Causes of Climate Change: Natural Forcings and the Enhanced Greenhouse Effect

Climate has always changed. Over Earth's history, ice ages have come and gone, sea levels have risen and fallen by more than a hundred meters, and global temperatures have swung by several degrees. Recognizing these natural changes is important, because it helps separate what the climate does on its own from what humans are now driving. The key concept is radiative forcing, any factor that alters the balance between incoming and outgoing energy. A positive forcing warms the planet and a negative forcing cools it. Natural and human forcings can be compared directly using this common measure.

The most important natural driver of long-term, slow climate change is the set of variations in Earth's orbit known as the Milankovitch cycles. There are three components. Eccentricity describes how the