It is obvious that things were different in the past, but somehow we tend to assume that the climate and weather were similar to what they are now (unless, of course, we are studying the last Ice Age, where the change in climate is the starting assumption). But the climate hasn't stayed constant over the last two millenia, and this isn't the first period of global warming. The last one occurred in roughly 1000-1300 CE, and is known as the Medieval Warm Period; summer temperatures averaged 1°C warmer than in the 20th century. During this time, Vikings settled Greenland and explored the eastern coast of North America; grapes grew in much of England and made so much good wine that the French tried to exclude it from the rest of Europe. Summer after summer of good weather led to bountiful crops, rising populations, and the development of villages on newly-cleared land in areas that were previously too marginal for crops. The general level of wealth and health resulted in an explosion of culture, sometimes called the 12th-century Rennaissance. Cathedrals rose higher than churches had before, with delicate stained-glass windows and elaborate stone carving; nobles at court had time to listen to and compose the poetry and music of the troubadours; illuminated manuscripts reached a high point of luminous beauty. All due to 1°C more warmth in the summers.
To learn more about how weather affects cultures, read The Little Ice Age: How Climate Made History 1300-1850, by Brian Fagan. It has a chapter about the Medieval Warm Period to set the stage before discussing the subsequent cold period, the Little Ice Age, and its effects on Europe.