Earth's temperature depends on radiation balance
Definition
Radiation balance
The comparison between the average power a body absorbs and the average power it emits, which determines whether its temperature rises, falls or stays constant.
- The Sun supplies Earth mainly with short-wavelength radiation, including visible light and some ultraviolet and infrared radiation.
- Some incoming radiation is reflected by clouds, the atmosphere, ice and light-coloured surfaces.
- The remaining radiation is absorbed by the atmosphere, land and oceans, increasing their internal energy.
- Earth emits energy to space mainly as long-wavelength infrared radiation.
- Earth warms when absorbed solar power exceeds emitted infrared power and cools when emitted power exceeds absorbed power.
- A stable average temperature requires the average incoming absorbed power to equal the average outgoing emitted power.
The atmosphere changes outgoing radiation
Definition
Greenhouse effect
The warming caused when atmospheric gases absorb infrared radiation emitted by the Earth and re-emit it in all directions, including back towards the surface.
- Greenhouse gases such as carbon dioxide, methane and water vapour allow much incoming visible radiation to pass through the atmosphere.
- They absorb some outgoing infrared radiation at particular wavelengths.
- They re-emit infrared radiation in random directions, including back towards Earth's surface.
- This reduces the net rate at which energy escapes to space, so the surface and lower atmosphere warm until outgoing power again balances absorbed incoming power.
- An increase in greenhouse-gas concentration strengthens this effect, although clouds, oceans, ice cover and surface reflectivity also affect Earth's energy balance.
Practical
Surface finish and thermal radiation
- Aim: investigate how surface finish affects the rate of infrared emission using otherwise identical containers.
- Apparatus: identical metal containers with matt black, white and shiny surfaces, equal volumes of hot water, lids, temperature probes or thermometers, measuring cylinder, stopwatch, heatproof mat and data logger if available.
- Independent variable: surface finish.
- Dependent variable: temperature decrease in a fixed time or the gradient of a temperature-time graph.
- Control variables: container size and material, water volume, starting temperature, lid, probe depth, position, timing and room conditions.
- Method:
- Place the identical containers in the same room position and add the same measured volume of hot water to each.
- Adjust the water so every container begins at the same temperature, fit identical lids and place each probe at the same depth without touching the container.
- Start timing together and record every temperature at fixed intervals until a clear fall has occurred.
- Repeat the investigation, calculate mean readings and plot temperature against time for each surface.
- Compare gradients over the same temperature range because radiation rate depends on the temperature difference from the surroundings.
- Expected pattern: a matt black surface is the best emitter and cools fastest, while a shiny light surface is a poor emitter and cools more slowly.
- Uncertainty and improvements: use simultaneous probes, identical containers and lids, equal starting temperatures, fixed probe positions, repeated trials and means; shield the apparatus from draughts without covering the tested surfaces.
- Safety: use a heatproof mat, keep containers away from bench edges, handle hot water carefully and keep electrical equipment dry.
Example
Interpreting cooling curves
- At the same temperature above room temperature, the matt black container has a steeper negative gradient than the shiny container.
- The steeper gradient means a greater rate of temperature decrease and therefore a greater rate of energy transfer.
- Because all other variables are controlled, the difference supports the conclusion that the matt black surface is the better infrared emitter.
Exam technique
Explain a change in Earth temperature
- Identify the change in absorbed or emitted power.
- State whether Earth gains or loses energy overall.
- Link the energy change to a rise or fall in average temperature until a new balance is reached.
Common Mistake
Do not describe greenhouse gases as a solid blanket or say they reflect all infrared radiation; they absorb selected wavelengths and re-emit radiation in all directions.
Self review
- What determines whether Earth's average temperature rises or falls?
- Which radiation does Earth mainly emit to space?
- How do greenhouse gases reduce the net energy loss to space?
- Which surface is usually the best infrared emitter?
- Why must cooling curves be compared over the same temperature range?
