Solar Energy: Importance and Controls on Solar Radiation Reaching Earth’s Surface

Solar energy is important because it provides a large, essentially inexhaustible energy inflow to Earth and can be harnessed to produce electricity and heat with substantially lower climate and health impacts than fossil fuels when deployed at scale. The IPCC finds that, if implemented properly, renewable energy sources—including direct solar energy—can reduce negative environmental and health impacts and contribute to sustainable energy supply and climate-change mitigation.

Solar radiation reaching the ground is the physical “input variable” that determines how much solar power (e.g., for photovoltaics) can be generated. The irradiance at Earth’s surface is shaped by (i) solar geometry (sun angle and distance), (ii) atmospheric attenuation (absorption and scattering by gases, aerosols, and clouds), and (iii) local effects such as surface reflection and the sky distribution of direct vs. diffuse radiation.3

Key learning terms in this section: Solar irradiance , Irradiance , GHI , DNI , DHI.

1) Why solar energy matters

Solar energy is a primary renewable resource because the Sun supplies a continuous energy flux to Earth. From a climate-mitigation perspective, the IPCC emphasizes renewable energy’s potential to reduce negative environmental and human-health impacts and to support sustainable development when deployed appropriately.

From an energy-system perspective, solar power contributes to decarbonization pathways by displacing fossil generation and by enabling distributed generation (e.g., rooftop PV), which can improve resilience and energy access in many settings (with system design depending on grid and storage needs). The IPCC SRREN assesses renewable energy sources’ role in climate mitigation and energy access/secure supply.

2) The goal: quantify “how much solar gets to the ground”

Solar radiation reaching the surface is typically expressed using irradiance components measured or modeled for solar-energy applications. A common reference is GHI, which can be decomposed as direct plus diffuse contributions: $\text{GHI}=\text{DHI}+\text{DNI}\cos(z)$ where $z$ is the Solar zenith angle.

Atmospheric effects and geometry determine how much of the top-of-atmosphere solar signal survives to become ground irradiance. NASA highlights that variable absorption by atmospheric gases and scattering/absorption by clouds and aerosols attenuate the signal transmitted through the atmosphere, motivating realistic solar-irradiance measurement and modeling.

Key learning terms in this section: Top-of-atmosphere irradiance , Solar zenith angle , Direct beam , Diffuse radiation.

3) Factors that influence solar radiation at Earth’s surface

3.1 Solar geometry (where and when the Sun is)

1. Solar zenith angle ($z$): Direct irradiance onto a surface depends strongly on the solar zenith angle because it changes the effective optical path length and projection factor. The decomposition into GHI uses $\cos(z)$ for the direct term.
2. Season and latitude: As Earth orbits, $z$ changes throughout the year; therefore, the same atmospheric conditions can yield different surface irradiance across seasons and latitudes (a geometry effect layered with atmospheric variability). Standard irradiance references and models include geometry dependence.

3.2 Earth–Sun distance and solar “input” variability

NASA explains that accurately measuring the Sun’s energy input at TOA is challenging because atmospheric gases and clouds/aerosols attenuate the signal transmitted through the atmosphere. In surface modeling, geometry and TOA variability are treated so that the remaining variability at the ground can be attributed to atmospheric conditions.

3.3 Atmospheric attenuation: absorption and scattering

For photovoltaic-relevant radiation, atmospheric processes are central. PVEducation summarizes the major impacts on solar radiation at Earth’s surface as: reduction due to absorption/scattering/reflection, changes in spectral content, introduction of a diffuse component, and local variations due to water vapor, clouds, and pollution.

Key mechanisms include:

• Rayleigh scattering (molecular scattering): Strongly wavelength-dependent; it contributes to the blue sky and redirects light, affecting the direct vs. diffuse balance.
• Gas absorption (e.g., ozone and water vapor): NASA’s atmospheric modeling references show that extinction arises from absorption and scattering, with distinct roles for ozone and water vapor across spectral regions.
• Aerosols and particulates: Aerosols both absorb and scatter radiation. Research on aerosol direct radiative effects reports that aerosols can decrease downwelling and absorbed shortwave radiation at the surface, with large impacts in polluted and desert regions.

3.4 Clouds: reduce direct beam; increase diffuse fraction

Clouds can substantially modify surface irradiance by blocking the direct beam while simultaneously enhancing diffuse radiation. Because clouds introduce a sky-wide scattered component, the measured outcome depends not just on total cloud amount but also on cloud optical thickness and type—affecting how the direct and diffuse fractions evolve. (This is why solar resource datasets typically track both DNI and DHI rather than a single number.)2