Reducing incoming energy by a small fraction
Since the pre-industrial era, Earth has warmed by about 1.2 C. Greenhouse gas radiative forcing is near 3.1 W/m2, while Earth absorbs roughly 238 W/m2 after albedo. SRM aims to offset part of that imbalance by reducing incoming solar radiation.
Deflect photons before they heat the surface
Solar Radiation Management covers methods that intercept or reflect a small portion of sunlight before or during atmospheric entry. The target scale is only about 1 to 2 percent, but the engineering and governance implications are planetary.
Space-based methods
Reflective structures in low Earth orbit or at Lagrange point L1 intercept sunlight before it reaches the atmosphere.
Atmospheric methods
Aerosols or brightened clouds increase atmospheric albedo and scatter part of the incoming solar flux.
Surface methods
White roofs, reflective roads, high-albedo crops and other land treatments raise surface reflectivity, mostly locally.
Orbital reflectors and an L1 sunshade
The source study compares a LEO reflector constellation targeted at the tropics with a single enormous solar screen at the Sun-Earth L1 point.
Solution 1: LEO reflector satellites
Reflector satellites in low Earth orbit would shade the tropical zone between latitudes +/-23.5 degrees, where insolation is strongest.
~2.17M km2 shadow area~21,700 satellites at 100 km2 each~$130B initial estimateAdvantages
Reversible, geographically targetable, no chemical impact on the atmosphere, and based on familiar solar panel/reflection technologies.
Challenges
Huge satellite counts, orbital congestion, debris risk, difficult maintenance and very high up-front cost.
Solution 2: L1 solar sunshade
A sunshade at L1, roughly 1.5 million km from Earth, intercepts sunlight before the last 1 percent of the Sun-Earth distance.
~3.4M km2 screen~1,040 km radius disk~3.4M tonnes at 1 g/m2Advantages
Global coverage from one point, no Earth-orbit debris, reversible operation, and more uniform distribution.
Challenges
Colossal area, mass delivery to L1, station keeping, remote maintenance and multi-trillion-dollar scale.
Lower-cost methods with harder side effects
Atmospheric options can be much cheaper and faster than space infrastructure, but they introduce chemical, regional and governance risks.
Stratospheric Aerosol Injection
Inject SO2 at 20-25 km altitude to form sulfate aerosols, modeled partly on the Pinatubo cooling effect. Estimate: 5-10 Mt SO2 per year to offset about 1 C, at roughly $2-8B per year.
Marine Cloud Brightening
Spray seawater droplets into low marine clouds to increase droplet concentration and cloud reflectivity. Potentially lower cost, but weather-dependent and regionally uneven.
Surface Albedo Enhancement
White roofs, reflective roads and crop albedo changes offer local heat benefits but weak global cooling, likely below 0.1 C planet-wide.
Multi-criteria view
No method wins on all dimensions. The more deployable options carry more environmental and political uncertainty; the cleaner space options are slower and vastly more expensive.
| Method | Effectiveness | Cost | Feasibility | Timeframe | Reversibility |
|---|---|---|---|---|---|
| LEO satellites | Moderate, tropical targeting | ~$130B initial + maintenance | Difficult, millions or tens of thousands of units | 20-50 years | Good |
| L1 sunshade | High, global coverage | ~$1,000-5,000B | Very difficult | 50-100 years | Good |
| Stratospheric aerosols | High, 1-2 C possible | $2-8B per year | Immediate | 1-2 years | Medium, 1-3 years |
| Marine clouds | Moderate, regional | ~$0.5B per year | Feasible | 5-10 years | Immediate |
| Surface albedo | Low, below 0.1 C | Variable | Easy | Gradual | Medium |
The technical problem is not the only problem
Solar geoengineering can mask temperature but cannot remove carbon dioxide, stop ocean acidification, or decide who gets to set the planetary thermostat.
Termination shock
If a large SRM program stops abruptly, masked warming can appear within years, faster than many ecosystems and societies can adapt.
Uneven regional impacts
Modeling suggests changes to monsoons and precipitation patterns. Some regions may benefit while others face drought or flood risk.
Ozone and chemistry
Sulfate aerosols can accelerate ozone depletion, especially at the poles, while CO2-driven ocean acidification continues.
Free-driver governance
Some SRM approaches are cheap enough that one state, company or wealthy actor could attempt unilateral deployment.
Experiments and proposals already exist
The field is not purely speculative: research programs, cancelled tests and controversial startups already reveal the governance difficulty.
SPICE, United Kingdom
A 2010-2012 stratospheric particle injection test concept cancelled over governance and patent concerns.
SCoPEx, Harvard
A proposed micro-scale stratospheric particle release suspended after opposition from local groups and scientists.
Space Mirrors, R. Angel
A 2006 L1 swarm concept using trillions of small discs to diffuse sunlight, foundational but far from deployable.
Marine Cloud Brightening
Research in the United States and Australia tests seawater spraying concepts, including coral reef protection trials.
Parker Solar Probe
Not geoengineering, but relevant for solar navigation, heat shields and extreme-space materials.
Make Sunsets
A controversial commercial balloon release effort illustrating the free-driver governance problem.
Proceed only as a complement, never a substitute
The source study frames SRM as a risk-managed complement to emissions cuts and adaptation, not a replacement for decarbonization.
Research before deployment
Advance observation, modeling, governance treaties and small-scale reversible tests before operational systems.
Prefer gradual cooling
Any intervention should avoid abrupt changes, with monitoring and off-ramps designed from the start.
Continue CO2 reduction
SRM does not solve carbon concentration or ocean acidification, so emissions cuts remain central.
Build international consent
Planetary-scale radiative control requires transparent, enforceable, equitable governance before any real deployment.