Methane

Methane, along with carbon dioxide, nitrous oxide and CFCs, is one of the long-lived greenhouse gases and influences the radiation budget of the atmosphere and thus the anthropogenic (man-made) greenhouse effect.

Table of Contents

1 Atmospheric concentration changes

After carbon dioxide, methane is the second most important anthropogenic greenhouse gas, with a radiative forcing of 0.48 W/m 2 compared to 1.66 for CO2. Over the last 650,000 years, atmospheric methane concentrations have ranged from 400 ppb during cold periods to 700 ppb during warm periods. It has increased since 1750 from 722 ppb to approx. 1803 ppb more than doubled in 2011. [1] The current level is unprecedented in the last 650,000 years (s. Fig. 1). While earlier data were from air bubbles trapped in ice or firn, methane concentrations have been measured directly in the atmosphere and in a globally representative manner since 1983. During this time, the methane concentration has once again increased significantly. However, the growth rate of methane increases has declined to near zero from the early 1980s to 2005. This decrease is not sufficiently understood in research until today. Hypotheses range from lower anthropogenic emission to changes in sinks. In the case of the most important sink, the reaction with the hydroxyl radical OH (s.u.), but no significant changes can be detected. The decline in anthropogenic emissions can be explained in part by the economic collapse of the former Soviet Union. However, changes in rice cultivation methods, especially in China, where new varieties require less long irrigation periods, are also a possibility. In addition, there may be prolonged periods of drought in the northern wetlands. [2] [3]

What is striking about the growth rate of the methane concentration is the strong annual fluctuations. Explanations have been attempted for some of these variations over the past 25 years. Thus, the drop in the rate of increase in 1992 with the onset of Mt. Pinatubo has been linked to this. Large amounts of aerosols and sulfur dioxide were ejected into the lower stratosphere during the volcanic eruption, which are believed to have negatively affected photochemical processes and the removal of CH4 by OH. It is also possible that lower temperatures and precipitation as a result of the Pinatubo eruption reduced emissions from wetlands. Conversely, the significant increase in the 1998 growth rate is believed by some researchers to be related to the warming caused by the 1997/98 El Nino, which may have promoted emissions from wetlands and biomass burning in the boreal climate zone. [4]

In 2007, there was a renewed turnaround in the trend of methane concentration (s. Fig. 2). Methane levels in the atmosphere rose again, reaching 1867.6 ppb in October 2018 and 1890 ppm in 2021. [5] The average rate of increase was 6.9 ppb per year in 2007-2015, [6] as high as 12.7 ppb in 2014, and the highest value to date since the mid 19080s of 14.7 ppb in 2020. [5] Whether the thawing of the permafrost in the Arctic is already having an additional effect here is unclear. [6] Other reasons may be increasing emissions from growing energy production in China and India, or stronger emissions from tropical wetlands. [3] According to a recent study [7], two-thirds of the sources of both natural and anthropogenic global emissions, which amounted to 559 Tg/year in 2003-2012, are in the tropics. The increase in methane emissions after 2007 is primarily from agricultural sources, mainly in Africa and Asia. Temporary strong growth also came from forest fires in Indonesia due to El Nino conditions in 2015/16. Emissions from fossil fuel extraction, transport, and processing, on the other hand, have been decreasing since 2000, as has biomass burning. However, this thesis is contradicted by a recent study by R.W. Howarth. [8] According to this, the increase in recent methane emissions is not primarily from biological sources such as tropical wetlands, rice cultivation, or livestock, as the proportion of carbon isotopes 13 C in atmospheric methane has been decreasing since 2009. Rather, the main source of the increase in methane emissions is the exploitation of shale gas, which is mainly extracted in the U.S. through fracking. Shale gas extraction had increased 14-fold globally between 2005 and 2015, with the U.S. accounting for 89% of that increase.

Another approach is to consider the methane sink through reaction with OH, which may have decreased due to a decrease in OH concentration, [9] although here, as with other processes, climate change may have played an amplifying role. [10] The uncertainty in this issue is largely due to the lack of data.

The current rise in methane and the unexplained nature of its causes pose a significant problem for the 2015 Paris climate agreement, in which the global community set a goal of limiting global warming to 2 °C, and preferably as low as 1.5 °C. To meet these limits, a very rapid reduction in methane emissions is essential. Thus, according to the RCP2 scenario.6 that could enable the 2°C target, in the case of methane a 500 ppb reduction from the 2005 baseline (1754 ppb) by 2100. In 2018, however, the value was already 100 ppb above the projection of RCP2.6 scenario (under which a 2°C limit would be possible) for this year. Another problem is that, according to recent research, methane is not 25, but 32 times as effective on climate as CO2. [10]

2 Sources and sinks

2.1 sources

Methane (CH4) is a greenhouse gas that is emitted from both natural (z.B. swamps, termites, forests) as well as anthropogenic sources (z.B. rice fields, landfills, or natural gas extraction and transportation) comes from. The gas is usually produced during decay processes under anaerobic conditions (d.h. under exclusion of air) with the participation of microorganisms.

During the 2000s, natural methane sources accounted for 35-50% of total global methane emissions. There is great uncertainty about the amounts emitted by individual sources. The main natural source is wetlands, from which 177-284 Tg [11] of methane escapes per year. [12] They are mainly located in the tropics, from which 70% of emissions from global wetlands originated in the early 2000s, and in boreal latitudes. [13] Wetlands are very sensitive to climate change, especially to higher precipitation levels. In addition, CH4 emissions from wetlands were observed to increase in response to higher atmospheric CO2 concentrations, which appears to be related to the fact that plant water demand decreases with higher CO2 concentrations, resulting in increased soil moisture. [12] Termite mounds are also an important source with 20-30 Tg/year. Other natural sources, the magnitude of which is unclear, are outgassing from the ocean and from the earth’s crust, natural forest fires, wild animals and methane hydrates at the continental margins of the oceans. Recently, significant outgassing has been measured from the upper layers of the offshore Arctic Ocean, the amount of which may be related to the melting of Arctic sea ice and thus would be anthropogenically influenced. [14]

50 to 65% of global methane emissions in the 2000s came from anthropogenic sources. [12] The most important anthropogenic source, 87-94 Tg/year, is livestock from ruminants, especially cattle, which produce methane during digestion. India, China, Brazil and the USA contribute the most to this. In 2003, India alone, the country with the highest cattle population in the world, emitted 11.8 Tg/yr. Another important anthropogenic source is wet rice cultivation with 33-40 Tg/year, on whose flooded fields anaerobic rotting processes take place. 90% of these emissions come from tropical Asia, primarily China and India. In addition, the extraction and transport of gas at 85-105 Tg/year and coal mining play an important role. Landfills and biomass burning are also mentioned as important sources. [12]

2.2 sinks

In the atmosphere, methane has a relatively short residence time of 9 years. [15] The most important sink is the chemical reaction with the hydroxyl radical OH in the troposphere:

This process removes 511 Tg of methane from the atmosphere per year. In addition, a small amount is taken up by the soil (30 Tg/year) and converted in the stratosphere by reaction with OH, Cl, and O (40 Tg/year). [16] The hydroxyl radical (OH), which controls not only methane but also other climatically and toxically important trace substances such as nitrogen oxides and carbon monoxide, is produced mainly by the photolytic splitting of ozone (O3 + hv> O + O2). Electronically excited O atoms subsequently react with water vapor to form hydroxyl radicals:

The most important sinks for OH in the global mean are the reaction with carbon monoxide (CO) and CH4. But it also reacts with a number of other trace gases. These reactions often lead to the formation of H2O radicals, through which OH is formed again via a reaction with O3 or NO. Because of this and other reactions, the OH concentration (and thus the reaction with methane) is also subject to fluctuations over time. Forest fires, which emit large amounts of carbon monoxide, play an important role in this context. For example, severe forest fires in Indonesia as a result of the 1997/98 El Nino are likely responsible for the minimum global OH concentration in recent decades. After the maximum around 1990 and the minimum around 1997/98, the OH concentration has increased again (Fig. 4).

3 Effects of climatic changes

As geological data show, methane sources and sinks are also influenced by climatic parameters such as temperature and humidity. This is especially proven for the transitional phases between warm and cold periods during the ice age. In the case of methane, it is primarily wetlands whose methane emissions are varied by climatic factors, but also z.B. Rice fields and the burning of biomass. The latter, as shown above, also shapes the OH concentration, the most important sink of methane. Methane is therefore not only an important greenhouse gas, but is itself in turn affected by climate change.

3.1 Wetlands

Methane emissions from wetlands are strongly influenced by temperature and water levels. Higher temperatures favor decomposition processes or. make them possible in the thawing of permafrost in the first place. High precipitation, and thus higher water levels, promote the anaerobic conditions under which methane formation occurs in the first place. Methane emissions from wetlands are the main cause of annual global variations, but also of multi-year trends in methane emissions.

For example, although the decrease in the growth rate of methane concentrations in the 1990s is primarily due to decreasing anthropogenic emissions caused by the collapse of industries in the former Eastern Bloc. Singularly played, however, also the temporary cooling by the Mt.-Pinatubo eruption in 1991 a role. Lower temperatures and reduced precipitation as a result of the volcanic eruption have probably suppressed methane emissions in the wetlands. [17] Methane emissions have also been decreasing for several years since 1999, especially in the tropical wetlands of Asia and South America. The reason was a major dry period. This climate-driven decrease temporarily masked rising industrial emissions from the economic boom in China and other countries. [18]

For future development, model simulations showed a 21% increase in methane emissions for a 2°C increase in temperature and 10% increase in precipitation. For a warming of 3.4 °C (as a result of a doubling of the atmospheric CO2 concentration), methane emissions from wetlands would increase by as much as 78%, according to model calculations. [19] Thawing of permafrost in the high northern latitudes in particular is estimated to be a significant source of methane in the future. However, warming and expansion of northern wetlands will also very likely lead to higher methane emissions. The total amount of methane stored in the Northern Hemisphere permafrost is estimated to be between 7.5 and 400 gigatons of carbon (Gt C). In contrast, the amount of methane in the atmosphere is only approx. 4 Gt C. [20]

3.2 Methane hydrates

Climate change could also attack an even much larger source of methane, namely methane hydrates in ocean sediments, which could threaten a tipping point in the climate system in the long term, d.h. An overturning of the current climate into a new condition. [20] [21] Methane hydrates are compounds of water and methane formed under high pressure and at temperatures around freezing point, which are found on the continental slopes of ocean floors at depths of ca. 400-1000 m. The amount of methane currently trapped there is very difficult to determine and is estimated to be between 700 and 10 000 Gt C. [21] Even a relatively small release would have a significant effect, given an atmospheric content of methane of about 4 Gt C in the atmosphere.

How could such a release of methane occur? Warming of seawater due to a global temperature increase could cause the ice-like methane hydrates to disintegrate and lead to the emission of methane. Methane can combine with dissolved oxygen in water to form carbon dioxide, which can then rise into the atmosphere together with unreacted methane. However, all processes occur on very large time scales. The warming of the atmosphere is passed on only very slowly to the lower water layers and to the sediments. The methane released there also takes a long time to reach the atmosphere. It may be transported by ocean currents or oxidized by bacteria already in the upper sediments.

However, geological data from ice cores suggest that the possibility of a major methane release from hydrates cannot be ruled out. Already in earlier epochs of the earth’s history, when there was a sudden warming, larger amounts of methane escaped from the hydrates. A massive methane release of this type is thought to have occurred about 55 million years ago in the Paleocene/Eocene (at the beginning of the Cenozoic Era). [22] The consequence was a strong temperature increase of 5-8 °C in the higher latitudes. The cause of this methane release is discussed to be both a warming of the ocean by 4-6 °C and tectonically induced landslides on the continental slopes. Rapid release of methane from gas hydrates on the seafloor is also thought to have occurred during glacial sudden warming episodes known as Dansgaard-Oeschger cycles. A warming of the sea water by 2-3.5 °C probably played a role here as well. The decay of the methane hydrates during the cold periods was also favored by a low water pressure, as sea level rose by approx. was 80 m lower. [23]

Two likely particularly sensitive locations on Earth have recently been the site of investigations into possible methane release, off the southeast coast of North America and on the Arctic shelves. A northwestward shift and warming of the Gulf Stream has been observed at the edge of the western North Atlantic over the past 5000 years. This has resulted in a warming of up to 8 °C in the ocean over the North American continental slope. This warming is currently driving destabilization of 2.5 GtC of methane hydrates. However, this is only about 0.2% of what may have been the Paleocene/Eocene temperature maximum 50 million ago. years, when the global mean temperature rose by 5-6 °C. [24]

Arctic methane hydrates are thought to contain a total of 900 GtC of methane. Bottom water temperatures in the Arctic shelves are expected to rise 1-2 °C over the next 100 years, according to model calculations. This amount could be amplified by a possible influx of warm water from the Atlantic Ocean. Of the methane released into sediment pore water in the Arctic due to warming, 50% is likely to be retained in the seafloor by anaerobic microbial oxidation. Methane released into the water column is converted to carbon dioxide by aerobic microbial oxidation, resulting in accelerated acidification of ocean waters. The total methane released into the atmosphere by melting Arctic hydrates is estimated at 162 million. t CH4 per year, which is well below the current input from anthropogenic activities of 600 million. t CH4 per year. Computer model calculations have shown that methane release from Arctic hydrates will have a negligible effect on the climate system over the next 100 years. Over a longer second period, however, global warming could be amplified by 0.8 °C due to methane release from Arctic hydrates. [25]

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