Methane as a greenhouse gas

Question 1

What are the main anthropogenic sources of atmospheric CH4?

Answer 1

• Fossil fuel production and use
• Agriculture and waste

Question 2

What are the main natural fluxes of CH4 into the atmosphere?

Answer 2

• Wetlands

Question 3

What are the main natural fluxes of CH4 out of the atmosphere?

Answer 3

• Chemical reactions in the atmosphere
• Soils

Question 4

What is a natural and anthropogenic (combined) flux of CH4 into the atmosphere?

Answer 4

Biomass and biofuel burning

Question 5

How many extra tons of methane are estimated to be added per year?

Answer 5

17 million

Question 6

What are methane hydrates?

Answer 6

A solid where methane is trapped in frozen water.

Question 7

Where are methane hydrates found?

Answer 7

• Mainly along continental shelves (due to fertility and biological production)
• Permafrost

Question 8

What is the composition of methane hydrates?

Answer 8

4CH4.23H2O

Question 9

Which conditions keep methane hydrates stable?

Answer 9

• Low temperatures
• High pressure (e.g., weight of sediment or water)

Question 10

How is sea level rise expected to affect methane hydrate stability? What would occur with falling sea levels?

Answer 10

Increase stability due to greater pressure.

Falling sea levels would produce instability, allowing methane release

Question 11

What is the name for the biological production of methane by microbes?

Answer 11

Methanogenesis.

Question 12

Where does methanogenesis typically occur?

Answer 12

Flooded soils with limited oxygen and other environments such as the rumen on a cow, wastewater treatment plants, lake sediments etc.

Question 13

How is nitrous oxide biologically formed?

Answer 13

Through the “breathing” of nitrogen compounds via both nitrification and denitrification. N2O is an intermediate in these processes.

Question 14

Where is N2O typically released from biologically?

Answer 14

Soils.

Question 15

What is an example of the biological aerobic uptake of N2O?

Answer 15

Aerobic methanotrophs (methane oxidisers):
- Use methane mono-oxygenase as a key enzyme
- CH4 + O2 -> biomass + ATP

Question 16

What is an example of the biological anaerobic uptake of N2O? What is this known as?

Answer 16

Some microbes in anoxic conditions “make” their own O2 by converting nitrous oxide into oxygen.
This is known as anaerobic methane oxidation.

Question 17

Where is anaerobic methane oxidation especially prevalent?

Answer 17

In the deep sea.

Question 18

How much methane can be consumed by biological oxidation before atmospheric release?

Answer 18

More than 600 Tg a year (maybe even ~800 Tg per year).
This highlights an area of potential methane uptake that could be utilised.

Question 19

What proportion of CH4 atmospheric inputs come from rice cultivation?

Answer 19

10%

Question 20

Why does rice cultivation produce methane?

Answer 20

Rice is cultivated by flooding, which creates anoxic conditions in the soil. This leads to methanogenesis, the biological production of methane by microbes

Question 21

How does methane produced in flooded rice paddies reach the atmosphere?

Answer 21

Most methane is transported directly through the plant’s vascular system. The plant roots reach deep into the sediments where methane is produced, acting like a chimney to move the gas straight into the atmosphere, bypassing methane-consuming bacteria in the upper sediments.

Question 22

What is one technique to reduce methane emissions from rice paddies that can half the methane emissions relative to current practices? What does this involve?

Answer 22

Intermittent irrigation. This involves allowing the soil to dry between flooding periods, which helps to reduce methanogenesis.

Question 23

By how much have global rice harvest areas increased by between 1950-2000?

Answer 23

40%

Question 24

By how much could intermittent irrigation reduce the CH4 emissions from rice paddies in comparison to continuous flooding?

Answer 24

This technique can reduce CH4 emissions by half relative to continuous flooding.

Question 25

What are some techniques other than intermittent irrigation that can be used to reduce methane emissions from rice paddies?

Answer 25

- promoting rice varieties that grow in drier soils - northern migration of rice cultivation to colder climates (where less methane is produced) - ridge and furrow technique to reduce the flooded area

Question 26

What are some practices/features of rice cultivation that can increase methane emissions?

Answer 26

- Using waste straw from old plants. - Irrigating with domestic sewage water, which can cause more CO2 and methane production. - Increased atmospheric CO2, which stimulates photosynthesis, bringing more CO2 into the soil and increasing methane potential.

Question 27

How does the balance between methane (CH4) and nitrous oxide (N2O) emissions relate to rice paddy management?

Answer 27

Methane has a relatively short atmospheric lifetime (12 years), while Nitrous oxide (N2O) stays in the atmosphere for about 120 years. Intermittent flooding reduces CH4 production but can increase N2O production. Intense intermittent flooding, while initially seeming beneficial, can lead to significant N2O accumulation over time, potentially resulting in a negative long-term climate impact greater than continuous flooding. There is a need to balance immediate CH4 reductions with the risk of long-term N2O increases.

Question 28

What form of intermittent flooding might be the most effective in the long term (remember balance of CH4 and N2O)?

Answer 28

Mild-intermittent -> with occasional draining. This has the best long term effect as not too much N2O is produced, but CH4 production is still limited slightly.

Question 29

Where is methane produced in lakes, and what consumes it? What can cause this consumption to by bypassed?

Answer 29

Methane is produced in the deeper, anoxic layers of lake sediments. As it diffuses up through the water column, it can be consumed by methanotrophs (methane-oxidising bacteria), which reduces the amount of methane released to the atmosphere. Plants can also transport methane directly, bypassing these bacteria.

Question 30

How does anoxia in a stratified water column affect the formation of methane in a lake, for example?

Answer 30

Methane production can occur in the anoxic water column (not just the sediments as previously).

Question 31

Are reservoirs and hydroelectric dams significant sources of greenhouse gas emissions? Which GHGs are important here?

Answer 31

Yes -> account for 1.3% of anthropogenic CO2 emissions 80% of gases released from reservoirs are methane, and 79% of radiative forcing contribution is from methane too.

Question 32

What are some consequences of reservoirs/hydroelectric dams other than GHG production?

Answer 32

- Changes to the entire ecosystem, including impacts on sediment and nutrient flows (e.g., reducing sediment, phosphorus, and nitrogen to the basin downstream). - Reservoirs can release the neurotoxin methylmercury (from flooded soils) that bioaccumulates in fish and humans.

Question 33

How is methane produced in reservoirs, and how does it get released?

Answer 33

Methane is formed when biomass decomposes anaerobically upon flooding the land to create the reservoir. Plant run-off also adds more material for decomposition. The gas is released when water is sent through the turbines of the dam, which releases the gas immediately, reducing the chance for methanotrophs to consume it.

Question 34

Where is the rate of methanogenesis highest in terms of climate?

Answer 34

Warmer climates (lower latitudes)

Question 35

How does thawing permafrost contribute to greenhouse gas emissions? Roughly how much carbon is stored?

Answer 35

Permafrost contains a vast amount of stored carbon (estimated ~1700 PgC). As permafrost thaws due to rising temperatures, previously frozen organic matter decomposes, releasing methane and CO2 into the atmosphere. This thawing can lead to the formation of expanding thermokarst lakes and slumps (regions of land entering the water).

Question 36

Do wetlands contribute to greenhouse gas emissions?

Answer 36

Wetlands produce methane due to anaerobic decomposition in waterlogged conditions. However, they slow decomposition rates and therefore reduce CO2 release.

Question 37

Are wetlands environmentally beneficial in terms of GHG emissions (based on presence vs absence)?

Answer 37

Studies suggest that the overall environmental impact is better when wetlands are present (anaerobic) compared to aerobic decomposition (without wetland) which produces large amounts of CO2 without CH4. Therefore, wetlands are considered crucial despite their methane release, and they also store large amounts of carbon in peat.

Question 38

Are wetlands aerobic or anaerobic?

Answer 38

Anaerobic