How can a trend help scientist make an prediction?

Scientists use something called computer models to make their predictions

A computer model is a computer program that has been designed to simulate a particular function and to make predictions of its expected behavior. Computer models are very useful to help us understand all sorts of events, lots of models have been developed and are used to explain lots of different events. Those who model climate over the next 50-100 years should have great interest in proof, testing and use of their models. The environmental revolutionaries want us to believe that every global warming prediction is correct, but computer models can easily draw wrong conclusions, it is easy for a computer model to be wrong, actually it is rather amazing that they ever make any correct predictions. Some events are simply to difficult to model, sometimes the results of a particular set of events are not known. One must then perform a complicated calculation each time those events must be used, rather than using the result of the mathematical equation to recreate a function. Some things cannot be modeled if you don't know on what they depend on.