Increasing the reflectance of a surface modifies the Earth’s energy balance and helps offset global warming. The Earth’s energy (im)balance is the result of a complex set of interactions between the Earth and sunlight, known as radiative forcing.

Incoming shortwave solar radiation (i.e., sunlight) may be reflected by the Earth’s surface or absorbed and re-emitted as longwave radiation. Increasing the solar reflectance of a surface in the built environment increases the ability of the Earth to reflect solar radiation and reduces the amount of the sun’s energy that is absorbed and converted to heat. Research over the last 25 years permits a quantitative comparison of the effect of increasing the albedo of the built environment and the impact of GHG mitigation efforts.

The radiative forcing of different GHGs are commonly compared to each other using a scaling factor called global warming potential (GWP). GWP is a ratio of how much energy a GHG will absorb compared to CO2 over a given timeframe. The Earth’s albedo is an important driver of radiative forcing. Raising the Earth’s albedo increases the percentage of sunlight that is reflected by the surface and reduces the amount of sunlight that is absorbed and re-emitted as thermal infrared radiation.

The Earth’s albedo is declining. Higher temperatures reduce the area covered by high-albedo surfaces such as sea ice, glaciers, and snow and increase the area covered by low-albedo water. Further, global urbanization and urban development result in the replacement of natural areas with darker, impermeable built environments.

In the absence of natural radiative forcings and feedbacks (e.g., volcanic eruptions) and anthropogenic radiative forcings and feedbacks (e.g., GHG emissions from vehicles or power plants), the annual total radiative energy leaving the Earth would equal the amount of the sun’s energy arriving on the Earth.

A destabilization of this balance causes global warming, in which the amount of radiation leaving the Earth is currently less than the amount of the sun’s energy arriving. This deficit, or equivalently the excess energy that remains, is called the Earth’s radiative imbalance, measured in flux units of W/m2. The Earth’s system adjusts to this excess energy by warming until the amount of departing energy equals the amount arriving.

Yes, there are many benefits to increasing the reflectance of roof, wall, and pavement surfaces. Increased surface albedo has been shown to reduce air temperatures inside of buildings, making them more comfortable for those inside and may reduce demand for cooling energy if the building is air-conditioned.

Research indicates that, when deployed at scale, higher albedo surfaces can reduce outdoor air temperatures which provides additional benefits by reducing smog formation and lowering electrical power demand. Lowering indoor and outdoor air temperatures, even by a modest amount, can yield substantial benefits to human health and improve the resilience of health, energy and transportation systems.

No. Solar geoengineering tends to include large-scale atmospheric- or space-based interventions such as stratospheric aerosol injection (SAI) and marine cloud brightening (MCB), as well as nascent ideas for both cirrus cloud thinning (CCT) and space-based technologies. It may also include ideas to modify the albedo of large surface areas such as water bodies or agricultural areas.

By contrast, AMBE primarily involves small-scale changes to our built environments, including roads, parking lots, rooftops, and walls, which total <1% of the Earth’s surface. A focus on the albedo of the built environment both reduces the global energy imbalance and provides a range of benefits to the places and people near the intervention, including reduced surface and air temperatures, improved thermal comfort and health outcomes, and reduced energy demand for cooling.