Consider an area of a fantasy world which experiences the following changes:
- Amount on rainfall, and thunderstorms, increases dramatically.
- Average temperature increases by 10-20 degrees Celsius over a period of two years (depending on how cold the location originally was; there is some variance in the temperature of the region).
What happens to forests, fields and livestock? Will the place be a decaying apocalyptic wasteland (with forests and fields rotting where they used to grow, and livestock and game animals dying of exhaustion), or will there be enough crops and remains of an ecosystem for people to survive without a significant loss of population?
In the long run species more adapted to the new climate will certainly take over, but I am more interested in what happens within the transitory period; up to five years, for example.
A climate change of 2 degrees is considered to significantly change agriculture; a change one order of magnitude will do more. On the other hand, many animals can survive such a change (some people take a vacation in the sunny south during the coldest winter, for example), if they have something to eat. I do not know about plants.
Before the changes above, the area is a more-or-less stereotypical fantasy setting - flora, fauna and climate are roughly similar to central to northern medieval Europe, as is culture. There is lots of uninhabited woodland and hills. The population is fairly small and scattered; this is a backwater of a larger civilization.
The area is limited by mountain ranges, a desert, and a large swampland, where the temperature gradually changes from the previously abnormally cold to that of the surrounding areas.
I'm running a fantasy roleplaying game, and due to a powerful nature spirit leaving and another filling the power vacuum, the mentioned environmental effects happen. These forces do not otherwise actively change the life of living things in the area. There is other magic, but I am not interested in what effects it might or might not have. I want to know what will happen if none of there powerful unrealistic forces take notice and act.
(In the context of roleplaying, this will give the player character room to do something about the events, or to ignore them, as they will, and with consequences.)