A Glass of Sunlight
The Earth acts like a giant battery. Almost 100% of its energy comes from the sun. Earth's ability to capture this energy is tremendously inefficent. Most of the energy is expelled from the earth, both before and after being used, as heat. Plants and photosynthetic protists are the only organisms on earth that are capable of sequestering the sun's energy as pyhsical matter (carbon). Thus the amount of carbon matter on earth can be seen as Earth's stored charge. When this "charge" is released, as occurs in the burning of fossil fuels, we can imagine the expenditure as energy that has left earth for good, or as sunlight that is lost forever. In the past, the earth has continuously gained energy, "charging" itself by increasing it's total mass of sequestered carbon. Our current levels of carbon consumption bring us to a threshold point at which we burn energy at the same rate as the sun provides it to the earth. For the first tiime since the evolution of life, the earth is losing energy.