Well, that topic is precisely the one that helps to clarify the speculative nature of science the best.
If you look back a number of years, you'll see that there was at one point an outcry about the impending disaster of global cooling. Some years later, it turned into global warming, as scientific research into the cause of global cooling developed further. I wouldn't be surprised to see global warming turn back into global cooling at some point in the near future.
The point is: all science is speculative. Some areas are more so speculative than others. What defines a scientific field as speculative is our ability to observe the subject of study. We can't very well go back 10,000,000 years to see when the earth was created, or if it even existed then, but we can speculate on that date using scientific measures that are constantly changing.
Look at quantum physics, for instance. You can't say quantum physics is anything
but speculative. If you do, you probably don't understand it that well... or you do understand it, but you believe that current theories don't contradict one another.
