It's a little hard to miss a climate event that happens every year and brings India 80% of it's rainfall. :-))
I'm guessing the advent of the Roman trade had more to do with the absorption of Egypt into the Empire and for the first time in centuries bringing relative stability, allowing people in that part of the world to venture out.
I am more to the thought that the Greeks in Egypt claimed the trade was overland and did the usual markup. There are many things that we do not know about the India trade except that it was about 2000 years old when the Romans popped up.