Reducing global emissions of carbon dioxide has been a major international goal since the 1990s, due to the prospect of significant climate change being brought about by rising levels of the gas in the atmosphere. However since this time little progress has been made in developing energy sources not dependant on burning hydrocarbons (the main source of atmospheric carbon dioxide), while demand for energy has grown across the globe. One possible way to counter this that has been suggested is the development of very large scale wind and solar power generation plants in the world's desert regions, though the likely impact of such plants on the climate is itself unclear.
In a paper published in the journal Science on 7 September 2018, Yan Li of the Department of Atmospheric and Oceanic Science at the University of Maryland, the Department of Natural Resources and Environmental Sciences at the University of Illinois at Urbana-Champaign, and the State Key Laboratory of Earth Surface Processes and Resources Ecology at Beijing Normal University, Eugenia Kalnay, also of the Department of Atmospheric and Oceanic Science, and of the Institute for Physical Science and Technology, at the University of Maryland, Safa Motesharrei, again of the Department of Atmospheric and Oceanic Science, and the Institute for Physical Science and Technology, and the Department of Physics, at the University of Maryland, Jorge Rivas of Rockville in Maryland, Fred Kucharski of the Earth System Physics Section at the Abdus Salam International Centre for Theoretical Physics, Daniel Kirk-Davidoff, again of the Department of Atmospheric and Oceanic Science at the University of Maryland, Eviatar Bach, again of the Department of Atmospheric and Oceanic Science and Institute for Physical Science and Technology, at the University of Maryland, and Ning Zeng, once again of the Department of Atmospheric and Oceanic Science at the University of Maryland, and of the Institute of Atmospheric Physics of the Chinese Academy of Science, publish the results of a study which used computer modelling to try to assess the impact of large scale wind and solar power generation on the the Sahara Desert and its surrounding regions.
The Sahara is the word's largest desert, and is in addition very sparsely populated, so that any future large scale wind or solar power projects would face little competition from other forms of Human land use. Li et al. modelled the potential impact of large scale wind and solar projects on both the Sahara and the more populated Sahel, a transition region between desert and wooded savanna to the south, using a model in which wind farms producing three terawatts of power per year and solar plants producing 79 terawatts of power per year were assessed for their impact.
Li et al. found that the wind farms would result in an average rise in ground temperature of 2.16 K, though this would be mostly due to a rising minimum temperature, which would go up by an average of 2.36 k, with the average maximum temperature rising only by 1.85 K. This is a previously observed phenomenon around wind farms, which mix air layers vertically, bringing down warmer air from above the surface at night. The wind farms also increased average daily precipitation in the Sahara by 0.25 mm per day (more than doubling the amount of rain in the desert, though this would not be in the form of very small amounts of rain each day), and in the Sahel region by 1.12 mm per day, as the increased ground temperature leads to more air rising above the desert (hot air rises), drawing more moisture laden air from elsewhere. This increases precipitation is predicted to lead to an increased vegetative ground cover, leading to a lower albedo (the ability of the ground to reflect heat and light, vegetation tends to absorb, whereas exposed rock and sand, particularly if light in colour, tends to reflect), increased surface air friction (which might reduce average wind speeds by up to 36%), and increased evaporation through transpiration, leading to more cloud cover and more rain.
Impacts of wind and solar farms in the Sahara on mean near-surface air temperature (kelvin) and precipitation (millimetres per day). The impacts of wind farms (A) and (B), solar farms (C) and (D), and wind and solar farms together (E) and (F), respectively, are shown. Only areas where changes are significant at the 95% confidence level (t test) are displayed on the map. Gray dots denote the location of wind and/or solar farms. At the bottom of each plot, the number after Δ represents the changes in climate (in either kelvin or millimetres of precipitation per day) averaged over areas covered by wind and solar farms. Li et al (2018).
Solar power projects were found to have a similar effect, raising average ground temperatures and increasing precipitation, though in this case the main driver was a decreasing albedo due to the solar panels themselves, which absorb rather than reflecting sunlight, leading to an average rise in daytime maximum temperatures of 1.28 K, while the minimum nighttime temperature rose by only 0.97 K. These solar projects were predicted to raise precipitation in the Sahara by an average of 0.13 mm per day, and in the Sahel by an average of 0.57 mm per day. This model did not produce a notable drop in average wind speeds.
Combining the wind and solar projects resulted in an average temperature rise of 2.65 K, but an average increae of precipitation of 0.35 mm per day in the Sahara and 1.34 mm per day. This is particularly significant as it results in an increase in average rainfall of almost 500 mm per year, significanlty altering the local climate.
Relative contributions of roughness change (Rough) and vegetation feedback (Veg) in the climate impacts of wind farms in the Sahara. Contributions in the temperature (A), (C), and (E) and the precipitation (B), (D), and (F) impacts are shown. The wind farm impact is produced by the initial roughness of wind turbines and the subsequent albedo changes due to vegetation feedback. At the bottom of each plot, the number after Δ represents the changes in climate (in either kelvin or millimeters of precipitation per day) averaged over areas covered by wind farms. Li et al (2018).
These predictions were based upon an average energy conversion rate of 15% for the solar panels, roughly what we would expect with today's technology, however Li et al. also note that we should expect solar panels to become more efficient in the future, and that as they do so the amount of ground-level warming they cause should drop, so that once their average efficiency passes 35% they would be predicted to cause a cooling at ground level, combined with a reduction in rainfall, resulting in a rather different impact on the climate of the Sahara.
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