Key Points

The Earth system comprises five interacting spheres: the Geosphere (solid rocks, soil), Hydrosphere (liquid water), Cryosphere (ice, snow), Atmosphere (air), and Biosphere (all life). A change in one sphere affects the others.

The Sun is Earth's main energy source, providing energy as solar radiation. The amount of solar radiation reaching Earth's surface is called insolation, which averages about $1 \text{ kWm}^{-2}$ under clear skies.

The solar constant is the average solar energy received per unit area at the top of the atmosphere, perpendicular to the Sun's rays. Its value is approximately $1.4 \text{ kWm}^{-2}$, or $1400 \text{ J s}^{-1} \text{m}^{-2}$.

Albedo is the fraction of solar radiation reflected by a surface. High-albedo surfaces like snow (0.8-0.9) reflect more light and stay cool, while low-albedo surfaces like oceans absorb more light and get warmer.

Due to Earth's spherical shape, solar rays strike the equator more directly, concentrating heat. At the poles, the same energy spreads over a larger area, resulting in lower temperatures and driving global winds and ocean currents.

Most weather phenomena occur in the troposphere (0-12 km), where temperature decreases with height. The stratosphere (12-50 km) contains the ozone layer, which absorbs UV radiation, causing temperature to increase with height.

The Earth's surface absorbs sunlight and re-radiates it as infrared heat. Greenhouse gases like carbon dioxide ($\text{CO}_2$), methane ($\text{CH}_4$), and water vapor trap this heat, keeping the planet warm enough for life.

Winds are the movement of air from a region of high pressure to a region of low pressure. These pressure differences are primarily caused by the uneven heating of Earth's surface by the Sun.

Ocean currents are driven by winds, temperature, and salinity differences. They play a major role in regulating global climate by transporting heat from the equator towards the poles, influencing weather patterns worldwide.

Biogeochemical cycles describe the movement of essential elements like carbon, nitrogen, and oxygen between the living (biotic) and non-living (abiotic) components of the Earth, ensuring nutrients are recycled.

The water cycle is the continuous movement of water through evaporation, condensation, precipitation, and runoff. It connects the hydrosphere, atmosphere, geosphere, and biosphere.

Carbon cycles through photosynthesis, where plants convert atmospheric $\text{CO}_2$ into organic compounds, and respiration, where $\text{CO}_2$ is released. Burning fossil fuels rapidly releases large amounts of stored carbon into the atmosphere.

Atmospheric nitrogen ($\text{N}_2$) is unusable by most organisms until it is 'fixed'. Nitrogen-fixing bacteria convert $\text{N}_2$ into ammonia ($\text{NH}_3$), which is then converted by other bacteria into nitrates ($\text{NO}_3^{-}$) that plants can absorb.

The oxygen cycle is balanced by two main processes. Photosynthesis by plants and phytoplankton produces oxygen ($\text{O}_2$), while respiration by living organisms and combustion consume it.

Human activities, especially burning fossil fuels, have increased atmospheric carbon dioxide ($\text{CO}_2$) by about 35% since 1960. This enhances the greenhouse effect, causing global warming, melting ice, and rising sea levels.

The overuse of nitrogen-based fertilizers in agriculture leads to runoff into water bodies. This causes excessive algae growth (algal blooms), which depletes oxygen and kills aquatic life in a process called eutrophication.

The ozone layer protects Earth from harmful UV radiation. Human-made chlorofluorocarbons (CFCs) caused a 'hole' in the ozone layer, but the Montreal Protocol, a global agreement, has led to its slow recovery.

Clearing forests reduces photosynthesis, decreasing the absorption of $\text{CO}_2$ and potentially altering local rainfall patterns. It also leads to soil erosion and loss of biodiversity due to habitat destruction.