During ice ages, rain fall pattern shift as wind and current patterns change as more ocean water is locked in ice. The Sahara, and much of the modern Middle East became extremely lush during the big ice ages, as did other warm deserts, according to all of the paleontological and archaeological evidence we have I live in a cold desert. I know that a change in temperature doesn't make a desert more fertile. Also, ice ages disproportionately affect the temperatures of the world. The closer to the poles, the more extreme the change. Towards the equator, other factors, such as the changed water and wind circulation patterns have a much larger effect. So in the Fire Nation, the change in temperature would have been much less noticeable.