Climate Science

Climate Science & Sunlight Reflection

The Earth’s energy system has a well understood set of energy interactions. About 30% of sunlight is reflected off of atmospheric particles, clouds and bright surfaces back out into space. Greenhouse gases trap heat and re-radiate it back toward the Earth. The increased concentration of greenhouse gases results in an energy imbalance of about 0.5%, leading to a steady (or a sudden, tipping-point-fueled) warming the Earth. (IPCC)

The Earth’s temperature is determined by this equation. Until emissions are reduced to zero, and enough carbon dioxide is removed, reflecting sunlight could stabilize the climate.

Reflecting more sunlight would cool the earth.

In addition to natural reflection, several engineering concepts are being explored to reflect more sunlight. For almost as long as scientists have known about the greenhouse effect, some have proposed methods to reflect sunlight.

Learn more about sunlight reflection & climate science:

SRM360

Intergovernmental Panel on Climate Change to Include SRM in Next Assessment Report

Reflective

How do we model sunlight reflection?

Environmental Defense Fund

Would a sunshade ‘solve’ global warming?

A sunshade is not a substitute for reducing carbon dioxide (CO2) emissions and removing CO2 from the atmosphere.

Ocean acidification: The ocean absorbs approximately 25%–30% of anthropogenic CO2 emissions. This CO2 reacts with seawater to form carbonic acid, which subsequently lowers the pH and reduces the availability of carbonate ions—a process known as ocean acidification (Fabry et al., 2008). Under high-emission scenarios, the resulting undersaturation of calcium carbonate can lead to the dissolution of shells and skeletons in ‘calcifying’ organisms, such as pteropods and corals, which form the critical base of marine food webs (IPCC, 2014; Kroeker et al., 2013)

Precipitation: If the world was cooled by shading but CO2 levels remained high, the atmosphere would have a smaller moisture carrying capacity, resulting in an overall decrease in global precipitation. (Kravitz, 2013; Bala, 2008; Tilnes, 2013)

Would a uniform reduction in sunlight produce uniform cooling? 

A uniform reduction in incoming sunlight would result in cooler tropical latitudes but relatively less cooling at higher latitudes. (Kravitz, 2021)

A sunshade could be constructed to ‘improve’ on uniform sunlight reflection, providing more shade during the summers in the each hemisphere.

How do we know what we know about the climate?

Thousands of scientists studying the atmosphere, oceans, biological processes, ice caps and more have created a comprehensive understanding of the climate. Climate models predict weather trends and long-term climate changes. For the past couple decades, scientists studying sunlight reflection have compared the results of different models to arrive at some general conclusions about how it would work. (Visioni, 2025)

Could rocket launches impact the atmosphere?

Yes. The scale of terrestrial launch operations required to create a sunshade would generate significant greenhouse gas effects and ozone damage, if all the materials for the sunshade came from Earth. (Dallas, 2020) More research is necessary to understand these effects, not just for the sunshade but for all significant space launch activity. Constructing a sunshade using common materials found in space, such as aluminum and silicon from the moon, would drastically reduce the number of launches required. Learn more: PSI’s literature review of launch and re-entry emissions.

Physics

Where would a sunshade be located?

A planetary sunshade would be located near the Sun Earth Lagrange 1 point (SEL1). This region of space is relatively gravitationally stable. Objects here orbit the sun with the same period as the Earth, meaning they reflect sunlight from the earth 24-7.

The force exerted by solar radiation pressure, the force of photons transferring their momentum into objects they hit, pushes objects away from the sun. This is particularly true for large surfaces pointed at the sun, like a sunshade. So the actual location of a potential sunshade would be sunwards of SEL1 — around 2.36 million km from Earth. (McInness 2010)

How big would a sunshade be?

Reflecting 1% of sunlight would cool the Earth by ~1ºC (Visioni, 2021). At SEL1’, a total of about 3 million km2 of shade area would be required to reflect 1% of sunlight. If the world chose to proceed with a sunshade project, the actual size would depend on the shading required by the decision-makers. Reflecting more or less sunlight would provide more or less of a cooling effect.

How would a sunshade maintain position?

The SEL1’ location is not perfectly stable. Without slight adjustments, known as station-keeping, objects will drift away. All sunshade concepts need to account for this, and most credible proposals do so with solar sailing concepts.

Using the force of solar radiation pressure, shade craft can adjust the angle of their surfaces, which changes their thrust vector. The field of solar sailing has demonstrated the concept.

Could we put a sunshade in low earth orbit?

Short answer: no. The advantage of the SEL1’ region is that objects produce constant shade on Earth. Any earth orbit, whether geostationary, medium Earth orbit, or low Earth orbit, only casts intermittent shade on the Earth for a fraction of the time. This much mass in low Earth orbit would vastly increase the risk of the Kessler Syndome, where more space debris impacts more satellites, creating an escalating cascade of destruction which could render low Earth orbit unusable for generations.

Medium Earth Orbit, or Geostationary Orbits are similarly not feasible. The higher an orbit, the less effective the mass/area of a sunshade is for shading, meaning that more material is needed for the same result. Geostationary orbit is stationary over a point on the Earth, but not stationary with regard to the sun. A sunshade in geostationary orbit would shade the Earth a tiny fraction of its orbital period.

What sort of shade would a sunshade cast?

A sunshade would cast even, diffuse shade across the Earth. At ~2.36 million km, the ‘umbra’ or dark region of shadow would extend a tiny fraction of the distance to earth, while the cumulative penumbra of the shade constellation would create very even dimming. Some macro-level gradations of shading are possible if the spacecraft move above or below the Sun-Earth line.

Could a sunshade be used to increase sunlight on some parts of the Earth?

No, a sunshade at SEL1’ could not increase the sunlight on Earth.

Could a sunshade be used to shade some regions more than others?

Some, to a limited degree. A uniform reduction in incoming sunlight would cool tropical latitudes more than higher latitudes. (Kravitz, 2021) Because of this result, research has explored how a sunshade could address this. (Sanchez – McInnes, 2015)

A sunshade positioned 2.36 million km sunward casts a penumbra whose width is governed by the Sun’s distance, and it’s size — roughly 9.3 milliradians. The width of the penumbra at Earth distance is about 22,000 km. Earth itself has a diameter of 12,700 km. A shade cone is therefore nearly twice as wide as the planet it is cooling.

By tilting or displacing the constellation slightly off the Sun–Earth line, the shadow footprint can be steered — delivering more cooling very general regions — the tropics, the Arctic, or the Antarctic. The boundary of the penumbra is not sharp: limb darkening means the Sun’s edge contributes less irradiance to begin with, so the shade diffuses gradually across the outermost few thousand kilometres of the shadow, producing a smooth intensity gradient rather than a hard cutoff.


Could a sunshade impact photosynthesis and other natural cycles?

Yes, but it’s complicated. Unmitigated climate change also poses serious risks to plant growth. Plant growth depends on a variety of factors, including temperature, sunlight, water availability, CO2 availability and more. A sunshade would impact some of these variables in ways that promote photosynthesis, and others that inhibit it. A 1% reduction in sunlight is far less variability than seasonal weather trends or the 11 year solar cycle. More research is needed, but we have a strong scientific foundation to start to understand the scale of this dynamic. (Samuels-Crow, 2025)

How would a sunshade impact solar energy production?

Similar to photosynthesis but simpler, there may be some impacts on the order of 1%. More research is needed to compare the climate tradeoffs between stabilizing temperatures and maximizing clean energy production.


The Planetary Sunshade Institute is a U.S.-based, registered 501(c)3 nonprofit organization.
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