Surface energy balance and albedo for QCE Geography Unit 3

What this dot point is asking

QCAA wants you to explain the biophysical mechanism by which land cover affects climate even before carbon is considered: the surface energy balance. Different land covers reflect, absorb and release the sun's energy differently, and they move different amounts of water and heat into the air. When you transform the cover, you change that balance and therefore the local and regional climate. The command word "explain" means you connect a cover change to a change in albedo or evapotranspiration to a temperature or moisture outcome. Strong answers use the terms albedo, evapotranspiration, sensible heat and latent heat correctly and apply them to real transformations.

The answer

The components of the surface energy balance

Incoming solar (shortwave) radiation hits the surface. Some is reflected straight back; the rest is absorbed and re-emitted. The energy that is absorbed leaves the surface in three main ways: as reflected and emitted radiation, as sensible heat (warming the air directly, felt as temperature) and as latent heat (energy used to evaporate water, which cools the surface). The split between these flows depends almost entirely on the land cover.

Albedo

Albedo is the proportion of incoming solar radiation a surface reflects, from 0 (perfect absorber) to 1 (perfect reflector). Fresh snow and ice have very high albedo and reflect most sunlight; forests have low albedo and absorb most of it; bare soil, crops and deserts sit in between. Changing land cover changes albedo. Clearing dark forest for pale cropland raises albedo, reflecting more energy. Melting bright snow to expose dark ground or ocean lowers albedo, absorbing more. Albedo is why the effect of deforestation on local temperature is not simple: less absorbed solar energy can offset some warming, but the loss of evaporative cooling usually dominates in the tropics.

Evapotranspiration and latent heat

Vegetation moves large volumes of water from the soil through leaves into the atmosphere by transpiration, and evaporates intercepted water from leaf and soil surfaces. Together this is evapotranspiration. Evaporating water absorbs energy as latent heat, which cools the surface much as sweating cools skin. Dense forest with deep roots and large leaf area transpires heavily and runs cool and humid. Removing that vegetation cuts evapotranspiration, so more of the sun's energy becomes sensible heat, the surface and air warm, and the local climate dries.

Deforestation and the energy balance

When tropical forest is cleared, evapotranspiration falls sharply, latent heat cooling is lost, and surface temperatures rise even though albedo also rises. Reduced transpiration means less moisture cycled into the air, which can reduce downwind rainfall, a process linking Amazon clearing to drier regional conditions. The energy balance effect is strongest locally and regionally, while the carbon effect is global.

Urbanisation and the urban heat island

Urbanisation is the most extreme land cover change for the energy balance. Vegetation and soil are replaced by dark, dry, sealed surfaces such as asphalt and concrete that absorb solar energy by day and release sensible heat slowly at night. With little vegetation, evaporative cooling almost disappears. The result is the urban heat island, where built-up areas run several degrees warmer than surrounding countryside, especially at night. Western Sydney summers and the dense inner suburbs of Brisbane show this clearly, and it raises energy demand, health risk and discomfort.

Examples in context

Example 1. Amazon deforestation. Clearing rainforest for pasture cuts evapotranspiration, raises surface temperature and reduces moisture recycled into the atmosphere, contributing to a drier regional climate even as albedo rises.

Example 2. Western Sydney urban heat island. Replacing market gardens and woodland with dark roofs, roads and car parks removes evaporative cooling and stores heat, so summer temperatures run well above the cooler, greener coast.

Example 3. Snow and ice albedo. Where warming melts bright snow to expose dark ground or water, albedo falls and more energy is absorbed, a positive feedback that accelerates further melting.