Earth is currently in a period of warming. Over the last century, Earth's average temperature rose about 1.1°F (0.6°C). In the last two decades, the rate of our world's warming accelerated and 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.

Scientists note that there are two new and different twists to today's changing climate: The globe is warming at a faster rate than it ever has before; and Humans are the main reason Earth is warming. Since the industrial revolution, which began in the mid-1800s, humans have attained the magnitude of a geological force in terms of our ability change Earth's environment and impact its climate system.

Humans are affecting Earth's climate system in many ways. For example, we transformed roughly 40 percent of Earth's habitable land surface to make way for our crop fields, cities, roads, livestock pastures, etc. We also released particulate pollution (called "aerosols") into the atmosphere. Changing the surface and introducing aerosols into the atmosphere can both increase and reduce cloud cover. Thus, in addition to driving up average global temperature, humans are also influencing rainfall and drought patterns around the world. While scientists have solid evidence of such human influence, more data and research are needed to better understand and quantify our impact on our world's climate system.