At the start of the Iron Age (around 800 – 750 BC) it was colder and wetter than at any other time in the last 11,000 years, and the cold, wet spell lasted until about 400 BC. The attached paper reviews the evidence for climate change and demonstrates how this forced a switch from predominantly arable to predominantly pastoral agriculture and led to the creation of early hillforts, and possibly the creation of common land in lowland areas.