Planet Earth is perfect. It orbits at the perfect distance from the sun; it tilts at just the right angle; and it has a moon just large enough to hold it in place. On top of that, the day-to-day workings of the planet naturally serve to nurture its plants and animals. This five-part series will show how the forces of nature - weather, ocean currents, solar energy, and volcanoes - drive, shape, and support Earth's great diversity of life. But what happens when nature's epic forces are knocked off course? As human activity impacts Earth's perfectly balanced ecosystems, what can we do to protect the fragility of life on our unique and precious planet?