Possible influence of low latitude wetland area changes on the Holocene global atmospheric methane concentration trend

Methane is an important greenhouse gas with a strong infrared radiation absorbing effect approximately 25 times higher than that of CO₂. The global atmospheric methane concentration (GAMC) has increased from ~715 ppb to ~1853 ppb since the Industrial Revolution, which may have led to an ~18% increase in the global total solar radiation absorption. Therefore, understanding the causes of variations in GAMC is a crucial issue in global climatic change research. The long-term trend of atmospheric methane concentration recorded in ice cores roughly followed the orbital cycles in the northern hemisphere solar insolation. However, since the mid-Holocene, these two have become ‘decoupled’. Currently, there is still considerable controversy regarding the explanation for this ‘decoupling’. Previous studies believed that the temperature increase during the Holocene led to the expansion of wetland areas in high latitudes of the Northern Hemisphere, emitting substantial amounts of methane, which played a dominant role in the increase in global methane concentration. However, as research progresses, the impact of hydrological cycles in low-latitude regions has gradually gained attention, with the dry conditions in the mid-Holocene likely resulting in reduced methane emissions from wetlands in low-latitude regions. In addition to the influence of these natural factors, numerous studies have also focused on methane emissions from early agricultural practices (rice cultivation), with the expansion of rice paddies in the late Holocene emitting large amounts of methane.

To discuss the impact of natural and anthropogenic factors on the trends in atmospheric methane concentration, we collect methane emission flux data from global wetlands and rice paddies (a total of 377 sites) for analysis and comparison. We believe that during the mid-Holocene, with primitive agricultural production techniques and a smaller population, methane emissions from rice paddies were insufficient to reverse the trend in global methane concentration, which was likely more influenced by natural factors. Additionally, we find that the Holocene GAMC trend closely follows the long-term trend in global low latitude wetland extent, as inferred from our lake-level reconstruction and from other existing hydroclimate records, implying that changes in tropical-subtropical wetland areas could play a major role in the natural global methane cycle. The reduction in the area of low latitude wetlands during the mid-Holocene, caused by increased submersion from sea level rise and by reduced subtropical water availability inferred from decreased lake levels, could have led to the observed mid-Holocene GAMC fall.