Reflective surfaces, or ground-based albedo modification (GBAM), is a method used to increase Earth's ability to reflect sunlight. This helps reduce the amount of heat that reaches Earth's surface. The IPCC describes GBAM as actions like whitening rooftops, changing how land is used (such as farming without turning over soil), covering large areas like glaciers or deserts with reflective materials, and altering the reflectiveness of oceans.
The most common example of a reflective surface is a "cool roof." Cool roofs are often white but can also be other light colors and are used on both homes and businesses. In some places, like California, laws encourage painting rooftops with light colors to reflect sunlight.
This method has limits because there is not enough surface area available to cover globally. It can reduce heat by 0.01 to 0.19 W/m² on average worldwide, depending on whether only cities or all areas are treated. This effect is much smaller compared to the heat increase caused by doubling the amount of carbon dioxide in the atmosphere, which adds about 3.7 W/m². While using reflective materials can be low-cost for small projects, it may become expensive when applied widely.
A 2009 report by the Royal Society said that covering 1% of Earth's land surface (about 10 m²) with reflective materials would cost about $300 billion each year. This makes it one of the least effective and most expensive methods studied. However, using reflective surfaces can lower the need for air conditioning, which releases carbon dioxide and worsens global warming.
Method
The IPCC 2018 report stated that reducing global temperatures has a small chance of success. However, scientists strongly agree that temperatures in specific regions could change by 1-3°C. Using reflective surfaces in cities and farms can help reduce the urban heat island effect.
Reflective surfaces can increase how much sunlight is reflected, which is called albedo. If the albedo in cities and farms increases by 0.04–0.1, it might help lower global temperatures by more than 1.0°C.
The reflective surfaces method is similar to a technique called passive daytime radiative cooling (PDRC). Both methods work on the ground, but PDRC focuses on helping Earth release more heat into space instead of only reflecting sunlight.
Types of reflective surfaces
Cool roofs in hot climates can provide both short-term and long-term benefits, including:
- Saving up to 15% of the yearly energy used for air conditioning in single-story buildings
- Helping reduce the urban heat island effect
- Lowering air pollution and greenhouse gas emissions, as well as reducing the warming effect of these emissions
Cool roofs help save energy during hot summers but may increase heating needs in cold winters. Because of this, the overall energy savings from cool roofs depend on the climate. A 2010 study of air-conditioned commercial buildings in the United States found that summer cooling savings usually outweigh winter heating costs, even in cold areas near the Canada–United States border. This results in savings for both electricity use and emissions. However, without proper cleaning and maintenance, the energy savings of cool roofs can decrease over time due to loss of reflectivity and dirt buildup.
A study in London during the 2018 British Isles heatwave showed that if all buildings had cool roofs, heat-related deaths (estimated at 655–920) could have been reduced by 249 (32%). Using a method to value human life, the benefits of avoiding these deaths were estimated to save £615 million.
Research has shown that heat from the sun is a major cause of damage to roof materials. High temperatures and large temperature changes, whether daily or seasonal, harm the lifespan of roof membranes. Reducing these temperature extremes can help protect roof materials. Covering membranes with materials that reflect ultraviolet and infrared light can reduce damage from heat and UV radiation. White surfaces reflect more than half of sunlight, while black surfaces absorb nearly all. Using white or white-coated roofing materials or white gravel on roofs seems best for reducing heat-related damage when roofs are exposed to sunlight.
If all flat roofs in warm areas were painted white, the increased reflectivity would reduce global warming effects equivalent to removing 300 million cars from roads for 20 years. This is because a white roof about 93 square meters (1,000 square feet) can offset 10 tons of carbon dioxide over 20 years. A 2008 study in Spain found that covering a large desert area with greenhouses made of polythene reduced temperatures by 1.6°C (2.9°F) over 20 years compared to nearby regions. Farmers there also whitewash their greenhouses in summer to cool crops.
When sunlight hits a white roof, most of it is reflected back into space. When sunlight hits a dark roof, most of it is absorbed and re-radiated as longer wavelengths, which are trapped by greenhouse gases in the atmosphere. A 2021 study in Malaysia found that white roof tiles significantly lower heat transfer and roof temperatures during the day but do not affect nighttime temperatures, as heat is released to the sky when there is no sunlight.
A 2012 study by Concordia University estimated that using cool roofs and pavements worldwide could create a cooling effect equal to removing up to 150 gigatonnes of carbon dioxide from the atmosphere—enough to remove all cars globally for 50 years.
White thermoplastic roofs (PVC and TPO) are highly reflective, reflecting 80% or more of sunlight and emitting at least 70% of absorbed heat. In contrast, asphalt roofs reflect only 6–26% of sunlight.
In addition to white thermoplastic materials, research is ongoing to develop cool asphalt shingles. These shingles are popular in North America, but consumer preference for darker colors makes creating reflective ones challenging. Asphalt shingles typically reflect only 4–26% of sunlight. Reflective shingles can reduce summer cooling costs and the urban heat island effect, even though they may slightly increase winter heating costs. To meet color preferences while improving reflectivity, materials, coatings, and pigments are being tested. Since only 43% of light is visible, reflectivity can be improved without changing color by increasing reflectance of ultraviolet and infrared light. High surface roughness in asphalt shingles, caused by small granules, also reduces reflectivity. Alternatives like flat rock flakes or dual-coated granules are being explored to improve performance.
Natural white gravel can also be used to create cool roofs and pavements.
Stainless steel roofs have the highest SRI (Solar Reflectance Index) ratings, ranging from 100 to 115. Some are water-repelling, staying clean and maintaining their SRI even in polluted areas.
A roof can be made reflective by applying a solar reflective coating. Information about over 500 reflective coatings is available from the Cool Roofs Rating Council.
Researchers at the Lawrence Berkeley National Laboratory found that a pigment used by ancient Egyptians, called "Egyptian blue," absorbs visible light and emits light in the near-infrared range. This could help keep buildings cool. They also developed fluorescent ruby red coatings with reflective properties similar to white roofs.
Green roofs provide…
Climatic variables
In some areas where heating is needed more than cooling, white reflective roofs might not save energy effectively. This is because the energy saved from using less cooling in summer could be offset by higher heating costs in winter. According to the U.S. Energy Information Administration's 2003 Commercial Buildings Energy Consumption Survey, heating uses 36% of energy in commercial buildings annually, while air conditioning uses only 8%. Energy calculators often show that dark-colored roofs save more energy yearly in cooler climates.
An ideal roof would prevent heat from entering in summer and prevent heat from escaping in winter. To achieve this, it would need a very high SRI (Solar Reflectance Index) to block all heat gain in summer and reduce heat loss in winter. High SRI roofs act as radiant barriers, similar to how a thermos bottle keeps liquids hot or cold. However, high emissivity cool roofs can increase energy use in winter due to heat loss through radiation, which reflective metal roofs like stainless steel do not experience.
Applications
In 2001, the Lawrence Berkeley National Laboratory (LBNL) studied how cool roofs reduce peak energy demand. They compared a black rubber roof on a Texas retail building with a new white vinyl roof. The white roof lowered the roof’s surface temperature by an average of 24 °C (43 °F), reduced air conditioning energy use by 11%, and cut peak energy demand by 14%. The black roof reached 75 °C (167 °F) in summer, but the white roof reached 52 °C (126 °F). Annual energy costs dropped by $7,200 or $0.07 per square foot, without considering taxes or other charges.
Scientists measured weather, building temperatures, and energy use before and after the roof change. The original black roof was replaced with a white vinyl roof that had the same insulation and heating, ventilation, and air conditioning (HVAC) systems.
Although a full year of data was collected, some data was removed because it did not meet study requirements. Only 36 days of data were used before the retrofit and 28 days after.
In 2009, Ashley-McGraw Architects and CDH Energy Corp studied a green roof, a dark EPDM roof, and a white TPO roof on a building in New York. The TPO and green roofs had lower temperatures than the dark EPDM roof. However, the TPO roof used 30% more energy for heating in winter, and the green roof used 23% more energy for heating.
In July 2010, the U.S. Department of Energy (DOE) began using cool roofs on its buildings when it was cost-effective. In October 2013, the DOE ranked cool roofs as 53 out of 100 for energy savings. Cool roofs work best in warm climates but may increase heating costs in colder areas. More insulation reduces the impact of cool roofs. The DOE requires cool roofs on new or replaced roofs when cost-effective.
Energy Star is a program by the U.S. Environmental Protection Agency and DOE to help save energy and reduce pollution. For low-slope roofs, Energy Star requires a solar reflectivity of at least 0.65 when new and 0.50 after weathering. Warranties for reflective roofs must match those of non-reflective roofs.
The Energy Star label only checks the roof’s surface, not the whole roof system, such as insulation or structure. A note on the Energy Star website says building owners should use the DOE’s "Roof Savings Calculator" to estimate energy savings, as results depend on factors like insulation, climate, and building design.
The Cool Roof Rating Council (CRRC) rates roofing products based on solar reflectance and thermal emittance. It lists over 850 products and tests them yearly to ensure accuracy. The CRRC does not set minimum requirements for reflectance or emittance but helps label products correctly.
The Green Globes system, used in the U.S. and Canada, evaluates buildings based on energy use. To earn a rating, roofs must have a solar reflectance of at least 0.65 and thermal emittance of at least 0.90. Up to 10 points can be earned for using vegetation or reflective materials on roofs.
The U.S. Green Building Council’s LEED system sets standards for sustainable buildings. LEED does not certify products but helps choose them. Unlike building codes, LEED standards are updated by members of the U.S. Green Building Council and are not publicly voted on.
Under LEED 2009, to earn a credit for reducing heat islands, at least 75% of a roof must use materials with a solar reflective index (SRI) of at least…
Urban heat island effect
An urban heat island happens when heat-absorbing structures, such as dark asphalt parking lots, roads, and black rooftops, along with little plant life, cause air temperatures to be 1 to 3 °C (1.8 to 5.4 °F) higher than nearby rural areas.
Green building programs encourage using cool roofing to reduce the urban heat island effect and the harmful smog it creates. Light-colored roofs reflect sunlight, which lowers the temperature increase and decreases the need for cooling energy and smog formation. A study by LBNL found that if methods to reduce this effect, including cool roofs, were used widely, the Greater Toronto Area could save more than $11 million each year on energy costs.