The heat budget of the earth

At the top boundary we compare incoming solar and outgoing radiation for the whole Earth system. In exams distinguish surface budget from top of atmosphere budget.

Higher albedo sends more solar energy back so less energy is absorbed. In exams keep albedo opposite to absorption.

Greenhouse gases trap longwave energy so the surface can be warmer than effective radiating temperature. Link this difference with greenhouse effect.

This term gives globally averaged absorbed energy after considering solar constant and albedo. In exams treat (1-\alpha) as absorbed fraction.

Earth intercepts sunlight as a disk but energy is averaged over the whole spherical surface. Remember the geometric averaging reason.

Radiative forcing is an external push to the energy balance. In exams treat it as the start of energy budget imbalance.

Positive forcing tends to increase net energy in the Earth system. In exams link positive with warming.

Higher reflection gives less energy to the Earth system. In exams link negative forcing with cooling effect.

Greenhouse gases can absorb more longwave energy and reduce direct outgoing. In exams understand the initial imbalance.

After warming outgoing energy can rise toward a new balance. In exams treat balance as a dynamic process.

Warm air can hold more water vapour and water vapour is a greenhouse gas. In exams treat it as positive feedback.

Loss of ice can increase absorption and strengthen warming. In exams remember albedo in polar amplification.

When ice decreases darker surface absorbs more energy. In exams link polar amplification with feedback.

Clouds modify both shortwave and longwave energy. In exams understand cloud effect according to context.

High clouds can sometimes show warming through longwave trapping. In exams note height and type.

The direct answer is A: low, thick clouds mainly cool the daytime surface through high shortwave reflection. Sunlight reaching Earth is mainly shortwave radiation. Thick low clouds reflect a large part of this incoming sunlight back to space, so less solar energy reaches the ground and the daytime surface warms less. The reasoning is: sunlight arrives, thick clouds reflect much of it, absorbed energy at the surface decreases, and the cooling effect becomes strong. Option A is correct because it describes high reflection of incoming shortwave radiation, also related to high cloud albedo. Option B is wrong because clouds are not completely transparent to longwave radiation; they absorb and emit infrared radiation. Option C is wrong because clouds do not create solar energy; the Sun produces it. Option D is wrong because reflection reduces heating but does not make surface temperature zero. Memory cue: thick low clouds usually block and reflect daytime sunlight, so think “high albedo, daytime cooling.”

Aerosols directly change the path and amount of radiation. In exams understand direct aerosol effect as radiation modification.

Aerosols can alter cloud properties and affect reflection. In exams link indirect effect with cloud changes.

Oceans store large energy and change temperature slowly. In exams treat oceans as climate heat reservoirs.

Oceans can store part of the extra energy within them. In exams check where the energy is going.

All extra energy does not immediately raise air temperature. Include energy storage and phase change in exams.

Latent heat transports energy with the water cycle. In exams treat it as part of atmospheric heat transport.

Evaporation stores energy in water vapour and can move it upward and away. Link humid tropics with latent heat in exams.

Energy partitioning changes both surface temperature and atmospheric energy flow. Keep both separate for temperature response.

With little moisture evaporation decreases and energy raises temperature. Link dry surface with sensible heat in exams.