Now that you mention Climate change... There two possiblities or three...
The world can turn into an endless drought*sp?*
The World can be completely destroy which is yet unknown... (maybe like no air or food)
OR... The world turns into an Ice land... so... Cirno may be a Fairy that becomes 'The End of the World'... only becomming stronger as the world becomes and returns to the time of the Ice Age...
>3> just came into mind