magic rule of 72

"The world's population is growing at about 1.2% per year, or 75 million people/year."

I just did the excel calculation, by multiplying by 1.012 accumulatively. 1.2% population accumulative growth results in the population doubling in around 60 years.

I did not need to waste my time with this, as there is a magic rule called the rule of 72 http://betterexplained.com/articles/the-rule-of-72/. So I could have saved time, and simply divided 72 by 1.2:

72 /1.2 = 60

We have 1.2 % population growth. I presume energy growth is higher than 1.2%. Does any one have any facts on this ?
Ok let me presume energy growth is 2 % a year. Using the magic 72 rule, we have 72 / 2 =36 years.

So in 36 years the world needs to find double the energy. This is enough to worry anyone!