The Earth is bathed in a flood of sunlight. A fierce inundation of photons--on average, 342 joules per second per square meter. 4185 joules (one calorie) will raise the temperature of one kilogram of water by one degree Celsius. If all this energy were captured by the Earth's atmosphere, its temperature would rise by ten degrees Celsius in one day. Luckily much of it radiates back to space. How much depends on albedo and the chemical composition of the atmosphere, both of which vary over time. A good portion of Earth's albedo, or reflectivity, is created by its polar ice caps. If polar ice and snow were to shrink significantly, more solar energy would stay on Earth. Sunlight would penetrate oceans previously covered by ice, and warm the water. This would add heat and melt more ice, in a positive feedback loop.