Geoscience Reference
In-Depth Information
Appendix 19.C: Model variants
Annual or
growth season
data
With respect to temperature, we initially considered annual
GDD as an alternative to growth season GDD (defined as April
to September in our study). However, given the Norwegian
climate, the difference between these two measures would
have been minimal, as there are few days where the tempera-
ture rises above 5°C between late autumn and early spring.
Conversely, in the case of precipitation, we considered growth
season data as an alternative to annual data, but an annual
precipitation figure seemed more appropriate than a grow-
ing season figure, since a significant proportion of precipita-
tion falling outside the growing season is likely to feed crops
during it. This is because a large share of precipitation during
winter months is likely to be released as water when the snow
melts in spring and early summer, even if some water is lost to
runoffs to rivers, and so on.
Inclusion of
CO
2
concentration
Data was obtained at the global level (in parts per million)
from the Scripps Institution of Oceanography.
*
Different data
formats for CO
2
concentration were explored: atmospheric
concentration in ppmv; transformation of atmospheric concen-
tration to a normalised series starting at 0 in 1957 and ending
at 56 in 2001; logarithmic transformation of atmospheric con-
centration; and finally, a quadratic term from a second-order
polynomial was fitted to atmospheric concentration through
regression. These data formats were included either alone as
part of the regressions, or in addition to the linear trend. It
turned out that the simple time trend behaved as well or better
in the regressions than the various CO
2
formats, so we chose to
only include the former in the main model. The major reason
for this finding is the dominating linear part of CO
2
concentra-
tion in the atmosphere.
Frost events
Frost events can be harmful to crops, grains in particular.
Wheat is especially sensitive to sub-zero conditions during
its vegetative period, when germination and leaf growth take
place. Cromey et al. [2] found that a late frost event reduced
yields 13-33% for the affected winter wheat crops in the
*
Atmospheric CO
2
concentrations (ppmv) were derived from flask and
in situ
air samples collected at the South Pole. Source: C.D. Keeling, T.P. Whorf and
the Carbon Dioxide Research Group, Scripps Institution of Oceanography,
University of California, La Jolla, California USA 92093-0444, July 25,
2002; http://cdiac.esd.ornl.gov/trends/co2/sio-spl.htm.
