Earth as a System: Energy, Matter and Life: Notes

The Earth works as one system made of five parts, called spheres: the atmosphere (air), the hydrosphere (liquid water), the cryosphere (frozen water), the geosphere (rock, soil and the interior) and the biosphere (all life). Matter and energy move between them all the time, and the Sun drives most of the movement.

One event usually touches several spheres. In the monsoon, heat from the Sun makes sea water evaporate (hydrosphere to atmosphere). Moist air flows over the hot land, rises and cools, and rain falls on hills and fields (atmosphere to geosphere and biosphere). A volcanic eruption begins in the geosphere, puts ash and gas in the atmosphere, can melt ice of the cryosphere and buries plants of the biosphere, and its haze can cool the weather for a time.

Sunlight is electromagnetic radiation, which travels through empty space. It has three main parts. Infrared (about half of the energy, approximate) is felt as heat. Visible light (about 40 percent, approximate) lets us see and powers photosynthesis. Ultraviolet (under 10 percent, approximate) can burn skin, and the ozone layer stops most of it.

At the top of the atmosphere a surface facing the Sun receives about 1361 W m⁻² (approximate). About 30 percent of the sunlight is reflected by clouds, ice and the surface (approximate). The rest, about 70 percent, is absorbed by the air, land and sea, which warm up and give out infrared radiation. Light surfaces such as fresh snow reflect more and stay cooler; dark surfaces such as tar absorb more and get hotter.

The warm ground gives out infrared radiation. Greenhouse gases such as carbon dioxide, methane and water vapour absorb some of it and send part back down. This greenhouse effect keeps the average surface temperature near 15 °C instead of about −18 °C (approximate). The effect is natural and useful; trouble starts when extra gases strengthen it.

Latitude. At the equator the Sun is nearly overhead. At higher latitudes its rays slant, so a beam of the same width covers a larger patch of ground, of length $1/\cos(\text{latitude})$ times the width. At 60° the patch is twice as long, so each square metre receives about half the energy.

Seasons. The axis of the Earth is tilted by about 23.5 degrees and points the same way all year. The hemisphere that leans towards the Sun gets more direct rays and longer days (summer). The other has winter. Distance from the Sun is not the cause: the Earth is nearest to the Sun in early January.

Carbon moves between the air, living things, the ocean and rocks. Plants and algae take in carbon dioxide by photosynthesis and build sugars and wood. Animals get carbon by eating. Respiration by all living things, and decomposition by bacteria and fungi, return carbon dioxide to the air. Some buried remains change over millions of years into fossil fuels: coal, oil and natural gas. Burning them, or burning wood, releases carbon dioxide at once. The ocean takes in and gives out the gas.

The air held about 280 ppm of carbon dioxide before the late 1800s and about 420 ppm as of the early 2020s (both approximate).

Nitrogen. Air is about 78 percent nitrogen gas (approximate), but plants cannot take it up. Nitrogen fixation changes it into ammonia or nitrates: bacteria such as Rhizobium in the root nodules of pulses, lightning, and fertiliser factories do this. Nitrification is the change of ammonia to nitrates by soil bacteria. Plants absorb nitrates and make proteins, and animals eat the plants. Decomposers turn dead remains and wastes into ammonia. Denitrification by bacteria in waterlogged soil changes nitrates back to nitrogen gas.

Water. The Sun’s heat causes evaporation and plants lose vapour by transpiration. The vapour cools and condenses into clouds. Precipitation brings rain and snow. Runoff and infiltration return water to rivers, groundwater and the sea. About 97 percent of the water on Earth is in the oceans (approximate).

Burning fossil fuels moves carbon that was stored for millions of years into the air within a short time. Deforestation releases the carbon stored in trees and leaves fewer trees to take carbon dioxide in. Rice fields and cattle give out methane. These add to the greenhouse gases, strengthen the greenhouse effect and raise the average temperature, by about 1.1 °C since the late 1800s (approximate, as of the early 2020s). Effects include melting ice, rising sea level and more extreme weather.

Too much nitrogen fertiliser washes into ponds and rivers and makes algae grow too much. When the algae die, decomposers use up the dissolved oxygen and fish die.

What we can do: use solar, wind and hydro power; plant and protect trees; use public transport, cycles or walking; use fertiliser only as needed and grow pulses; save energy and water.