Metrics are (with obs-based estimates in brackets):
  permafrost area (17.46 million square km)
  area of permafrost / area northwards of zero degree isotherm (0.47)
  soil temperature at 1m - soil temperature at surface (-0.53 degrees C)
  soil temperature at surface - air temperature (6.15 degrees C)
  annual amplitude at 1m / annual amplitude at the surface (0.40 unitless)
  annual amplitude at the surface / annual air temperature (0.57 unitless)

Observed permafrost extent is from http://nsidc.org/data/ggd318.html: Brown, J.,
O. Ferrians, J. A. Heginbottom, and E. Melnikov. 2002. Circum-Arctic Map of
Permafrost and Ground-Ice Conditions, Version 2. Boulder, Colorado USA. NSIDC:
National Snow and Ice Data Center.  When calculating the global area of
permafrost the grid cells are weighted by the proportion of permafrost within
them.

Annual mean air temperature is from: Legates, D. R., and C. J. Willmott, 1990:
Mean seasonal and spatial variability in global surface air temperature. Theor.
Appl. Climatol., 41, 11-21.  The annual mean is calculated from the seasonal
mean data available at the Met Office.

The soil temperature metrics are calcuated following: Charles D. Koven, William
J. Riley, and Alex Stern, 2013: Analysis of Permafrost Thermal Dynamics and
Response to Climate Change in the CMIP5 Earth System Models. J. Climate, 26,
1877–1900. (Table 3) doi: http://dx.doi.org/10.1175/JCLI-D-12-00228.1 The
locations used for Table 3 were extracted from the model and the modelled
metrics calculated.  The observed metrics were extracted from Table 3.

The soil temperature metrics represent the ability of the model to accurately
simulate the coupling between the soil and the air temperatures at high
laititudes which reflect the accuracy of the snow physics and the soil
hydrology.

The permafrost extent represents a rough evaluation of the land surface physics
in the northern high latitude. A more in depth evaluation would also include a
comparison with active layer thicknesses.
