A new study by Hashem Akbari, a professor in the Department of Building, Civil and Environmental Engineering, offers a simplified way of calculating the degree to which surface reflectivity, known as albedo, can offset carbon dioxide (CO₂) emissions. The method, which builds on previous research, makes it easier to calculate these effects at regional and national scales using widely available weather data.
"This method allows policy practitioners to view problems on a regional basis," Akbari says. "It reveals the CO2 equivalency of increasing the albedo of a square metre of surface in a particular location. Surface reflectivity does not remove any CO2 from the atmosphere, but it does cool the Earth as though there were less CO2 in the atmosphere.
"It doesn't solve the problem of climate change, but it buys us time," he adds.
Hashem Akbari: "The beauty of cool surface technology is that we have thousands of years of experience with it, and we do not know of any negative impacts associated with it."Previous method refined
It's long been known that using highly reflective materials for urban surfaces like roofs and pavement can help lower local temperatures and mitigate climate change. Brighter surfaces reflect more solar radiation back into the atmosphere, reducing building energy use and easing the urban "heat island" effect, which makes cities hotter than surrounding areas.
This reflectivity is measured as "albedo," on a scale from zero to one, where zero means no reflection and one means all incoming solar radiation is reflected. Most urban areas fall between 0.1 and 0.3, meaning they absorb between 70 to 90 per cent of the sun's radiation.
A previous paper cowritten by Akbari and professor Damon Matthews and researcher Donny Seto, both in the Department of Geography, Planning and Environment, estimated that raising the albedo of one square metre of surface by 0.01 could offset about 2.5 kilograms of CO₂ emissions.
The new research, published in the journal Urban Climate, refines this idea by accounting for local conditions such as sunlight intensity and cloud cover, allowing for more precise, location-specific estimates.
Global data for local knowledge
To develop the updated model, Akbari analyzed data from roughly 4,400 weather stations worldwide. By combining solar radiation measurements taken at Earth's surface with estimates of incoming solar energy at the top of the atmosphere, the researchers were able to determine how much sunlight is reflected back into space in different regions.
The method allows practitioners to estimate CO₂ equivalency using standard climate and weather datasets rather than complex simulations.
The findings show that the climate benefit of reflective surfaces varies significantly by location. On average, increasing albedo by 0.01 can offset about 1.8 to 2 kilograms of CO₂ per square metre globally, with some regions reaching up to 5 kilograms depending on atmospheric clarity. The effect is strongest in less cloudy regions, particularly around 20 to 30 degrees north and south of the equator, where more solar energy reaches the ground.
No negative impacts
Akbari says this simplified approach could help policymakers and planners better quantify the climate value of "cool roofs" and pavements. By translating reflectivity into CO₂-equivalent reductions, the method can be used in developing carbon accounting systems, climate policies and even carbon markets.
"The beauty of cool surface technology is that we have thousands of years of experience with it, and we do not know of any negative impacts associated with it," Akbari says. "There is very little incremental cost, it improves ambient conditions within the city while it cools the planet, and it puts money in your pocket."
Read the cited paper: "A simplified method to calculate atmospheric CO2 equivalency for changing surface albedo"
