Julie K. Shoemaker

What Role for Short-Lived Climate Pollutants in Mitigation Policy?

Parallel strategies must focus on long- and short-lived pollutants, but not at the cost of reducing pressure for action on CO2. Short-lived climate pollutants (SLCPs) include methane (CH4), black carbon (BC), tropospheric ozone, and hydrofluorocarbons (HFCs). They are important contributors to anthropogenic climate change, responsible for as much as one-third of the current total greenhouse forcing (1). An emerging strategy, which we refer to as hybrid climate mitigation (HCM), emphasizes reducing SLCPs in parallel with long-lived carbon dioxide (CO2) so as to achieve climate goals, as well as health and food security benefits, associated with some of the SLCPs. Proponents of HCM argue that we should focus substantial effort on reducing SLCPs now, as we wait for sufficient political will to reduce CO2 emissions (2–4). But others (5) worry that any strategy involving SLCPs risks delaying efforts to reduce CO2, the main greenhouse gas most important for long-term warming if emissions continue as projected.

Forest ecosystem changes from annual methane source to sink depending on late summer water balance

Forests dominate the global carbon cycle, but their role in methane (CH4) biogeochemistry remains uncertain. We analyzed whole-ecosystem CH4 fluxes from 2 years, obtained over a lowland evergreen forest in Maine, USA. Gross primary productivity provided the strongest correlation with the CH4 flux in both years, with an additional significant effect of soil moisture in the second, drier year. This forest was a neutral to net source of CH4 in 2011 and a small net sink in 2012. Interannual variability in the summer hydrologic cycle apparently shifts the ecosystem from being a net source to a sink for CH4. The small magnitude of the CH4 fluxes and observed control or CH4 fluxes by forest productivity and summer precipitation provide novel insight into the CH4 cycle in this globally important forest ecosystem.

Subsurface characterization of methane production and oxidation from a New Hampshire wetland.

We measured the carbon isotopic composition of pore water carbon dioxide from Sallies Fen, a New Hampshire poor fen. The isotope profiles are used in combination with a one-dimensional diffusion-reaction model to calculate rates of methane production, oxidation and transport over an annual cycle. We show how the rates vary with depth over a seasonal cycle, with methane produced deeper during the winter months and at progressively shallower depths into the summer season. The rates of methane production, constrained by the measured delta(13)C(dic) profiles, cannot explain high methane emission during the summer. We suggest that much of the methane produced during this time comes either from the unsaturated peat, or from the top 1-3 cm of saturated peat where episodic exchange with the atmosphere makes it invisible to our method.