When I first floated this one I had several people challenge the exponential piece. They were right to do so but I am standing by the statement.
There are any number of ways this can be verified but the easiest is to read the financial reports of the companies that specialize in these systems. For the last several years quarter on quarter they have shown consistent unit and revenue growth. You might not see this in individual purchasing companies but I bet in the larger ones you will see the same type of growth if the entire company is looked at. Certainly I see it in the budgets I have seen.
If you want to get away from financial indicators then look at the number of PLC's and RTU's that are being aggregated in your historians and monitoring centers. I know a couple of oil companies that are devoting whole building floors to the systems and maintaining them.
This is also backed up by power industry news that shows clear areas where growth is available.
I will make one concession though. Even though the current growth rate appears exponential we haven't yet reached the asymptote of the curve and even if we do it doesn't mean that there isn't a plateau that will occur later.
Regardless right now automation, SCADA, DCS and PCN systems are undergoing the same explosive growth that all other areas of the information industry have and are. There is a several year lag due to differences in the capital cycle and implementation and usage but the overall trend is identical. This means that we can expect more and more interconnectivity, accessible systems and most importantly more direct control of functions that previously could not be easily controlled remotely. Furthermore we should expect these cycles to shorten in length and to converge towards the technological adoption rates seen elsewhere. No more 20 year cycles 10, 8, 5 or even 2 (when looking at software portions) will begin to emerge.
This obviously has security implications but it also means great benefits are being accrued.