Environmental Science

Earth's Spheres

Atmosphere: Layers of gases surrounding Earth that provide essential gases ( $O_2$ , $CO_2$ ), shield against ultraviolet (UV) radiation, and stabilize planetary temperatures.
Lithosphere / Geosphere: The solid Earth; specifically, the lithosphere comprises the upper portion of the crust and mantle, representing a subsection of the larger geosphere.
Hydrosphere: All liquid and frozen water present on Earth (including oceans, rivers, and ice).
Biosphere: All life on Earth, distributed across and supported by the atmosphere, lithosphere, and hydrosphere.

Atmospheric Composition and Structure

Gas Composition:
- Nitrogen ( $N_2$ ): $\approx 78\%$
- Oxygen ( $O_2$ ): $\approx 21\%$
- Argon ( $\text{Ar}$ ): $\approx 1\%$ ( $0.96\%$
- Carbon Dioxide ( $CO_2$ ): $0.04\%$ ( $< 1\%$
- Trace gases (including methane, $CH_4$ ) and variable concentrations of water vapor ( $H_2O$ ).
Vertical Layers:
- Troposphere ( $0\text{ to }12\text{--}18\,\text{km}$ , $15\,^\circ\text{C}\text{ to }-56.5\,^\circ\text{C}$ ): Layer closest to Earth where weather occurs and the vast majority of terrestrial life resides.
- Stratosphere ( $11\text{ to }50\,\text{km}$ , $-56.5\,^\circ\text{C}\text{ to }-2.5\,^\circ\text{C}$ ): Houses the ozone layer; upper limit of life is restricted to rare airborne fungal spores and high-flying avian species (such as the Ruppell's vulture).
- Mesosphere ( $40\text{--}50\text{ to }80\text{--}90\,\text{km}$ , $-2.5\,^\circ\text{C}\text{ to }-86.5\,^\circ\text{C}$ ): Temperature decreases with altitude; meteors burn up here.
- Thermosphere ( $80\text{--}90\text{ to }800\,\text{km}$ , $-86.5\,^\circ\text{C}\text{ to }1200\,^\circ\text{C}$ ): Displays auroras; temperature increases sharply due to solar radiation.
- Exosphere ( $800\text{ to }3000\,\text{km}$ , $1200\,^\circ\text{C}$ ): Outermost atmospheric fringe transitioning into space where satellites orbit.
Atmospheric Pressure & Temperature Dynamics:
- Atmospheric pressure decreases continuously as altitude increases.
- Temperature alternates between decreasing and increasing depending on the specific atmospheric layer.

Atmospheric temperature and pressure profiles by altitude

Biogeochemical Cycles

Carbon Cycle Dynamics:
- Carbon Sinks (Storage): Photosynthesis, soil absorption, ocean diffusion, limestone formation, and fossil fuel formation.
- Carbon Sources (Release): Cellular respiration, animal flatulence, organic decomposition, and combustion of fossil fuels.
- Photosynthesis Equation: $\text{Carbon dioxide} + \text{Water} \xrightarrow{\text{Light / Chlorophyll}} \text{Glucose} + \text{Oxygen}$ $6CO_2 + 6H_2O \rightarrow C_6H_{12}O_6 + 6O_2$
- Aerobic Respiration Equation: $\text{Glucose} + \text{Oxygen} \rightarrow \text{Carbon dioxide} + \text{Water}$ $C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O$
- Deforestation Impacts: Elevates atmospheric $CO_2$ by eliminating photosynthetic sinks, releasing stored carbon rapidly via tree burning, and releasing carbon slowly through decaying vegetation.

Nitrogen Cycle Stages:
- Atmospheric nitrogen ( $N_2$ ) is chemically inaccessible to living organisms due to a triple covalent bond.
- Nitrogen Fixation: Atmospheric $N_2$ is reduced into ammonia ( $NH_3$ ) by nitrogen-fixing bacteria or abiotic lightning energy.
- Nitrification: Soil bacteria oxidize ammonia into nitrites ( $NO_2^-$ ) and subsequently into nitrates ( $NO_3^-$ ) for plant uptake.
- Ammonification: Decomposers break down dead biomass and metabolic waste, returning ammonia ( $NH_3$ / $NH_4^+$ ) to the soil.
- Denitrification: Specialized anaerobic bacteria convert nitrates ( $NO_3^-$ ) back into gaseous $N_2$ , completing atmospheric return.

Hydrological (Water) Cycle:
- Evaporation: Liquid surface water vaporizes into atmospheric gas.
- Transpiration: Water absorbed by plant root systems travels upward and evaporates into the air through foliage.
- Condensation: Gaseous water vapor cools and condenses into airborne liquid droplets, forming clouds.
- Precipitation: Condensed atmospheric water returns to Earth in liquid or solid states.
- Percolation: Gravity-driven movement of liquid water filtering downward through soil and rock layers to replenish groundwater reserves.

Climate, Weather, and Global Climate Change

Natural Greenhouse Effect:
- Incoming solar radiation warms the surface; terrestrial infrared radiation emitted back into space is absorbed and re-radiated by greenhouse gases.
- Operates naturally over millions of years to maintain planetary temperatures capable of sustaining life.
- Primary constituents: Water vapor ( $H_2O$ , contributing $95\%$ of natural greenhouse heat retention), carbon dioxide ( $CO_2$ ), methane ( $CH_4$ ), and nitrous oxide ( $N_2O$ ).
Enhanced Greenhouse Effect:
- Anthropogenic warming mechanism triggered since the Industrial Revolution, primarily from burning fossil fuels (coal, oil, natural gas), deforestation, and industrial cement production.
- Driven predominantly by excess anthropogenic $CO_2$ , amplifying thermal entrapment annually.
Weather vs. Climate:
- Weather: Short-term, localized atmospheric variations (e.g., a winter morning frost of $-2\,^\circ\text{C}$ in Ballarat or a five-day heatwave surpassing $40\,^\circ\text{C}$ in Adelaide).
- Climate: Long-term statistical averages and patterns across decades or centuries (e.g., high average annual rainfall of $11{,}000\,\text{mm}$ in Cherrapunji or an overall historical extreme of $-89.2\,^\circ\text{C}$ at Vostok Station, Antarctica).
- Drivers of climate variations include solar radiation intensity, the axial tilt of Earth, differential land/water distribution, and mountain topography.
Climate Variations & Empirical Evidence:
- Glacial Periods: Prolonged colder epochs characterized by extensive ice sheet coverage.
- Interglacial Periods: Extended phases of global warming resulting in contracted ice mass.
- Observable Indicators of Climate Change: Measured global mean temperature increase (approximately $1.1\,^\circ\text{C}$ rise since pre-industrial times), worldwide glacial retreat, elevated sea levels, polar ice core samples, and ancient pollen records.
- Mitigation Measures: Transitioning to public transportation, active commuting (cycling, walking), and energy conservation (efficient appliance use) to curb fossil fuel demand.