On a global scale, the sources of methane, both natural and anthropogenic, are numerous and scattered. In order to distinguish and quantify them, it is first crucial to measure atmospheric CH4 levels on a regional scale with sufficient resolution to study the seasonality of flows. This task is carried out by a worldwide network of several dozen stations located from the South Pole to Greenland.
The pCH4 records show similar increases over recent decades, but with a marked difference between the two hemispheres. The excess in the Northern Hemisphere (> 50 ppb) is linked to the fact that the main natural and anthropogenic sources are located in the North. The balance between emissions in the northern hemisphere and CH4 degradation in the atmosphere, notably by -OH radicals, shows that annual accumulation in the northern hemisphere is around 275 Tg, corresponding to the inter-hemispheric gradient of pCH4.
Records from stations located on the continents show high-frequency variability, in contrast to the regular, multi-year and seasonal trends from stations close to the ocean. This continental variability suggests the existence of powerful CH4 sources linked to the terrestrial biosphere and human activities.
