A rise in temperature would shift all agricultural zones polewards.
There is less light and less land area poleward.
And less of
that land has fertile soil. Much of northern Canada, Scandinavia, and Russia is
either bare rock, or else bedrock with a very thin skin of soil over it. Spruce
trees can survive on such limited soil, but food crops cannot (farming was
attempted, and failed). Much of the far north of Canada (north of the tree line)
is barren rocky desert (the area is also known as "the barren lands", or "the
barren grounds").
I am less familiar with Russia, but in Canada and
Scandinavia, the northern limit of agriculture is determined by the underlying
geology, not by temperature. If you overlay geological and agricultural maps of
Canada you will see that they match very closely. While some small amount of
fertile land may become arable, any significant agricultural gains in Canada
(for example), would need to come from increased yields on existing land, not
from any mythical "new lands".
Counterbalancing a lengthened growing
season would be less water. The prairie regions of Canada (the western interior
provinces) are comparatively dry. For agriculture they depend heavily on winter
precipitation stored as snow which melts in the spring to provide moisture in
the soil. If the climate was warmer, much of this precipitation would either
fall as rain, or else melt over the winter in a series of thaws. This
precipitation would then not be available in the spring to grow crops. To this,
add the increased evaporation caused by higher summer temperatures. The net
result of higher temperatures could be less food, not more. The more optimistic
predictions are that increased crop yields on some lands will offset the loss of
farmland caused by drought in drier regions. The more pessimistic views are that
agriculture will suffer a net loss.
Scandinavia would probably be less
prone to becoming drier, and so agriculture there on existing land may benefit
from the longer growing season. However, it has so little arable soil that this
would have no significance on a global scale.
Russia seems to have
become prone to extreme summer heat and drought and extreme winter cold in
recent years. This is probably due to the size of the Asian continent which
would amplify seasonal effects. The central Asian deserts would expand in a
warmer world. It would not be realistic to depend on increased food production
from Russia.
In short, anyone who is looking for a warmer climate to
increase food production in the north is delusional. Statements that arctic
countries would "benefit" from a warmer climate are generally based on hopes
that a longer arctic shipping season would make oil, gas, and mining in coastal
regions there more economical. However, those industries are already suffering
in the interior regions as shorter winters are making trucking over ice roads
impossible or uneconomic in some areas. The people who actually live in the
north see "global warming" as something that is already having a net negative
effect on their lives.
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