LONGLY. Regurgitated info mostly.
tl;dr: Simply stated, ASOIAF is traditional cyclic sci-fi of the eternal battle of competing evil Red versus goodly Green forces.
THE COMBATANTS:
In this corner - RED --Redly fire-eating shapeshifting Dantesc Infernolistic 7th-hellish scientifically-inclined dragon-beings seeking continuance and advancement of status as Red Godly immortals who experimentally solicit genetic traits of lower vassalages efficiently and effectively through time - fed on (mostly) towards eventual creation of perpetual master-raced nazideathlifers --a foreverlasting Red-Dead World ruled by the rarely if ever pastly-successful expressioned doomly-evil Thee Prince that was Promised;
versus:
In this corner - Green --Greenly-oriented granola-eating nature saviors perpetually sentenced to counter Red forces with the rarely expressed Last Hero Jon (or in a way Bran, either or, they occur together, rarely, you could have both if you like) - Jon is mortally immortal and has and will physically die several times (for now - like Dany his T#=3), Bran is immortally mortal-ish for now life Bloodraven but with a few extra's thrown in for the current time, and once he is done greenseeingly influencing events he needs to further his future purposes by seeding Meera in a few years continuing his greenseeing bloodline spermbank in history for the future survival of the mankindly-influential greenseer --as it will be needed in the future. Meera will then be magically hidden so no Redforce can find her and kill what she bears-- a green tower of life, or a hidden Whitefish Cave of Life, perhaps with floating flotsam and jetsom of weirwood logs and ebony rocked creekbeds-surely the light in this cave is the lifeforce of all nature itself, contracting the dark depths of deathly dark shadows of the House of Black and White).
HOW AND WHY:
Bloodlines and events have cyclically lined up in 283AC-300AC for this to happen-- it may have happened in +/-260AC also; again as in past times as it will again in the future--maybe in Southeros even in future or past days. Redside lost at Old Valyria in 0AC and a prophetically-emplaced past-visiting red dream of a Red-seer in Daenarys Targaryen of old (her character is that role and now recycled more advanced) resulted in survival-migration of Red-sided forces-- thus Westerosi Old Valyrians-- the Targaryens in a 12-years before DOOM bloodline movement. Perhaps since there's been several attempts at wars to come with each generation.
The Ending
In the end of this story (hopefully), Free Folk with Aemon Steelsong, the true legacy result of the Song of Ice -Mance and Dalla's son, or Val's in other attempts, he Aemon being all things goodly green and life-ly, rules what is left in Northeros after total destruction of the forces of Redly-blooded unnatural humanly/animal exploitation--Targaryens. Most likely Aerys' bones, or Rhaegars are somewhere in the Summer Islands, as well as Ashara's and Ned's will find their way there also (or someplace). So yes, climate change is real and it is now and ASOIAF is most obscurably-hearted a cumulative memed-tribute to Jack Vance with a hopeful future of sorts. ASOIAF is seemingly a grey-Vancian work with all of the most complex kitchen sinkery of literary deviced tropes thrown in you will ever find anywhere. And truly wonderfully so!
A few words on the Fandom Appeal of ASOIAF
To broader ranges of fans -- eithr right/left-brained equally, pages are permeated with qualities of pure fantasy --giants and faerys-and the Children of the Forest are in fact faery-like; magically-possessed and possessing dragons and such. Scientifically-inclined aspects appeal to enamassed bean-counter types: genetically controlled breeding/birthing as each generation of the great houses are experimentally controlled through selective breeding attempts with purposeful end-results of eliciting very specific biologically-defined traited offspring-the Prince that was Promised- who I suspect is Tommen, now controlled by his grandfather reincarnated and now fleshly-bodied as the High Septon, himself just the latested body used ---- this is all countered by Green forces and Jon, and Bran, both in the interim, but later resulting in the ascension of the hidden Aemon Steelsong as the finale hopeful ending future. Magical "things" exist such as horns, swords and even the bones of the dead themselves are important--so the whereabouts and use of Ned's missing bones and who has those bones (is just another important part of the story just as dragon skulls in the crypts of the Red Keep are important--bones contain genetic spirits of sorts and are used for furtherence of bloodlines (bone meal, bone marrow all important-- they are family seeds for future places and future wars in future locations of recurring characters-- most likley Southeros or elsewhere, as needed).