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Methane is a greenhouse gas that is a primary component of natural gas. Its warming potential is 28-34 times that of carbon dioxide, and its levels have increased 150% since 1750, mainly due to anthropogenic activity. 
Having spent most of the past millenium around 680 parts per billion (ppb), methane has jumped to 1876 ppb over the past 250 years. The increase started around the Industrial Revolution, a time where human’s fossil fuel consumption increased exponentially.
Natural sources are found below ground, whether continental or seabed, formed mainly by geological processes (like magmatic or water-rock reactions). Biological processes involve bacteria or archaea, often found in soils and landfills, that degrade organic matter and emit methane as a side-product.
Anthropogenic sources, responsible for the recent enhancement in atmospheric methane, include landfills which promote micro-organic methanogenesis, oil and natural gas systems, coal mining, agricultural activity, fuel combustion, wastewater treatment and more.

A worrying trend is that of permafrost thaw in the Arctic circle, where frozen organic matter gets exposed to heat, helping bacteria degrade nearly 12,000 years worth of dead plants and animals.

A more reassuring point is that methane’s presence in our atmosphere if far shorter-lived than that of CO2 (12 to 15 years and 20-200 years respectively). However, methane is naturally degraded to water and CO2 by chemical reactions, thus leaving greenhouse gas behind either way. Moreover, the chemicals that degrade it in the atmosphere are running out as the exponential growth of methane is overreacting with them. This means that it could linger longer than it used to in the future.

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