The Sun delivers an enormous amount of energy to Earth every second, but our planet doesn’t just soak it all up. It reflects some, absorbs some, and sends a portion back out to space as heat. This continuous exchange – between incoming solar energy and outgoing thermal radiation – is what scientists call Earth’s radiation budget. When this budget is balanced, temperatures remain stable. When it’s not, climate changes. Understanding how this balance works, and what can tip it, is fundamental to understanding climate science.
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Solar radiation and albedo
At the top of Earth’s atmosphere, the Sun delivers roughly 340 watts of energy per square meter when averaged across the entire planet’s surface. This incoming energy – known as solar or shortwave radiation – spans ultraviolet, visible, and near-infrared wavelengths. But not all of it reaches the ground.
A significant fraction is reflected back to space before it ever heats anything. The measure of how much energy a surface reflects is called albedo, expressed on a scale from 0 (total absorption) to 1 (total reflection). Earth’s overall albedo currently sits between 29% and 30%, meaning roughly a third of incoming sunlight is reflected away without contributing to warming.
But different surfaces behave very differently. Forests have a low albedo of around 0.15, and open ocean is even lower at 0.05-0.10, absorbing most of the solar energy that strikes them. Snow and ice, by contrast, reflect 80-90% of incoming radiation. This difference has major implications: when ice melts due to warming, the darker land or ocean beneath it is exposed, absorbing more solar energy and driving further warming. This self-reinforcing cycle is known as the ice-albedo feedback.
Of the solar radiation that isn’t reflected, about 70% is absorbed – partly by the atmosphere and partly by Earth’s surface – and this absorbed energy ultimately drives weather systems, ocean circulation, and life itself.
Terrestrial emission and the greenhouse effect
Once Earth’s surface absorbs solar energy, it doesn’t just hold onto it. It re-emits that energy as longwave infrared radiation – essentially heat – back toward space. This outgoing radiation is what balances the incoming solar energy and keeps the planet from continuously warming.
Without an atmosphere, Earth’s average temperature would be approximately −18°C – well below freezing across most of the planet. The reason it’s actually around +15°C is the natural greenhouse effect. Certain gases in the atmosphere – primarily water vapor, carbon dioxide (CO₂), methane (CH₄), and nitrous oxide – are largely transparent to incoming visible sunlight but absorb outgoing infrared radiation. They then re-emit that heat in all directions, including back down toward the surface.
Increasing concentrations of greenhouse gases such as CO₂ and methane restrict the outward passage of emitted radiation, effectively thickening the atmospheric “blanket” and causing the lower atmosphere and surface to warm. This is the mechanism behind human-driven climate change: not a weakening of the Sun, but a reduction in how efficiently Earth can shed its heat.
Outgoing longwave radiation (OLR) is a critical parameter for monitoring the greenhouse effect, and it is tracked globally using satellite instruments like NASA’s CERES (Clouds and the Earth’s Radiant Energy System) aboard the Terra and Aqua satellites.
Net radiation and climate balance
Earth’s radiation budget comes down to a straightforward accounting: absorbed solar radiation (ASR) on one side, and outgoing longwave radiation (OLR) on the other. When these two are in balance, Earth’s climate remains stable. When they are not, the planet either warms or cools.
Right now, Earth is not in balance. Data from NASA’s CERES instruments show that Earth’s energy imbalance increased from approximately +0.42 W/m² in 2005 to +1.12 W/m² in 2019, driven by rising greenhouse gas concentrations, declining ice cover, reduced cloud cover in some regions, and increased atmospheric water vapor. This excess energy – more coming in than going out – is accumulating primarily in the oceans and is a key driver of rising global temperatures.
The consequences extend well beyond air temperature. Energy imbalances drive changes in precipitation patterns, intensify storms, accelerate glacier retreat, and raise sea levels. Even a small, sustained imbalance compounds over time into significant climate shifts. History shows the reverse is also possible: large volcanic eruptions can inject reflective particles into the stratosphere, temporarily tipping the budget toward cooling by blocking incoming sunlight for months or even years.
These dynamics play out through climate feedback mechanisms – processes where an initial change triggers further changes that either amplify or dampen the original effect. The ice-albedo feedback is one example. Water vapor is another: as temperatures rise, more water evaporates, and since water vapor is itself a greenhouse gas, it traps additional heat, amplifying warming further.
Role of clouds and aerosols
Clouds and aerosols add significant complexity to the radiation budget. Both can warm or cool the planet, and their net effects depend on their type, location, altitude, and physical properties.
How clouds influence the budget
Clouds affect the radiation budget in two competing ways. On one hand, they are highly reflective – low, thick stratocumulus clouds reflect a large portion of incoming solar radiation back to space, producing a cooling effect. On the other hand, clouds also absorb and re-emit infrared radiation from Earth’s surface, trapping heat like greenhouse gases do – a warming effect that is most pronounced in high, thin cirrus clouds.
Which effect dominates depends on cloud altitude and thickness. High, thin clouds let most sunlight through but trap outgoing heat efficiently, producing net warming. Low, thick clouds block sunlight strongly but emit heat at nearly the same rate as the surface below them, producing net cooling. Averaged across all cloud types globally, clouds currently contribute a net cooling of roughly 13 W/m² – a substantial effect that moderates warming. However, how clouds will respond to a warming climate remains one of the largest sources of uncertainty in climate projections.
How aerosols influence the budget
Aerosols – microscopic particles suspended in the atmosphere including dust, sea salt, volcanic ash, sulfates, and soot – affect the radiation budget both directly and indirectly.
Indirectly, aerosols act as cloud condensation nuclei – the tiny particles around which water droplets form to create clouds. A higher concentration of aerosols can produce more, but smaller, cloud droplets for the same amount of water, increasing cloud reflectivity and causing additional cooling – an effect known as the cloud albedo effect. This indirect influence means that aerosol emissions don’t just affect the atmosphere directly; they reshape the clouds themselves.
Aerosol effects show negative radiative forcing overall, contributing to cooling, but their interaction with clouds is poorly understood and carries the largest uncertainty in future climate projections. The U.S. Department of Energy’s Atmospheric Radiation Measurement (ARM) program is one of several major initiatives actively working to narrow these uncertainties through real-world atmospheric measurements.
Taken together, clouds and aerosols represent powerful, if complex, regulators of Earth’s radiation budget – capable of amplifying or dampening climate change depending on how they evolve in a warming world.
What do you think? As human activities continue to alter atmospheric greenhouse gas concentrations and aerosol emissions simultaneously, how confident can we be in predicting the net effect on Earth’s radiation budget? And if aerosols are currently masking some of the warming from greenhouse gases, what might happen to global temperatures if air pollution is significantly reduced in the coming decades?
References
- https://science.nasa.gov/ems/13_radiationbudget/
- https://nap.nationalacademies.org/read/18988/chapter/4
- https://scied.ucar.edu/learning-zone/how-climate-works/energy-budget
- https://geo.libretexts.org/Courses/Coalinga_College/Introduction_to_Earth_Science_(C-ID:_GEOL_121)/06:_Climate_and_Climate_Change/6.05:_Earth's_Heat_Budget_and_the_Greenhouse_Effect
- https://www.eoportal.org/other-space-activities/earth-radiation-budget
- https://csl.noaa.gov/research/erb/about.html
- https://en.wikipedia.org/wiki/Earth's_energy_budget
- https://www.metlink.org/wp-content/uploads/2020/11/iop_radiation_budget.pdf
- https://science.nasa.gov/earth/earth-observatory/clouds-and-radiation/
- https://www.esa.int/Applications/Observing_the_Earth/FutureEO/EarthCARE/Clouds_and_aerosols_in_the_climate_system
- https://en.wikipedia.org/wiki/Cloud_forcing
- https://www.gfdl.noaa.gov/aerosols-and-climate/
- https://www.nature.com/scitable/knowledge/library/aerosols-and-their-relation-to-global-climate-102215345/
- https://www.eorc.jaxa.jp/EARTHCARE/about/gw_cloud_aerosol_e.html
- https://www.energy.gov/science/doe-explainsclouds-and-aerosols
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