What type of business will be skyrocketing within the next 20 years?

If you want to have your own business someday -- or if you just want to be able to hold your own or even get ahead in a fast-changing world -- you need to learn how to learn. That should be your primary goal in college. As Casey Stengel supposedly said, "Never make predictions, especially about the future." It's very hard to get it right. So what can you do? You can prepare yourself to handle whatever comes your way. If your goal is learning how to learn, then what subject you major in is less important than learning how to soak up any new subject you might need to learn in the future.

The best way I know of to do that is to take courses in a variety of academic areas. Think of each one as a particular way of looking at the world. Economists look at things one way, with a particular set of tools they bring. Physicists bring a different set of tools, and art historians see things in a totally different way. Each approach has its strengths and weaknesses, but if you have a wide variety of tools in your personal toolchest, you'll be able to learn new subjects more easily because you'll be used to looking at the world in a variety of different ways. By the way, this also can work well for people who aren't planning on starting their own business; the average American worker changes careers many times between entering the workforce and retirement. Do you think their college major is directly applicable throughout their working life? It may be for some, but not for most!

The other important thing that you may learn if you follow this approach is what kinds of things you enjoy learning about and participating in, and what kinds of things you don't. That's valuable information (not to mention that it can point you to the major you should pursue). Pity the poor person who spends X years learning to become a Y -- only to find out that they don't like the kind of work that Ys do. As an entrepreneur, if you know what kinds of work you enjoy and what you don't, you'll be able to build your team to be strong where you're not.

So what type of business will you create? "Never make predictions". But if you have learned how to learn, then once you decide what you need to learn in order to start your business, you'll be well equipped to handle whatever you need to learn at that point.

Rob, that's such an interesting question. I completely agree Geoff's answer above, that's the smartest starting point.

I'll share my personal perspective as it relates to Geoff's answer. I've been working in "skyrocketing" industries my entire career (digital advertising, mobile payments, and now 3D Printing), and nothing I studied in school could have prepared me for it. I studied policy analysis & management (sort of like economics), and though this isn't the answer to your question, I'd like to share what I got from my education that was particularly valuable for a career in growth industries:
1) An understanding of basic economics - supply/demand, impacted by the world around you
2) An understanding of people's psychology (through classes, friendships, and you can read a lot of it in books)
3) An understanding of balances sheets - I took an accounting class that was not directly applicable, but if you're gonna start a business, you have to understand how basic finance works

I took classes in everything from basic computer programming to architecture, and the exposure to these different things helped me learn more about my own interests and passions - for starting a business, this is important.

As for the "skyrocketing" industries, there are a lot of smart people who's job it is to identify them - if you're interested in learning more, just do a google search. For example, in 2012, Wall St Journal published this list http://online.wsj.com/news/articles/SB10000872396390444813104578018940187057924 and in 2013, Business insider published this: http://www.businessinsider.com/americas-fastest-growing-industries-2013-6.