Parsimony has many names, in philosophy it is best known as Occam's Razor. Basically, the best, of the available, theory makes the fewest assumptions (occams is fewest additional concepts). Its something you need to know by 3rd year, so i just gave ya a head start ;P.
Gravity per se, like Global Warming is not a theory but an observation. Theories come from explaining what is causing it, and thats what I debate. I dont think anybody can argue that the wolrd is warming up, that is supported by facts, but why is the earth warming up? Is it because the CO2 we put in the air? Well, geologically, there was a time where far more CO2 was in the atmosphere, I forget the name of the time period, but I know it was when the trees and swamps that now are used as fossil fuels existed(gimme a break, its been 3 years since i did any geology).
What about the Dark ages, where it is hypothesized that the warmest temperatures occured?
These 2 things, provide circumstantial evidence against the C02 theory of global warming.
I, like many people in the science department at my university believe that Global Warming is caused by increases in solar output. Pollution is still a terrible thing, we are poisoning ourselves and other living things, but it might not be the answer to global warming.
With respect to scientific theory; dont make me break out Aristotle on your ass, ;P.