Prediction of future Alaskan lake methane emissions using a small-lake
model coupled to a regional climate model
Abstract
Methane emissions from lakes will increase with climate warming.
However, CH4 these emissions are not presently in the surface schemes of
Global Climate Models (GCMs). Because climate projections depend on
future atmospheric CH4 concentrations, a positive feedback loop is not
simulated. To address this issue, a one-dimensional model was developed
to simulate future CH4 diffusive and ebullitive fluxes from four Alaskan
lakes. The model was hindcast for validation (1976-2005) and forecast
for prediction (2071-2100) with one-way coupling to raw meteorological
data from the CanESM2 ensemble GCM. Three climate warming scenarios
(RCPs 2.6, 4.5 and 8.5) simulated bottom water to warm by up to
2.24{degree sign}C, increasing the CH4 flux from the lakes by 38 -
129%. However, RCP 2.6 and 4.5 led to stabilized temperatures and CH4
emissions by 2100, at levels of 0.63 - 1.21{degree sign}C and 38 -
67%, respectively, above the 1976-2005 averages. The CH4 diffusion
parameterization was transferable between the four lakes; however,
different ebullition parameterizations were required for the two deeper
lakes (~6-7 m mean depth) versus the two shallower lakes
(~1-3 m mean depth). Relative to using observed
meteorological forcing, which had a cold bias (-0.15 to -0.63 {degree
sign}C) and RMSE of 0.38 to 0.90 {degree sign}C, the GCM-forced
models had a warm bias (+0.96 to +3.13{degree sign}C) and marginally
higher RMSE (1.03 to 3.50{degree sign}C) compared to observations. The
results support continued efforts to couple CH4 lake-emission models to
GCMs without downscaling meteorological data, allowing feedback between
CH4 dynamics and future climates to be modelled.