In the climate change debate, we often hear the argument that the climate has been changing since time immemorial. This is true, but if modern climate change differs from pre-historic climate cycles, the statement by itself is empty. We need to know how modern climate change compares with that of the past.
Geological observations and computer climate-models have revealed the basic controls and responses involved in past climate variability. This is how we know that the climate has always varied. We also know that this happened because of small changes in the forcing of climate that resulted from long-term changes in Earth’s orbital position around the Sun, small changes in the actual solar output, and occasional injection of volcanic dust veils into the atmosphere. These small triggers could result in large climate responses because of feedback processes.