Article

Surface Albedo Modification

Surface albedo modification describes a set of ideas to produce a cooling effect by increasing the amount of light reflected away from different surfaces. At a local scale, they can be an effective form of adaptation, but could they be scaled up to achieve a global cooling effect?

Key takeaways

  • Using methods to reflect sunlight away at a local scale can be effective at providing cooling. For example, painting buildings white in hot countries.​​
  • However, scaling up such methods would not produce a substantial global cooling effect.
  • If it were possible to make a significant fraction of the land surface much more reflective, for example, the deserts, this would lead to substantial reductions in monsoon rainfall.

On 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, 818% 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 

Surface albedo

Reflectivity, also known as albedo, ranges from 0 to 100%. Highly reflective surfaces have an albedo closer to 100% while surfaces that absorb sunlight have an albedo closer to zero.

Sources: Petrov M, Industry 4.0, 2022 (trees, white paint); Duhis AH et al., IJPEDS, 2023

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 

A field of densely packed yellow barley crop stretching out into the distance.

Crops ​such as​ barley reflect more sunlight than bare soil or trees​.

The 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 existing14 or bioengineered13 could help to decrease local temperatures during the growing season.10 Modelling suggests 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 melting17 and causes more warming over time. 

Two people shown from a distance stood on white ice, with several blue melt ponds around them.

Freshwater ponds darken the surface of melting sea ice in the Arctic.

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 suggests 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. The 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. 

Open questions

  • How much does the urban heat island effect exacerbate climate impacts in urban areas, and could urban brightening alleviate this?
  • Would it be practical to add increasing reflectivity to the goals of agricultural management? If so, what trade-offs would there be with other goals?
  • Should local surface albedo modification ideas be treated as a sunlight reflection method, or would the scale of their climate effects be too small?

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Endnotes

  1. Petrov M. (2022). The evolution of albedo values of the Earth-atmosphere system under the influence of carbon dioxide pollutant concentrations. Industry 4.0.7(1):36-41. https://stumejournals.com/journals/i4/2022/1/36
  2. Irvine PJ, Ridgwell A, Lunt DJ. (2011). Climatic effects of surface albedo geoengineering. Journal of Geophysical Research Atmospheres; 116. https://doi.org/10.1029/2011JD016281 
  3. Royal Society (Great Britain). (2009). Geoengineering the climate: science, governance and uncertainty. Royal Society. Available at https://royalsociety.org 
  4. Ahmed Memon R, Leung DY, Chunho L. (2008). A review on the generation, determination and mitigation of Urban Heat Island. https://doi.org/10.1016/S1001-0742(08)60019-4 
  5. Ouyang Z, Sciusco P, Jiao T, et al. (2022). Albedo changes caused by future urbanization contribute to global warming. Nature Communications; 13. https://doi.org/10.1038/s41467-022-31558-z 
  6. Santamouris M, Yun GY. (2020). Recent development and research priorities on cool and super cool materials to mitigate urban heat island. Renewable Energy; 161: 792–807. https://doi.org/10.1016/j.renene.2020.07.109 
  7. Xu X, AzariJafari H, Gregory J, et al. (2020). An integrated model for quantifying the impacts of pavement albedo and urban morphology on building energy demand. Energy Build; 211. https://doi.org/10.1016/j.enbuild.2020.109759 
  8. Jandaghian Z, Akbari H. (2021). Increasing urban albedo to reduce heat-related mortality in Toronto and Montreal, Canada. Energy Build; 237. https://doi.org/10.1016/j.enbuild.2020.110697 
  9. Yang J, Wang ZH, Kaloush (2015). Environmental impacts of reflective materials: Is high albedo a ‘silver bullet’ for mitigating urban heat island? Renewable and Sustainable Energy Reviews; 47: 830–843. https://doi.org/10.1016/j.rser.2015.03.092 
  10. Kala J, Hirsch AL, Ziehn T, et al. (2022). Assessing the potential for crop albedo enhancement in reducing heatwave frequency, duration, and intensity under future climate change. Weather Clim Extrem; 35. https://doi.org/10.1016/j.wace.2022.100415 
  11. Singarayer JS, Ridgwell A, Irvine P. (2009) Assessing the benefits of crop albedo bio-geoengineering. Environmental Research Letters; 4. https://doi.org/10.1088/1748-9326/4/4/045110 
  12. Slattery RA, Vanloocke A, Bernacchi CJ, et al. (2017). Photosynthesis, light use efficiency, and yield of reduced-chlorophyll soybean mutants in field conditions. Front Plant Sci; 8. https://doi.org/10.3389/fpls.2017.00549
  13. Genesio L, Bassi R, Miglietta F. (2021). Plants with less chlorophyll: A global change perspective. Global Change Biology; 27: 959–967. https://doi.org/10.1111/gcb.15470 
  14. Breuer L, Eckhardt K, Frede HG. (2003). Plant parameter values for models in temperate climates. Ecol Modell; 169: 237–293. https://doi.org/10.1016/S0304-3800(03)00274-6 
  15. Sieber P, Böhme S, Ericsson N, et al. (2022). Albedo on cropland: Field-scale effects of current agricultural practices in Northern Europe. Agricultural and Forest Meteorology; 321. https://doi.org/10.1016/j.agrformet.2022.108978 
  16. Kashiwase H, Ohshima KI, Nihashi S, et al. (2017). Evidence for ice-ocean albedo feedback in the Arctic Ocean shifting to a seasonal ice zone. Sci Rep; 7. https://doi.org/10.1038/s41598-017-08467-z 
  17. Field L, Ivanova D, Bhattacharyya S, et al. (2018). Increasing Arctic Sea Ice Albedo Using Localized Reversible Geoengineering. Earths Future; 6: 882–901. https://doi.org/10.1029/2018EF000820 
  18. Rantanen M, Karpechko AY, Lipponen A, et al. (2022). The Arctic has warmed nearly four times faster than the globe since 1979. Communications Earth and Environment; 3. https://doi.org/10.1038/s43247-022-00498-3 
  19. The IPCC characterises characterises the Arctic Ocean as sea-ice free if sea ice has an area of less than 1 million km2. This projected state persists across all Shared Socioeconomic Pathways (SSP) considered by the IPCC.
  20. IPCC. Climate Change 2021. (2021). The Physical Science Basis Contribution of Working Group 1 to Sixth Assessment Report of the Intergovernmental Panel on Climate Change; Cambridge University Press. https://doi.org/10.1017/9781009157896 
  21. Zampieri L, Goessling HF. (2019). Sea Ice Targeted Geoengineering Can Delay Arctic Sea Ice Decline but not Global Warming. Earths Future; 7: 1296–1306. https://doi.org/10.1029/2019EF001230 
  22. Webster MA, Warren SG. (2022). Regional Geoengineering Using Tiny Glass Bubbles Would Accelerate the Loss of Arctic Sea Ice. Earths Future; 10. DOI: 10.1029/2022EF002815. https://doi.org/10.1029/2022EF002815 
  23. Cherlet M, Hutchinson C, Reynolds J, et al. (2018). World Atlas of Desertification. Luxembourg: Publications Office of the European Union, 2018. https://dx.doi.org/10.2760/06292 
  24. Schäfer S, Lawrence M, Stelzer H, et al. (2015). Final report of the FP7 CSA project EuTRACE The European Transdisciplinary Assessment of Climate Engineering (EuTRACE). Available at: https://www.rifs-potsdam.de/ 
  25. Crook JA, Jackson LS, Osprey SM, et al. (2015). A comparison of temperature and precipitation responses to different earth radiation management geoengineering schemes. Journal of Geophysical Research: Atmospheres; 120: 9352–9373. https://doi.org/10.1002/2015JD023269

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Kimberly Samuels-Crow (2024) - "Surface Albedo Modification" Published online at SRM360.org. Retrieved from: 'https://srm360.org/article/surface-albedo-modification/' [Online Resource]

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