Earth Systems and the Geosphere
The big picture
Earth & Environmental Science treats Earth as a set of interacting reservoirs: the geosphere (solid Earth, crust to core), atmosphere, hydrosphere (including cryosphere), and biosphere. Energy from the Sun and internal heat from radioactive decay and primordial heat drive all transfers. Matter cycles; energy flows one way and is degraded as heat.
Concepts you must command
- Reservoir, flux, residence time. Residence time = reservoir size ÷ flux in (or out) at steady state. A large reservoir with a small flux has a long residence time.
- Feedback. Negative feedback damps change (e.g. silicate weathering consuming CO₂ as temperature rises); positive feedback amplifies it (ice–albedo feedback).
- Open vs closed systems. Earth as a whole is essentially closed to matter but open to energy; most subsystems are open.
- Scale and deep time. Geological processes span milliseconds (seismic waves) to billions of years (mantle convection).
Traps in questions
- Confusing reservoir size with flux rate: the ocean holds far more carbon than the atmosphere, but atmospheric turnover is much faster.
- Calling a feedback positive because it is 'good'. Positive means amplifying, not beneficial.
- Forgetting that the geosphere can be a source or sink depending on the timescale (volcanic CO₂ outgassing vs weathering uptake).
- Mixing energy (flows, conserved) with matter (cycles, conserved).
Worked example
If the atmosphere contains about 800 Gt of carbon and the land-plus-ocean sink removes about 4 Gt per year, the residence time is 800 ÷ 4 = 200 years. Note this is a bulk average; individual CO₂ molecules exchange far faster. Use the rule: residence time = stock ÷ throughput, and always state the units.
Rule of thumb
When asked to explain a change, name the reservoir, the flux altered, the direction of the feedback, and the timescale. Four-part answers score full marks.