Write a Short Story Set in a Dystopian Future

Write a short story set in a dystopian future where climate change has ravaged the planet, and governments have collapsed. The story follows the journey of a young scientist named Ava, who possesses a unique genetic trait that makes her immune to the toxic environment. Ava must navigate the treacherous landscape, avoiding roving gangs of marauders and rival factions vying for control. As she delves deeper into the ruins of a once-great city, she discovers a hidden underground bunker containing ancient knowledge and technology that could potentially reverse the effects of climate change. However, she soon realizes that the bunker is not as secure as she thought, and she must make a difficult decision to save humanity or sacrifice herself for the greater good. Please ensure the story has a clear three-act structure, well-developed characters, and a gripping plot twist.