Methane gas is bad for the atmosphere
A puzzling increase in methane in the air threatens the Paris climate target
In the early 2000s, the level of greenhouse gas in the air appeared to have stabilized. But it has been increasing again since 2007. Scientists ponder the causes.
The gas escapes from rice fields and cattle stomachs: methane is a greenhouse gas more than 20 times as effective as carbon dioxide. Nevertheless, the emission of methane from man-made sources is only the second most important driver of climate change. This is due to the much lower concentration compared to carbon dioxide.
Since the beginning of industrialization, the methane content of the air has increased by around 150 percent. Causes include fossil fuel use, intensification of agriculture, and other human activities. At the turn of the millennium, methane levels seemed to stabilize. However, since 2007 researchers have recorded a significant increase again, which has accelerated again since 2014. This is cause for concern. Because the more methane accumulates in the atmosphere, the more difficult it will be to meet the two-degree target agreed in the Paris Climate Agreement.
The methane in the air is increasing again
Researchers can currently only speculate about what the renewed rise in methane is due to. It is clear that the complicated equilibrium of methane sources and sinks must be disturbed. Either the release of methane increases or the methane breakdown in the atmosphere slows down. Or both take place at the same time.
There is a lack of measurement data
Scientists find it difficult to answer these questions mainly because of the lack of data. The mean methane content in the air is recorded well with satellites. However, statements about the sources of methane, which are often very variable in terms of time and space, can hardly be made, explains Euan Nisbet, Professor of Geosciences at the Royal Holloway College of the University of London: in long-term measurements close to the ground, especially in the tropics, which are a huge methane factory, ”says Nisbet.
It is estimated that half to two thirds of global methane emissions are caused by humans. Every year around 350 million tons of methane are released into the atmosphere through fossil fuel use, agriculture and landfill. Significant sources include coal mines in China, leaky gas pipelines in the USA, and rice fields and cattle herds in China and India.
The main natural source of methane is wetlands. They emit around 30 percent of the world's methane. Large wetlands can be found, for example, in Canada and Russia, in the Amazon region, along the Congo and the Zambezi, and on Borneo. The extent of wetlands often varies considerably between the dry and rainy seasons. And at high latitudes, the soils freeze in winter. This makes it difficult to estimate the emissions from these regions precisely.
Isotopes serve as evidence
In the analysis, researchers take advantage of the fact that methane contains a carbon atom that occurs predominantly in two variants: as the lighter isotope C-12 and the heavier C-13. Chemically, the two behave identically. Depending on where the methane comes from, however, their proportions differ. In water-rich soils, swamps, rice fields, water sediments and the stomachs of ruminants, microorganisms produce methane by decomposing organic material in the absence of oxygen. The methane produced in this way contains less heavy C-13 than methane, which is produced in the extraction and combustion of natural gas, coal and oil.
Since 2007, researchers have been measuring a growing proportion of C-13 in addition to ever higher methane concentrations. This could indicate that microorganisms in wetlands, cattle stomachs, or landfills are producing more light methane due to climate change. Because the warmer it is, the faster microorganisms decompose organic material in the soil or at the bottom of rivers and lakes. "Higher temperatures increase methane production and thus the emissions of most natural and artificial systems," says Tonya DelSontro, biogeochemist in the F.-A. Forel des sciences de l’environnement et de l’eau of the Université de Genève. This is supported by the fact that the sharp rise in methane content from 2014 to 2017 coincided with above-average temperatures.
Heavy methane from fuels
Researchers suspect that at least some of the extra methane comes from fossil fuels. Because with the methane content, the proportion of ethane, a component of natural gas, in the atmosphere has also increased. Although there are currently great uncertainties in the estimates of fossil methane sources, according to Nisbet, the national greenhouse gas balances indicate a rapid increase in fossil emissions.
If more heavy methane from fossil fuels gets into the atmosphere, the biogenic emissions of light methane must have increased disproportionately. There is hardly any other explanation for the trend in the ratio between C-12 and C-13. Or the C-13 content in newly developed gas fields and coal mines is lower than in older ones. "We have very little information for the coal industry from China and India, the two largest coal-producing nations," says Nisbet.
Another possible explanation would be a decrease in natural and intentionally set fires. Slash and burn, bush fires and fires in tropical peat soils produce methane, which contains a relatively high amount of C-13. Less widespread fires in recent years may have offset the increase in heavy methane emissions from fossil fuels.
Perhaps the breakdown is weakening
It is also conceivable that methane accumulates in the atmosphere because it is broken down more slowly. The average lifespan of a methane molecule in the atmosphere is nine years before it is destroyed by other, short-lived molecules. If their content decreased, methane would last longer in the atmosphere and its concentration would increase. This mechanism seems to be responsible for the increase in methane at best, but it cannot be ruled out. A less effective “self-cleaning” of the atmosphere in response to climate change would be deeply worrying, as Nisbet and colleagues wrote in a study published in February.
The methane increase observed since 2007 was so unexpected that it is not included in the emission scenarios on which the Paris Climate Agreement is based. If the methane content continues to rise so sharply, global carbon dioxide emissions must fall much more rapidly and earlier than previously assumed so that the two-degree target can be met.
"Even if the increase in methane does not have to be caused by fossil fuels - the fossil emissions are huge and the obvious starting point for reducing emissions," says Nisbet. In any case, anthropogenic emissions would have to decrease, since humans could hardly control the natural ones, it is said in specialist circles. Methane emissions from wetlands, so DelSontro, continued to rise in the course of climate change with great certainty.
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