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Earth can be studied like a machine, a very complicated one indeed. In this view, it can be subdivided in many parts, each one a subsystem that can also be interpreted as a machine interconected to many others to form the entire Earth system. One of this parts is the biosphere, and a subsystem of the biosphere refers to human activity. Since human activity can be approached by economics, it is appropriate to talk about economics as part of the entire Earth system, as a piece of the biosphere machinery. This is the view of Ecological Economics.
As it happens to any machine, Earth has to obey the second law of thermodynamics, entropy can only increase with time. And what is the power source of the Earth machine? Easy, the solar radiation. Everything that has ever occurred, occurs, or will occurs could only take place in Earth because of solar radiative energy. Even these lines that I am writing, would not be here if not because the sun shines.
That is why it is so important to be aware of the energy cycles of Earth to understand everything, including economics. Think about it: the energy that powers the device you're using now, and ultimately the brain that is thinking and taking decisions now, every single joule of this energy was once photons leaving the sun towards Earth. Think of this interconnectedness and believe, and worry, and care about Earth, the biosphere, the econosphere and humankind, because everything is One!