“So, What’s Up with the Weather?”
There are now many different published reconstructions of global temperature that include this period of time (Figure 1.3). These data, predominantly from measurements on land in the Northern Hemisphere, confirm the existence of a warmer period around 1,000 years ago known as the Medieval Climate Anomaly (MCA) and a period of distinct regional cooling that started around 400 years ago known as the Little Ice Age (LIA). This is a clear record of natural variation in climate that is not the result of human activity. Even so, the implications are disturbing: If small natural changes in global temperature have such a large impact on society, what can we expect to happen as a consequence of a larger increase in global temperature projected by the end of the 21st century?