The guns are the implication that the rest of the world isn't exactly idyllic. When climate change gets worse and resources dwindle, people get desperate, nations get violent, but that is not the focus of this manga. The guns however are a powerful symbol that evoke violence. I'm not entirely sure it was worth including them in the manga, as it is quite jarring, but still, it's a practical thing for people in that era to have around. The fact that the story is set in Japan adds another dimension to it all, as guns aren't exactly common over there, but it further emphasises how climate change has impacted everyones way of life drastically.