In the 1990s James Lovelock proposed the Gaia hypothesis. This proposes that the whole of life on Earth acts as a single system, maintaining both the composition of the atmosphere and the temperature at the surface. Both conditions seem to be maintained by negative feedback loops. Basically this is a balance of processes that says: “when there is too much of something (like oxygen) make less of it; and when there is too little of it, make more.” This process leads to dynamic stability. The idea of feedback loops comes from the study of complex systems. These have been studied since the beginning of the 20th century. Though the study of complex systems does not give precise results, it does show how such systems work in a general way. This is useful because most of the world around us is far too complex to be mathematically predicted.
The negative feedback system that seems to function on Earth is compared to Venus, where a runaway positive feedback loop (“if you make more of this, then make even more”) seems to have raised the surface temperature of the whole planet without control, and long ago evaporated all of its water into space.
The Gaia hypothesis was modeled in the Daisyworld computer programs, that show how two components self create a negative feedback loop that regulates the temperature. Bacterial populations in the oceans of the early
Earth are related to the Daisyworld model to speculate how their inputs and outputs of gases could have been regulating the temperature for billions of years. The cycling of oxygen is shown in a similar negative feedback system. There are many cycles of gases and nutrients going on across earth, and they are mostly connected. The question is asked if there are other emergent properties of the earth system beyond the regulation of the temperature and the oxygen level.