Earth is currently in a period of warming. Over the last century, Earth's average temperature rose about 1.1 F (0.6C). In the last two decades, the rate of our world's warming accelerated. Scientists predict that the globe will continue to warm over the course of the 21st century. Is this warming trend a reason for concern? After all, our world has witnessed extreme warm periods before, such as during the time of the dinosaurs. Earth has also seen numerous ice ages on roughly 11,000-year cycles for at least the last million years. So, change is perhaps the only constant in Earth's 4.5-billion-year history.