Forests and photovoltaic (PV) fields both possess significant potential for mitigating climate change, either through carbon uptake via photosynthesis or by reducing emissions in energy production. However, both contribute to the global heat load by darkening the surface, making it crucial to compare the time required to offset this warming impact. This is because the dark surfaces (low albedo) of photovoltaic fields or forests absorb more sunlight, transferring it into the atmosphere as heat.
In our new study, published in PNAS Nexus, we explored which land use—trees or solar panels—more rapidly offsets the increased heat they generate due to surface darkening. We demonstrated that photovoltaic fields outperform afforestation as a global climate-change mitigation strategy. We assessed changes in surface reflectance (albedo) at a solar field in a dryland, in conjunction with measurements in an afforestation area at a research station on the northern edge of the Negev desert, both located in Israel. These observations were combined with measurements of energy, heat, and CO2 fluxes using a method called Eddy Covariance. Eddy Covariance involves ultra-fast wind measurements of turbulent wind (eddies) with CO2 and water concentration in the air, along with air temperature. This combination of measurements allowed us to calculate the amount of energy (in the form of water vapour and warm air) entering the atmosphere, as well as the CO2 absorbed in natural ecosystems. For solar panels, we integrated this with the solar energy converted into electricity production, considering it equivalent to the CO2 uptake by forests, and factored in the life cycle of solar electricity in terms of its carbon footprint. Finally, we incorporated datasets from global sources to include two additional climate zones, temperate and tropical.
These datasets enabled us to calculate the break-even time required to balance the positive radiative forcing (atmospheric heating effect) due to reduced albedo and the negative radiative forcing (atmospheric cooling effect) due to carbon emission suppression of PV power generation or carbon sequestration by forests. In semiarid lands, photovoltaic fields break even and begin offering climate change mitigation benefits after about 2.5 years, more than fifty times faster than afforestation. In humid lands, the gap is narrower, but solar panels maintain the advantage. These results are crucial as there are increasingly large project ideas for photovoltaic fields, such as Europe’s plans to build them in Spain or even the Sahara, as well as afforestation plans for naturally tree-free areas.
The difference between humid and dry land has two reasons: (1) Humid lands have a darker background, meaning that installing photovoltaic fields or planting forests is not equivalent to doing so in a desert but rather a grassland. Grasslands are inherently darker than bare ground, resulting in a smaller change in energy uptake of solar radiation when transformed. (2) Forests in humid zones are much more productive than in drylands, as they are not limited in water for growth. This means that CO2 assimilation is more significant in humid lands, leading to a more substantial reduction in atmospheric CO2.
In conclusion, although photovoltaic fields offer substantial benefits in terms of CO2 mitigation, it is crucial to acknowledge the additional social and ecosystem benefits of forests. Forests continue to absorb CO2 after reaching the break-even point, making them highly important as carbon sinks, especially in humid climates where they can have a lifetime of centuries. However, exclusively relying on afforestation to mitigate emissions globally is not feasible due to the limited available land. This emphasizes the importance of protecting existing forests at all costs and suggests that a mixed approach would be ideal for future climate mitigation planning. Drylands are well-suited for electricity production through photovoltaic fields, focusing on emission reduction, as they are not highly productive in terms of plant growth. In contrast, mitigation strategies in more humid climates should likely emphasize natural ecosystems as carbon sinks. Through this study, we aim to encourage decision-makers to consider both climate and current land use when planning climate mitigation strategies.
Photo caption: One of the authors working on the measurement system. (Photo credit: Jonathan Mueller)
Read the paper
Rafael Stern, Jonathan D Muller, Eyal Rotenberg, Madi Amer, Lior Segev, Dan Yakir, Photovoltaic fields largely outperform afforestation efficiency in global climate change mitigation strategies, PNAS Nexus, Volume 2, Issue 11, November 2023, https://doi.org/10.1093/pnasnexus/pgad352
For more information, contact Jonathan Mueller at jdmuller@sun.ac.za