Earth is also a system. It consists of four major parts called "spheres." Earth's spheres include:

• the lithosphere, which contains all of the planet's rock;
• the hydrosphere, which contains all of the planet's water;
• the biosphere, which contains all of the planet's living organisms; and
• the atmosphere, which contains all of the planet's air.

These four major spheres are closely connected. In some places they blend so much that it is difficult to distinguish between them. Consider the soil. Soil contains minerals from rock, water, microorganisms, and air.

Earth's spheres are so closely connected that a change in one sphere often “causes” a change or impact in one or more of the other spheres. An event is a change or impact that can occur as a result of natural processes, such as an earthquake or hurricane. An event can also occur as a result of human activities, such as an oil spill or air pollution. Not only can an event cause changes or impacts to occur in one or more of Earth's four spheres, it can also be the “effect” of such changes. The two-way, cause and effect relationship between an event and a sphere is called an interaction. Interactions also occur between the spheres; for example, a change in the atmosphere can cause a change or impact in the hydrosphere, and vice versa.

Interactions that occur as the result of events such as floods or tornadoes usually have localized impacts. The reason is that the damaging forces of these events--flood waters and funnel clouds--can only travel a small distance from their point of origin. On the other hand, the effects of events such as El Niño or ozone depletion may cause interactions that are observed globally. The El Niño event--a change in the ocean currents off the coast of Peru--can cause changes in weather patterns all the way across North America; while ozone depletion above Antarctica may cause increased levels of ultra-violet B radiation around the world.

The effects of an event may be positive or negative. At first glance, all the effects of a forest fire appear to be negative. Fires (event) burn the trees (biosphere), the barren soil (lithosphere) becomes more susceptible to erosion, and the highly erodible soil gets washed into streams (hydrosphere) where it chokes aquatic organisms. However, some effects of events are good--even effects from events such as forest fires. For example, the seeds of some trees must experience extremely high temperatures in order to germinate. Such trees benefit from forest fires.

Though the aftermath of an event may linger for years, the actual direct impacts can be seen either in the short term or over the long term. The initial effects of a flood are generally short term. The flood waters usually subside within a few days or weeks. The effects of events such as ozone depletion, on the other hand, may be felt for generations.

Understanding the interactions between and among Earth's spheres and events enables people to predict the outcomes of events. Being able to predict outcomes is useful when, for example, developers wish to know the environmental effects of a project, such as building an airport, before they begin construction. Understanding the interactions that occur within Earth's system also helps people prepare for the effects of natural disasters. Consider volcanic eruptions. Understanding the force and nature of a volcanic eruption permits scientists to predict how far and in what direction the lava could flow or where ash may fall.

Studying the interactions between and among an event and Earth's spheres is called Earth system science (ESS). In Earth system science, there are ten possible types of interactions that could occur within Earth's system. Four types of interactions are between an event and each of the Earth's spheres:

event lithosphere

event hydrosphere

event biosphere

event atmosphere

In addition to the interactions above, there are six interactions that occur among Earth's spheres:

lithosphere hydrosphere

lithosphere biosphere

lithosphere atmosphere

hydrosphere biosphere

hydrosphere atmosphere

biosphere atmosphere

Again, the double-headed arrows () symbolize that the cause and effect relationships of the interactions can go in both directions. For example, "lithosphere hydrosphere" refers to the effect the lithosphere could have on the hydrosphere, as well as the effect the hydrosphere could have on the lithosphere.

These six types of sphere interactions are illustrated in dark gray in the Earth System Diagram below (the four event sphere interactions are also included in this diagram; they are depicted in light blue):

The ten types of interactions that can occur within Earth's system can occur in cycles called feedback loops. Feedback loops control or modify input, or the cause of a change. Positive feedback loops reinforce or intensify the effects of an input, while negative feedback loops lessen or diminish the impact of an input. Be sure not to confuse positive and negative feedback loops with positive and negative effects of a change.

Earth's temperature and the subsequent formation of sea ice are controlled by a positive feedback loop. A drop in Earth's temperature (input) can result in the formation of sea ice. Sea ice reflects the sun's light back toward space. An increase in sea ice would result in more of the sun's light being reflected back into space. This would result in further cooling of Earth's atmosphere and the formation of more sea ice. Thus, the presence of sea ice--because of a cool climate--reinforces the further cooling of Earth's climate (input) and the formation of more sea ice.

Earth's temperature is also controlled by negative feedback loops. Water vapor and other heat-trapping gases such as carbon dioxide (CO₂) are released (input) into the atmosphere from power plants and automobiles. These heat-trapping, or greenhouse, gases can lead to an increase in Earth's average temperature. An increase in Earth's temperature can lead to more evaporation and ultimately increased cloud cover. Clouds reflect the sun's energy to space before it can reach Earth's surface. The result is global cooling. Thus, increased cloud cover that results from global warming can lead to global cooling, which lessens the impacts of greenhouse gases on Earth's temperature.