Geoengineering the Siberian wastes would inevitably lead to the destruction of the entire planet. I can crunch the numbers if you want, but you really need to watch out with permafrost and albedo, especially over such an area. (Also, airstreams. Changing too much of Western Siberia would probably collapse the Siberian pressure field, which would break down Europe's climatical system. )
Changing too much at once would decimate the world, yes, but careful action over a few thousand years in only certain areas to allow for production to keep up with population, rather than the whole mass at once, would prevent a doomsday scenario. If this is untrue, send me a message containing the tolerances that it could take.
((I feel like planes for dragons would be motorized systems and semi-hang gliders to attach to their wings to help them go long distances more easily, rather than actual pressurized things. Tanks would be slow, purposefully heavy and very armored, but with good tracking systems, meant mainly to counter dragons, who could decimate light vehicles and infantry. Warfare is changed, as is SO much by the addition of dragons))
That seems reasonable.
I think I should mention that the purple nation on the map was the Czar's empire. The northernmost blue in Asia and northernmost blue in North America are 'The Ice Realms,' a nation comprised nearly entirely, if not entirely of Ice dragons, containing nearly the entirety of that type's world population.