Surface Albedo Modification

O​n average, Earth reflects about 30% of the sunlight that reaches it. ​The reflectiveness of a surface​, which scientists call ​“​albedo”, can vary due to several factors,​ such as its colour and texture, with lighter colours and smoother surfaces reflecting more sunlight.​​ 

​​The proportion of sunlight that is reflected by a surface​ is typically about 12% for worn asphalt, ​​6% for the world’s oceans, 8–18% for forests, 40% for deserts,¹ and up to 90% for snow and ice.

​​Some researchers have suggested that using techniques to increase the reflectivity of surfaces – ​ranging from​ cities​ to​ cropland, sea ice, glaciers, and deserts – ​could help to tackle climate change.​^2​ ​​ 

Though surface albedo modification ​can help​ produce local cooling effects​, ​​they likely have ​limited potential to lower global temperatures.​³​ 

Brightening urban surfaces  

Cities are hotter than ​their ​surrounding areas – a phenomenon known as the “urban heat island effect”. Urban surfaces ​​​such as​​ ​roads or roofs tend to be dark, reflecting ​away ​little sunlight. ​This effect warms cities but has a trivial effect on the global temperature​.^4,​5​  

​​Urban albedo modification involves brightening dark city surfaces by ​making buildings, roads, and roofs more reflective​​​.​² For example, many Mediterranean cities have white buildings to help reflect away sunlight during intensely hot summers.​​ 

​​Evidence shows that​​ brightening ​surfaces ​in cities ​could decrease the energy demands associated with cooling buildings in hot cities,​^6​ provide relief ​from heat ​in densely built cities,​^7​ and may help prevent some heat-related deaths.​⁸​ 

​​However, urban albedo enhancement cannot significantly decrease temperatures at a global scale because cities only account for about 1% of Earth’s surface. Replacing all urban paving and roofing materials with more reflective options would ​only ​decrease global temperatures by up to 0.11°C.​^2​ 

​​While urban albedo modification could be effective in some areas​, applying this approach to all cities around the world ​would be​ impractical. Further, research suggests that, in some cases, increasing urban albedo could have harmful impacts on local to regional rainfall. For example, modelling shows that in some regions, decreased urban temperatures could lead to significant reductions in regional rainfall, with associated impacts on water resources and ecosystems.​^9​ 

Brightening crops 

Crops and grasses reflect more sunlight than trees, and certain strains of crop plants are more reflective than others.​¹⁰​ Crop reflectivity enhancement ideas aim to lower temperatures by ​maximising​ crop reflectivity.​^11​ 

​​T​he amount of chlorophyll – the green pigment that allows plants to absorb sunlight and convert it to food – in crop plants varies. Plants with less chlorophyll​ ​generally reflect more sunlight​.^​​12​ Lower amounts of chlorophyll can lead to lower crop yields, but ​research suggests that more reflective crops could be bred without​ compromising crop yields.​¹³​ 

More reflective crops – whether existing​¹⁴​ or bioengineered​¹³​ ​– ​could help ​to ​decrease local temperatures during the growing season.​¹⁰​ ​Modelling​​ ​suggest​s​ this strategy would be especially effective in decreasing the intensity of heatwaves in North America and Eurasia.​¹⁰​ 

Models suggest that crop albedo enhancement could decrease global temperatures by up to 0.23°C.​²​ However, cooling at this scale would require replacing most agricultural crops​ globally​ with more reflective crop types,​²​ which ​would require impractical levels of global cooperation between landowners.​ 

​​Though enhancing the reflectivity of agricultural land would require large-scale interventions for global change,​ research suggests ​there could be significant local and regional cooling with smaller interventions.¹⁵

​​​Specifically, studies have shown that enhancing crop albedo in northern Europe could decrease regional average temperatures by around 1​​°​​C, provide relief during summer heatwaves, preserve soil moisture, and enhance soil carbon retention​​.​​​^15​​​ 

Sea-ice brightening 

Global warming is driving a sharp decline in the amount of Arctic sea ice in summer, leaving more open water exposed.​ ​Water reflects much less sunlight than ice, increasing temperatures further and causing even more ice to melt.​¹⁶​ The ice that regrows each winter is younger, thinner, and reflects less sunlight, which enhances sea-ice melting​^17​ and causes more warming over time. 

Arctic temperatures are rising faster than temperatures in other parts of the world,​¹⁸​ threatening summer sea ice​ further​. Due to rising temperatures, the Arctic could become practically sea-ice free¹⁹ by 2050 or even earlier.²⁰ 

Sea-ice brightening is an idea to increase the reflectivity of summer sea ice,​ with the aim of​ ​minimising​ melting. Researchers ​have ​suggested​ doing this by covering the surface of the ice with tiny glass “microspheres”​¹⁷​ or by adding seawater to the ice surface in winter to encourage thicker ice​, which​ could persist in summer, to grow.​²¹​ 

​​A study by​ proponents ​of the technique ​suggest​s that​ glass microspheres could increase sea-ice reflectivity and longevity.​¹⁷​ ​However, other​ researchers ​have found that​ the group made unrealistic assumptions in their study. Th​e second​ group found the glass microspheres could slightly decrease the reflectivity of springtime sea ice, accelerating sea-ice loss.​^22​ 

​​Further research suggests enhancing​​​​​​ winter sea-ice growth with seawater might be more effective than glass microspheres for retaining sea ice in the summer. ​However, like other surface albedo modification ideas, it would have local rather than global impacts.​²¹​ 

Brighter deserts 

​​A drawback of most​​ ​surface albedo modification ideas ​is that they ​have limited potential to produce global cooling ​because​ they can​ only be applied to a small fraction of the Earth’s surface. ​In contrast, some researchers have argued that​ brightening deserts – by blanketing them in reflective materials – could have a much bigger impact on global temperatures.​^23​  

However, ​other scientists have pointed out that ​covering desert surfaces in this way is impractical and disruptive.​ ​First, desert albedo modification​ ​would require trillions of dollars to implement and requires long-term maintenance to keep the reflective surfaces dust- and debris-free.²⁴ ​Second​, suggested strategies ​could endanger fragile desert ecosystems.​²⁴​ 

​​In addition​, if it were feasible to brighten deserts at a global scale, this would lead to substantial shifts in rainfall around the world, due to the cooling being concentrated in desert regions.​^25​ Climate model simulations of this idea deployed across all suitable deserts have found that it could decrease monsoon rainfall by as much as 45% in some regions.​^2​ 

Local interventions with limited global cooling potential 

When researchers and policymakers discuss sunlight reflection methods, they often implicitly or explicitly reserve the term for ideas that could lower global temperatures significantly.³ From this perspective, desert albedo modification, with its potentially detrimental local and global impacts, is the only surface albedo modification approach that would count as a sunlight reflection method. 

Other surface albedo modification ideas, such as urban or crop albedo enhancement, cannot scale up to have a significant global cooling effect and so do not present the same kind of global challenges.² However, these approaches might play an important role in helping communities adapt to the impacts of climate change, such as intensifying heat extremes.