Assuming this is for school, I'm sure your teacher wants you to put some effort into it yourself. A model is a pattern for how various things in a system are connected in terms of cause and effect. So, for example, you might start your model by thinking of how agriculture could change if the climate continued to warm up like it is now. What if the drought in the mid-west would continue like is now for 2-3 more years or longer? Then draw out the implications with arrows in a diagram. For example, corn would cost more. What happens if corn costs more? Gas prices rise. Meat costs more. You get the idea? That's a model about food, but health can be tied in too. Food costs rise, poor people can't afford to eat. Etc.