Old methane sources less important in modern climate warming

The atmospheric release of ancient stores of methane in thawing permafrost or from beneath Arctic ice may not impact future climate warming as strongly as previously believed, a new study finds. Rather, emissions of the greenhouse gas from current activities are more important for our immediate future. Methane (CH4) is a potent greenhouse gas with a global warming potential many times more powerful than carbon dioxide. Currently, natural emissions account for nearly 40% of total CH4 emissions. However, vast quantities of old and cold CH4 are locked in climate-sensitive reservoirs like permafrost and as hydrates beneath ice sheets. As the Earth continues to warm, these thousands-of-years-old stores have the potential to be released into the atmosphere and trigger further abrupt warming. But the sensitivity and overall impact of these sources remain unresolved, adding uncertainty to climate change predictions. Michael Dionyisius and colleagues investigated the climatic contributions of these old carbon sources during the last deglaciation - a period roughly analogous to our modern warming atmosphere. Using tiny bubbles of air trapped inside Antarctic ice cores, Dyonisius et al. measured the old, or "radiocarbon dead," CH4 in the atmosphere throughout the end of the last ice age. The authors found that CH4 emissions from old carbon reservoirs were small; instead, the vast majority of atmospheric CH4 during this period was from contemporary sources, such as decomposition or burning of new organic material. The results suggest that old CH4 emissions in response to future warming will likely not be as large as others have suggested. Intriguingly, the authors found that CH4 emissions from biomass burning in the pre-industrial Holocene were comparable to those today. The findings suggest an underestimation of present-day CH4 or a yet unknown two-way anthropogenic influence on modern fire activity. In a related Perspective, Joshua Dean discusses the findings in greater detail.

Credit: 
American Association for the Advancement of Science