Climate change as a global biophysical process altering atmospheric, hydrospheric, lithospheric and biospheric systems

What this dot point is asking

NESA expects you to treat climate change as a biophysical process operating at the global scale and producing effects in every other process. Strong answers recognise that climate change is not a separate environmental issue; it is the integrating driver of contemporary change across the four spheres.

The driving mechanism

Greenhouse gases (carbon dioxide, methane, nitrous oxide, water vapour, fluorinated gases) absorb outgoing long-wave radiation from Earth's surface and re-emit it. The thicker the greenhouse gas layer, the more heat is retained in the lower atmosphere and oceans.

Atmospheric CO2 was around 280 ppm before the Industrial Revolution. The 2024 annual mean at Mauna Loa was 425 ppm, the highest in at least 3 million years (paleoclimate proxy data). Methane is at 1,932 ppb, around 2.6 times pre-industrial levels.

Global mean surface temperature has risen around 1.3 degrees C since 1850-1900. Australia has warmed faster than the global average, around 1.5 degrees C since 1910. The Bureau of Meteorology State of the Climate (2022) is the standard reference.

Sphere-by-sphere impact in Australia

Atmosphere

• Extreme heat. Heatwaves longer, hotter, more frequent. Penrith recorded 48.9 degrees C on 4 January 2020. Marble Bar's record of 160 consecutive days above 37.8 degrees C (1923-24) is being approached more frequently.
• Fire weather. Forest Fire Danger Index reached "catastrophic" levels in new areas including Canberra and Sydney during 2019-20. CSIRO modelling projects another 20-30 percent increase in dangerous fire days by 2050.
• Rainfall regime shifts. Southern Australia drying (10-20 percent decline in cool-season rainfall since the 1990s). Northern Australia wetter and more variable. Tropical cyclones may become fewer but more intense.

Hydrosphere

• Sea level rise. Global sea level up 22 cm since 1900 and accelerating (3.7 mm/year over 2006-2018). Around Australia, Fort Denison has risen 12 cm since 1914.
• Ocean temperature. Australian marine surface temperatures up 1.0 degrees C since 1900, with hotspots off Tasmania (2.0 degrees C warming) reshaping marine ecosystems.
• Ocean acidification. Surface ocean pH has fallen by 0.1 units globally since pre-industrial. The Great Barrier Reef shows around 11 percent reduction in coral calcification rates compared to 1990.
• Ice loss. Although Australia has no permanent ice sheet, our region's contribution from melting Antarctic land ice is the dominant driver of regional sea level. Antarctic mass loss has accelerated since 2002.

Lithosphere

• Coastal erosion. Higher sea levels and more energetic storms accelerate cliff retreat and beach erosion. NSW Department of Planning data shows around 60 percent of the NSW open coast is in net erosion. Old Bar Beach (NSW) has lost around 80 m since 1980 and properties have been condemned.
• Permafrost thaw. Not Australian, but globally significant (releases methane and CO2 from permafrost soils, a positive feedback).
• Soil moisture and salinity. Drier southern soils combined with rising sea levels are accelerating coastal salinity intrusion in Queensland, NSW, and Victorian estuaries.

Biosphere

• Coral bleaching. Mass bleaching events on the Great Barrier Reef in 1998, 2002, 2016, 2017, 2020, 2022, 2024. 2016 bleached 67 percent of northern reefs; 2024 hit the largest spatial extent on record.
• Fire-driven loss. Black Summer 2019-20 killed or displaced 3 billion vertebrate animals. Up to 35 species had more than 30 percent of habitat burned.
• Species range shifts. Eastern tropical fish species recorded in Tasmania (long-spined sea urchin and others) at southern range extensions of 100-300 km in two decades.
• Phenology shifts. Wattles flowering earlier, migratory bird arrivals shifting, breeding seasons changing.

Why this matters for the four-sphere framework

Climate change is the canonical example of cross-sphere interaction. CO2 in the atmosphere drives ocean warming and acidification (hydrosphere), which bleaches coral (biosphere), which dissolves to alter ocean chemistry (back to hydrosphere), which alters sea-floor sediments (lithosphere), affecting future fish habitat (biosphere).

The HSC question that asks "examine cross-sphere interactions" is essentially asking you to write about climate change with rigour.

Projections to 2100

The IPCC Sixth Assessment Report (AR6, 2021) provides the canonical reference. Under low-emission scenarios (SSP1-2.6), global warming reaches 1.6 degrees C by 2050 and 1.8 degrees C by 2100. Under high-emission (SSP5-8.5), 2.4 degrees C by 2050 and 4.4 degrees C by 2100. Australian projections (CSIRO) follow the global trajectory but with stronger southern Australia drying and more extreme fire weather.

Adaptation requirements: coastal property setbacks (Wollongong, Byron Bay), water security infrastructure (desalination, dam upgrades), fire management investment, agricultural transitions away from heat-vulnerable crops.

Mitigation pathway: Australia's 2022 Climate Change Act locks in 43 percent reduction by 2030 and net zero by 2050. Whether emissions actually fall depends on sectoral policy: the Safeguard Mechanism (2023 reforms), Capacity Investment Scheme for renewable generation, electric vehicle uptake, and land-sector carbon credits.