Arctic Methane: Can We Detect a Change in Emissions?

Abstract:

Warming in the Arctic has proceeded faster than anyplace on Earth. Our current understanding of Arctic biogeochemistry implies that changes in terrestrial fluxes of carbon can be expected as the Arctic warms. Vast stores of organic carbon are thought to be frozen in Arctic soils, and if mobilized to the atmosphere, would have significant impacts on global climate.

Observations of greenhouse gases in the Arctic and globally have been collected for several decades. Analysis of this data does not currently support significantly changed Arctic emissions of CH₄, however it is difficult to detect changes in Arctic emissions because of transport from lower latitudes and large inter-annual variability. Modeling systems can help untangle the Arctic budget of greenhouse gases, but they are dependent on underlying prior fluxes and wetland distributions, as well as possible transport model biases, leading to significant uncertainties.

We discuss our current understanding of the Arctic carbon budget from both top-down and bottom-up approaches. We show that current atmospheric inversions agree well on the CH₄ budget. On the other hand, bottom-up models vary widely in their predictions of wetland emissions, with some models predicting emissions too large to be accommodated by the budget implied by global observations. Large emissions from the shallow Arctic ocean are also inconsistent with atmospheric observations. We also discuss the sensitivity of the current atmospheric network to what is likely small, gradual increases in emissions over time by examining modeled and observed spatial and seasonal variability. Spectral decomposition of CH₄ observations reveal subtle but significant changes in the amplitude and phase of the seasonal cycle. We attribute these changes, using isotope data and an atmospheric transport model, to changes in natural and anthropogenic sources.