It would be more enlightening to do some research on the reliability of proxy data that is used to determine the earth's temperatures prior to more modern measurements. Here is a hint, it isn't very reliable. We are essentially guessing on temperatures from centuries ago, especially since we are talking about 1 degree here and there to show a warming trend. Even if they were accurate, if we are to believe the age of the earth from scientists, we have an extremely small sample size to to determine a trend. Too small to make a reasonable determination.
Also, man made global warming is an even bigger leap of faith.