China and Asia, each contributing 10-15 million tons of CH4. Then North America is likely to contribute to 5-7 million tons, including 4-5 million tons from USA.

In Africa and Asia (except China), the agriculture and waste sector contribute the most, followed by the fossil fuel sector. This is the opposite for China and North America, where the increase in the fossil fuel sector is largest than the one in the agriculture and waste sector.

Europe seems to be the only region where emissions have decreased, by between -4 and -2 million tons, depending on the approach used for the estimation. This decrease is mainly related to the agriculture and waste sector.

Temperature increase in the boreal regions leads to permafrost thaw and changes in ecosystems, by creating thermokarstic lakes for example. However, methods based on atmospheric observations do not find any signal of increasing emissions in these regions for the moment.

Currently, methane concentrations increase in the atmosphere at a rate of about 8-12 ppb/yr since 2014, as fast as in the 80s.

In 2017 and 2018, the growth rates are estimated at 8.5 et 10.7 ppb/yr, 2 of the 4 highest rates since 2000. This trend is almost as high as in IPCCscenario (AR5) leading to a 3-4 °C increase in mean global surface temperature by 2100, not compliant with Paris agreement objectives. This trend needs to be confirmed in the next few years and further analyzed in terms of Paris Agreement objectives.

Methane has a lifetime of about 10 years in the atmosphere, much shorter than CO2.

In Wellington, the Science Media Centre asked experts to comment on these findings.

Dr Jocelyn Turnbull, Radiocarbon Science Leader, GNS Science:

“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:

“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:

“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.

Sources:  Scimex and the Science Media Centre





Author: Bob Edlin

Editor of AgScience Magazine and Editor of the AgScience Blog

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