At the end of Helaman 10, Nephi is given authority to use Jehovah’s power. He is told that what he sealed on earth would be sealed in heaven, that if he declared that something should be so, Jehovah, in heaven, would make it so. What Nephi seals is a famine, and what likely caused the famine was sealing the rains.
In a Mesoamerican setting, there are two seasons, a rainy season and a dry season. The water that comes in the rainy season is the time of planting and the growing of crops. Droughts could become severe in many parts of Mesoamerica, and have been suggested as a major cause of the fall of the Maya civilization. Thus, a drought would be desperate. It led to a lack of food, which, of course, defines a famine.
An interesting question would be why Nephi chose a debilitating famine rather than an invasion. At times, warfare could also cause famine because crops might be trampled in the fields. The answer, or at least a possible answer, might lie in the idea that the Nephite government had been filled with Gadiantons. If they continued to have contact with the north, it is conceivable that they could call upon the north to aid in a Lamanite invasion, and thus increase the number of Gadiantons in the region.