Yes, Earth has warmed and cooled repeatedly. Climatologists know this, and they can quantify it. They have many lines of evidence: ice cores, varies, deep sea oozes, the geological record, fossil forums and diatoms. They have modeled changes in Earth’s obliquity, worked out the precession of the equinoxes, and much more. When all non-anthropogenic factors are quantified, there is a small but nevertheless significant amount of change that can only be ascribed to human activity.
Studies of oxygen isotopes recovered from ice cores 100,000 years old, and measurements of ratios of carbon dioxide to oxygen in the air bubbles trapped in this ancient ice, taken in conjunction with changes in species of microfossils (often highly temperature-sensitive), lead to the same conclusion: Earth’s climate is changing at unprecedented speed, and the change has been accelerating for the last few thousand years. There are diaries kept by individuals, journals kept by amateur naturalist societies, weather records from weather stations worldwide, for the last couple of centuries, telling the same story: as humans have industrialized, significant shifts have occurred in such things as planting dates, first sightings of bird species, the date that ice breakup occurs in Alaska.
I really like this old cartoon from 2010:
http://greenmonk.net/2010/01/07/what...d-for-nothing/