Quick questions on The greenhouse effect and climate: VCE Physics Unit 1 Year 11

Approximately 1370 W m$^{-2}$ at Earth's mean orbital distance. This is the energy flux through a surface perpendicular to the sun at the top of the atmosphere.

Earth intercepts solar power on a cross-section $\pi R^2$ but distributes it over a surface area $4 \pi R^2$. So the average flux at the surface (before atmospheric effects) is $1370 / 4 \approx 342$ W m$^{-2}$.

Earth reflects about 30 percent of incoming solar radiation (clouds, ice, deserts). Albedo $\approx 0.30$. The absorbed fraction is $0.70$, giving about 240 W m$^{-2}$.

In equilibrium, Earth emits as much energy as it absorbs. Use Stefan-Boltzmann ($P/A = \sigma T^4$) to find the effective radiating temperature: $T_{\text{eff}} \approx 255$ K (or $-18$ degrees C).

- Water vapour feedback. Warmer atmosphere holds more water vapour, which is a greenhouse gas. Roughly doubles the direct CO2 forcing. - Ice-albedo feedback. Less sea ice means less reflection of sunlight, more absorption, more warming, more melting.

- Stefan-Boltzmann. Warmer surface emits more radiation ($T^4$ scaling), tending toward equilibrium. - Cloud feedbacks. Mixed; some clouds reflect more sunlight (cooling), others trap more infrared (warming). Sign uncertain.

Photovoltaic (PV) cells convert sunlight directly to electricity (photoelectric effect at semiconductor band gaps). Efficiency 15 to 25 percent for commercial silicon PV.

Kinetic energy of wind converted by turbines. Power $\propto \rho A v^3$ (proportional to cube of wind speed). Wind farms produce 20 to 50 percent of theoretical maximum (Betz limit 59 percent).

Gravitational potential energy of water converted by turbines.

Already discussed. Low CO2 emissions but waste and safety concerns.

Earth's internal heat (largely from radioactive decay in mantle).

Critical for intermittent renewables. Lithium-ion is current dominant technology.

Solar constant (1370 W/m$^2$) is at the top of atmosphere perpendicular to sun. Average surface flux (240 W/m$^2$ after albedo) accounts for the spherical geometry and Earth's reflection.

Earth absorbs only about 70 percent of incoming sunlight.

The natural greenhouse effect keeps Earth warm; the enhanced greenhouse effect (from human emissions) is causing current global warming.