Hasn’t the Earth heated and cooled all on its own in the past?
Yes. Many times. There are many natural variations and events that impact the Earth’s climate including changes in the Earth’s orbit around the Sun, changes in the tilt of the Earth on its axis, changes in solar output, volcanic activity, shifting of continental tectonic plates, impact of meteors and comets, and, yes, the natural release of greenhouse gases. These natural influences on the Earth have pushed the climate into states of mind-numbing cold ice ages and withering hothouses.
The majority of these variations influence the climate on a scale of thousands of years. Changes in solar output can impact climate on the scale of decades, and single events such as volcanic eruptions and meteor impacts can have large influences over shorter periods of time. What makes the current warming trend unique is that it cannot easily be attributed to any of these natural influences. Changes in the Earth’s orbit and tectonic plate shifts take too long, and we haven’t had any major volcanic eruptions or meteor impacts to influence the climate in the short term. As for short term natural influences, the most likely candidate would be changes in solar output. However, measurements have not shown solar output varying enough to impact Earth’s climate enough to explain the current warming trend, especially in the last 30 years. The only way to explain the current warming of the Earth’s climate is to incorporate the release of greenhouse gases by the activities of mankind.
If looking back through history at the incredible range of states of the Earth’s climate provides anything, it should demonstrate exactly what the planet is capable of doing, and Earth itself is quite content to be either frozen over or boiling hot. However, such states may not be so suitable for mankind. The unsettling part is that mankind is pushing portions of the climate into uncharted territories, and, as the past has demonstrated, the Earth is quite capable of responding in very harsh ways.