It sounds arcane but methane, which apparently arises from sources as rich and varied as
- melting Siberian permafrost
- gas production and
- both ends of the humble cow
Anyhow the good news is that, after rising to 150% of its pre-industrial levels, the global atmostpheric methane level hasn't budged for the past seven years, and may even start falling. This may even be a result of various conservation attempts over the years, including farmers bottling the stuff as it comes out of cows (let's not picture this in our minds) and then selling it or using to power their milking parlours.
Not, in other words, like this cartoon: