Geoscience Reference
In-Depth Information
Figure 9.19.
Cumulative Net Ecosystem Exchange (NEE) for low, medium, and high
warming rates without permafrost carbon and the ensemble mean cumulative NEE with
permafrost carbon flux. The grey bar represents uncertainty and the arrow marks the
time at which the permafrost carbon flux becomes positive (adapted from Chaefer et al.,
2011
).
SiBCASA includes biogeochemistry and sophisticated representations of soil ther-
modynamics, soil hydrology, and snow pack dynamics designed to simulate perma-
frost dynamics. Details of the model and the experimental design are fully described
by Schaefer et al. (
2011
).
Key results from the study are summarized in
Figure 9.19
, which shows cumu-
lative NEE summed for the years 1900 through 2200, with a negative NEE repre-
senting a net cumulative carbon uptake by the land. Without a PCF (the three lines
at the bottom representing low, medium, and high warming rates), the cumulative
NEE starts at zero and turns negative, indicating enhanced plant growth in perma-
frost regions driven by longer growing seasons, higher temperatures, and increased
atmospheric carbon dioxide concentrations. In contrast, as shown in the upper line,
accounting for the PCF changes the permafrost regions from a net carbon sink to a
source relative to the atmosphere. The line shows the ensemble mean of the simu-
lations with the grey shaded area indicating the uncertainty. Their best estimate of
PCF strength indicates a cumulative flux of 190 ± 64 Gt of permafrost carbon to
the atmosphere by the year 2200, equivalent to an increase in atmospheric carbon
dioxide concentration of 87±29 ppm. The subsequent study by A. MacDougall, C.
Avis, and A. Weaver (
2012
), using the University of Victoria Earth System Climate
Model (ESCM), comes to broadly similar conclusions.
9.9
Summary of Model Errors
As is evident from preceding discussion, models are prone to errors from a variety
of sources. A major thrust of the various MIPs is to examine such errors with the
