Millennia ago, ancient farmers cleared land to plant wheat and maize, potatoes and squash. They flooded fields to grow rice. They began to raise livestock. And unknowingly, they may have been fundamentally altering the climate of Earth. A team from the University of Wisconsin-Madison studied the findings based on a sophisticated climate model that compared our current geologic time period, called the Holocene, to a similar period 800,000 years ago. It shows the earlier period, called MIS19, and was already 2.3 degrees Fahrenheit (1.3 C) cooler globally than the corresponding time in the Holocene, around the year 1850. This effect would have been more noticeable in the Arctic, where the model shows temperatures were 9-to-11 degrees Fahrenheit colder.