Question

Estimates (ref. 1) for emissions of methane to the atmosphere are given in the table below and the current atmospheric concentration is \(1.77 \mathrm{ppmv}\). Calculate its residence time. $$ \begin{array}{|lc|} \hline \text { Sources of atmospheric methane in million tonnes per year } \\ \hline \text { Wetlands and other natural sources } & 160 \\ \text { Fossil-fuel-related sources } & 100 \\ \text { Other anthropogenic sources of biological origin } & 275 \\ \hline \end{array} $$ There may be \(10^{14} \mathrm{t}\) of methane hydrate \(\left(\mathrm{CH}_{4} 6 \mathrm{H}_{2} \mathrm{O}\right)\) in the permafrost below the ocean floors. If \(1 \%\) of this were to melt per year, what would be the increased concentration of methane (ppmv \(y^{-1}\) ) in the atmosphere neglecting any removal processes? What sinks for methane would play a role in reducing this concentration?

Short answer

Methane's residence time is approximately 3.37 days. If 1% of methane hydrates melt, atmospheric concentration would increase by about 358,416 ppmv/year. Sinks, including hydroxyl reactions and soil oxidations, reduce methane levels.

Step-by-step solution

Calculate Total Methane Emissions

Add the emissions from each source to find the total methane emissions per year:\[170 \, \text{(Wetlands)} + 100 \, \text{(Fossil-fuel-related)} + 275 \, \text{(Other anthropogenic)} = 535 \, \text{million tonnes/year}.\]

Calculate Residence Time of Methane

Methane's residence time can be calculated using the formula: \[\text{Residence Time} = \frac{\text{Atmospheric Concentration}}{\text{Emission Rate}}.\]Convert atmospheric concentration from ppmv to million tonnes. Use the conversion: - 1 ppmv \( \approx \) 2.79 million tonnes.Current atmospheric concentration:\[1.77 \, \text{ppmv} \times 2.79 \, \text{million tonnes/ppmv} = 4.94 \, \text{million tonnes}.\]Residence time:\[\text{Residence Time} = \frac{4.94 \, \text{million tonnes}}{535 \, \text{million tonnes/year}} \approx 0.0092\, \text{years} \approx 3.37 \, \text{days}.\]

Calculate Increased Methane Concentration from Melting Methane Hydrate

First, calculate 1% of the methane hydrate:\[1\% \times 10^{14}\, \text{tonnes} = 10^{12}\, \text{tonnes} = 1,000,000\, \text{million tonnes}.\]Estimate increase in atmospheric concentration:\[\text{Increase in ppmv}: 1,000,000 \, \text{million tonnes} \times \frac{1}{2.79 \, \text{million tonnes/ppmv}} \approx 358,416.15\, \text{ppmv/year}.\]

Identify Methane Sinks

Various sinks include: 1. **Atmospheric reactions:** Methane is primarily removed by reacting with hydroxyl radicals (OH) in the atmosphere. 2. **Soil absorption:** Certain soils can absorb and oxidize methane. 3. **Marine environments:** Bacteria in ocean waters can consume methane. These processes respectively decrease methane concentrations in the atmosphere.

Key concepts

Atmospheric Methane Concentration

Atmospheric methane concentration refers to the amount of methane gas present in the Earth's atmosphere. This is usually measured in parts per million by volume (ppmv). Methane, although present in smaller quantities compared to carbon dioxide, is a potent greenhouse gas that significantly impacts climate change. Its concentration in the atmosphere is influenced by both natural and human-made emissions.
Currently, our atmosphere has a methane concentration of 1.77 ppmv. This level is the result of various sources, including natural emissions from wetlands and anthropogenic activities such as fossil fuel extraction and agricultural practices. Understanding this concentration is crucial as it helps scientists assess the greenhouse effect's progression and potential impact on global warming. The atmosphere's capacity to contain methane is a key factor in determining its overall concentration at any given time.

Residence Time of Methane

The residence time of methane refers to how long a methane molecule typically stays in the atmosphere before being removed. This is an important concept because it gives insight into how long methane can contribute to the greenhouse effect once it is emitted.
To calculate the residence time, we use the formula:\[ \text{Residence Time} = \frac{\text{Atmospheric Concentration}}{\text{Emission Rate}}. \]Based on current data, methane has a very short residence time of 3.37 days given the current emission levels and atmospheric concentration. This indicates that while methane is a powerful greenhouse gas, its potential atmospheric build-up is more persistently managed by natural processes that remove it. Calculating residence time helps in understanding how swiftly actions aimed at reducing methane emissions could make a tangible impact on its atmospheric presence.

Methane Sinks

Methane sinks are natural processes and environments that help in removing methane from the atmosphere. These are crucial for regulating the atmospheric concentration of methane and mitigating its effects as a greenhouse gas.
Several critical methane sinks exist:

• Atmospheric Reactions: Methane is primarily broken down by hydroxyl radicals (OH) in the atmosphere in a chemical process that occurs naturally. This is the most significant methane sink and plays a major role in controlling its atmospheric levels.
• Soil Absorption: Some terrestrial soils have the capability to absorb methane and convert it into less harmful substances through microbial oxidation processes.
• Marine Microbes: Certain bacteria in oceanic waters can consume methane, thus preventing it from being released into the atmosphere. These bacteria are especially active near underwater methane emissions sources, such as methane hydrates.

These methane sinks work together to lessen its atmospheric concentration, contributing to a balance crucial for maintaining lower global temperatures.

Methane Hydrate

Methane hydrate, often located beneath the sea floor and within permafrost regions, is a solid compound where a large amount of methane is trapped within a crystalline structure of water, forming an "ice-like" substance. These hydrates potentially hold immense quantities of methane gases and represent a significant store of carbon.
If disturbed, such as by ocean warming or land movements, these hydrates could release methane into the atmosphere. Even a small release rate, such as 1% per year, could drastically increase atmospheric methane concentrations, by as much as 358,416 ppmv/year, markedly influencing the greenhouse effect.
Understanding methane hydrates' location, behavior, and stability is therefore vital for predicting potential impacts on climate change. Scientists closely monitor these reserves to anticipate and mitigate sudden releases that could lead to rapid climatic alterations.