To forecast and plan for the future, the traditional place to look has been the past. This, after all, is one of the reasons why we study history. There are two ways the past can help.
The first is when there is a long-running trend, which started many years ago and which looks likely, given certain circumstances, to continue in the future. Consider, for instance, the rise of the Chinese economy, or the growing acceptance of gay people in mainstream society. These changes are not smooth. They do not always fit convenient graphs. There is often plenty of noise around the signal. Sometimes they blip, make unexpected leaps, up, down, back, forward. Sometimes the Chinese economy surges. Sometimes it stalls. Every now and then, local laws make significant steps towards tolerance or intolerance, swiftly altering gay rights. But it is clear that these are both long-terms trends from the past that are likely to continue in the future.
The second way the past can help is by showing how the world works. As the historian Peter Stearns wrote in his excellent Why Study History? paper, "history offers a storehouse of information about how people and societies behave". This is also how weather and electricity and population forecasters do their jobs: they observe and analyse a set of circumstances from the past, and see what outcomes they led to. Using those insights, they create models that describe how the world works and how change happens. Then, by viewing the information they have about the present through those models, they work out how many people there will be, how much electricity will be needed, and whether it is going to be sunny tomorrow.
There are five key questions that the past can help us with: