Earth and Space: Cycles on Earth

The water cycle, or hydrological cycle, describes the continuous journey of water on, above, and below the Earth's surface. This process has no starting point, but it's often easiest to begin with the largest bodies of water. The Sun's energy heats water in oceans, lakes, and rivers, causing it to turn into vapour or steam. This process is called evaporation. Plants also release water vapour into the air through their leaves in a process called transpiration. As this warm, moist air rises, it cools, and the water vapour turns back into tiny liquid water droplets, forming clouds. This is called condensation. When these droplets become too heavy, they fall back to Earth as rain, snow, sleet, or hail, a process known as precipitation. The water then collects in rivers, lakes, and oceans (collection), or soaks into the ground (infiltration), ready to start the cycle all over again.

The carbon cycle is the process by which carbon atoms are exchanged between the Earth's major reservoirs: the atmosphere, oceans, land, and living organisms. Plants play a vital role by absorbing carbon dioxide (CO₂) from the atmosphere for photosynthesis to create glucose (food). This carbon is then transferred through the food chain when animals eat plants. Both plants and animals release CO₂ back into the atmosphere through respiration. When organisms die, decomposers like bacteria and fungi break them down, releasing carbon back into the soil and atmosphere. Over millions of years, some carbon from dead organisms is converted into fossil fuels (coal, oil, gas). The burning of these fuels (combustion) releases large amounts of stored carbon into the atmosphere as CO₂.

Nitrogen is essential for all living things as it's a key component of proteins and DNA. However, the most abundant form of nitrogen, nitrogen gas (N₂) in the atmosphere, is very unreactive and cannot be used directly by most organisms. The nitrogen cycle is the process that converts this unusable N₂ into forms that plants can absorb. This conversion is called nitrogen fixation. It happens naturally through lightning strikes and, more importantly, by nitrogen-fixing bacteria found in the soil and in the root nodules of legume plants. Other bacteria (nitrifying bacteria) then convert this fixed nitrogen into nitrates (NO₃⁻), which are easily absorbed by plant roots. Animals get their nitrogen by eating plants. When plants and animals die, decomposers break them down, returning nitrogen to the soil as ammonium compounds. Finally, denitrifying bacteria in the soil convert nitrates back into nitrogen gas, which returns to the atmosphere, completing the cycle.

Human activities are significantly altering the natural balance of Earth's cycles, often with negative consequences. The burning of fossil fuels for energy and transport releases huge quantities of carbon dioxide, enhancing the greenhouse effect and driving global warming. Deforestation has a double impact: it reduces the number of trees available to absorb CO₂ through photosynthesis, and the burning of forests releases stored carbon back into the atmosphere. Deforestation also disrupts the water cycle by reducing transpiration, which can lead to changes in local rainfall patterns. In agriculture, the extensive use of nitrogen-based fertilizers adds massive amounts of reactive nitrogen to the environment. When excess fertilizer washes off fields into rivers and lakes, it causes eutrophication – a process where algal blooms thrive, then die and decompose, using up all the oxygen in the water and creating 'dead zones' where fish cannot survive.