At every outlet, HR experts shout messages of workforce disruption like doomsayers predicting the end of the world. What demise do they warn us to prepare for? Not fire and brimstone. It's the millennials. The truth is, we're up against some changes and challenges--but millennials aren't the only disruptive type of worker poised to infiltrate our workplace. There are many changes we need to be preparing to take on. Here's what we're up against:

An increase in millennials--who are more transitory. (The jury is still out on exactly whose fault that is.)

You've heard a lot about this younger generation: how your workforce will be almost entirely made up of them by 2020, and how 91 percent of them expect to stay with your company for fewer than three years. How will that impact the way you do HR? Well, for one thing, you're going to have to re-think your retention strategy. If a three-year stay is average, you're going to have to start thinking more short-term too. Your time line for ramp up, knowledge transfer, culture acclimation, employee engagement, job satisfaction, identifying misalignment and everything else in the employee life cycle is getting chopped. How are you going to be more effective and efficient in getting return from your employee investment in a significantly shorter amount of time?

An increase in contingent workers--who aren't around as much. Not only are you losing years off employee careers with your company, you're losing the convenience of employees as a captive audience for at least 8 hours a day. Companies are using contingent workers more than ever before: 83 percent of executives say they're currently increasing the amount of contingent workers in their companies--an increase that's projected to reach 23 million contingent workers by 2017 (a 26 percent increase since 2013). Prepare for a lot of contractors (who aren't fully invested in your company since they have many other clients) and part-time employees. Have you thought about how you can more effectively interact with employees to maximize your time with them?

An increase in remote workers--who aren't physically present at all (or whose physical presence is only simulated). When your employees aren't around, managing them can be more difficult. 34 percent of companies said more than half of their full-time workforce will work remotely by 2020, and 25 percent said more than three-quarters would not work in a traditional office by 2020. The way we approach strategic HR outcomes like employee engagement, communication, alignment, reward and recognition, culture and productivity is about to change radically because we won't be able to do them the way we have with our physically present employees. Do you have a plan to unite a completely segmented workforce?

An increase in need for knowledge workers and an increase in the workforce skills gap--meaning more intense training and difficult recruiting than you've ever had to do before. If you thought recruiting and training was difficult now, brace yourself. A worldwide shortage of skilled college educated workers of more than 38 million people is predicted by 2020. So, before we even get to the difficult task of managing employees, we'll have to find competent ones and train them to be excellent. Making effective learning and development--as well as performance and talent management--a key component of HR will be necessary, but it will also solidify HR's position as a strategic necessity. Are you ready to battle it out for a shrinking pool of qualified candidates? And then train them further once you've secured them?

So what will our workforce really look like in 2020? Let's say you have 20 employees:

18 of them will have been at your company for three years or fewer.

10 of them will be working remotely.

At a 6.5 increase per year, your contingent workers will increase by 32.5 percent. So if you have three part-time or contracted workers right now, you'll have at least four (and another one every five years) by 2020.

And every one of those workers will have been a fight to get since the economy will have a deficit of 38 million skilled employees, making your recruiting investment even more valuable than it currently is.

These are the numbers if you're not planning to grow in the next five years (which probably isn't very realistic).