Chapter 13: Earth as a System: Energy, Matter, and Life

The Earth works as one system of interacting spheres: the geosphere (rocks, soil, landforms, interior), the hydrosphere (liquid water), the cryosphere (ice and snow), the atmosphere (air) and the biosphere (all living things and their habitats). The Sun is the main source of energy, helped by the Earth's hot interior. Energy and matter flow between the spheres, so a change in one affects the others.

Solar radiation reaches the Earth as electromagnetic waves that cross a vacuum at the speed of light, 3 × 10⁸ m/s (sound needs a medium). About 99 percent of the Sun's energy lies in the UV, visible and infrared range. The ozone layer absorbs UV, visible light drives photosynthesis and infrared warms the surface. The radiation reaching the ground is insolation. The solar constant is about 1.4 kW per square metre at the top of the atmosphere, and a clear sky gives about 1 kW per square metre at the surface.

Albedo is the fraction of solar radiation that a surface reflects (from a Latin word for whiteness). Snow has 0.80 to 0.90, ice 0.50 to 0.70 and crushed rock 0.25 to 0.30, while black soil and ocean water have low albedo. High albedo stays cooler and low albedo absorbs more and warms up. Every object re-radiates heat; concrete gives out stored heat at night, so cities become an urban heat island, while vegetation cools by shade and transpiration.

Because the Earth is a sphere, sunlight falls almost overhead near the equator and at a slant near the poles. Direct rays put their energy on a small area, while slanting rays spread it over a large area. So the equator stays warm and the poles stay cold. The tilt of the axis and the Earth's revolution give seasons and changing day length. This uneven heating drives winds and ocean currents.

The atmosphere, 78 percent nitrogen and 21 percent oxygen, is held by gravity. The troposphere (0 to 12 km) holds nearly all weather and cools about 6.5 °C per km. The stratosphere (12 to 50 km) holds the ozone layer, which absorbs UV so that temperature rises with height and the air stays calm. Space begins near 100 km. The atmosphere absorbs part of the incoming radiation and traps outgoing infrared with greenhouse gases (CO₂, CH₄, water vapour). This greenhouse effect keeps the Earth warm, but excess CO₂ causes global warming.

Wind is air moving from high to low pressure, and pressure differences come from uneven heating. By day sun-warmed slopes give a valley breeze, and at night cold dense air gives a mountain breeze. Globally, hot air rises at the equatorial low, sinks near 30° (sub-tropical highs), rises again near 60° (sub-polar lows) and sinks at the poles. Rotation deflects winds to the right in the north and to the left in the south. Winds drag surface water into ocean currents; temperature, salinity and continents shape them, and rotation makes circular gyres. Currents carry heat and nutrients.

Matter keeps moving between living and non-living parts in biogeochemical cycles. In the water cycle water evaporates, condenses, falls as precipitation and soaks in. In the carbon cycle plants take in CO₂ by photosynthesis, and respiration and decay return it (fast cycle, days to years), while buried remains become fossil fuels (slow cycle, millions of years) and oceans absorb CO₂. In the nitrogen cycle bacteria fix N₂, nitrifiers make nitrate, plants assimilate it, decomposers ammonify and denitrifiers return N₂. In the oxygen cycle respiration and combustion use O₂ and photosynthesis restores it.

Burning fossil fuels and deforestation raised CO₂ from 315 to 420 ppm since 1960 and strengthened the greenhouse effect. Extra CO₂ makes sea water more acidic, harming plankton and coral, while warmer oceans absorb less CO₂. Excess fertiliser causes eutrophication, algal blooms that use up oxygen. Deforestation cuts photosynthesis and transpiration, lowers local rainfall, changes albedo, erodes soil and destroys habitats. Vehicle fumes form ground-level smog and ozone. Cooperation such as the Montreal Protocol, solar and wind energy, tree planting, saving water and Mission LiFE help restore the balance.