A verdant forest is one of the most iconic symbols of the power of nature, from the abundance of plant and animal life that shelters among its thick vegetation to the positive impact it has on Earth’s climate, thanks in part to photosynthesis, which removes carbon dioxide from the air, thereby mitigating the effects of global warming. Cutting down tropical evergreen forests has played a significant role in exacerbating the climate crisis, and many environmental initiatives focus on rehabilitating destroyed forests or planting new trees. The problem is that, even if we were to cover the entire surface of the planet with trees, the resultant massive photosynthetic force would still not suffice to absorb the huge surplus of carbon dioxide — the major greenhouse gas — that has been pumped into the atmosphere during the past 150 years of human activity.

There is another way of dealing with the climate crisis, which, unlike the forests, is neither natural nor green, at least not in the literal sense of the word. This artificial solution consists of erecting fields of dark-colored solar panels. Obviously, the production of electricity from solar power has a positive impact on climate balance, since it replaces power stations that use fossil fuels such as coal and gas, thereby reducing harmful emissions of greenhouse gases that accumulate at increasing concentrations in the atmosphere.

But both the green, natural forest and the artificial, dark “solar forest” produce other effects, some of which can be problematic from a climate perspective. They are both relatively dark, which means that they absorb a large proportion of the radiation from the Sun (making them “low albedo” surfaces in the professional jargon) and, as a result, they heat up. Some of this energy is used for photosynthesis in natural forests or to produce electricity in solar “forests” — but most returns to the atmosphere as fluxes of energy, heating it up. In contrast, the light-colored desert soil, for example, reflects a significant portion of the sunlight back into space, which does not add to the accumulated heat in the atmosphere. (Such soil is known as a surface with a “high albedo.”)

What, then, would be the most effective use of a certain plot of land in terms of the climate crisis: planting a forest, which is a natural means of absorbing carbon dioxide from the atmosphere, or erecting fields of solar panels, which reduce the emission of carbon dioxide into the atmosphere? This dilemma has long been debated by decision-makers around the world.

Now, for the first time — based on findings from arid areas and on comprehensive measurements of the energy flow exchanged between the ground and the atmosphere — we may have an answer to this question, thanks to a new study led by Dr. Rafael Stern, Dr. Jonathan Muller and Dr. Eyal Rotenberg from Prof. Dan Yakir’s lab at the Earth and Planetary Sciences Department of the Weizmann Institute of Science. The study, published today in PNAS Nexus, was coauthored by Madi Amer, also from Prof. Yakir’s lab, and Dr. Lior Segev of Weizmann’s Physics Core Facilities Department.

A century of photosynthesis

The first stage of the study involved comparing the impact of a forest situated on the border of an arid area to that of a field of solar panels, or a solar farm, in an arid environment. Arid areas are characterized by a large amount of sunlight and a relative paucity of plant diversity and biomass, which makes them especially suited for large solar farms. Such fields already exist in Israel in the Arava and the Negev, and the government has plans to erect more in Jordan through an international collaboration. Elsewhere in the world, huge solar projects are under way, for example, in the deserts of China, and the European Union has long discussed plans to build solar farms in the Sahara. The Weizmann researchers traveled down to the Arava in a truck carrying a mobile measuring station, specially designed by Yakir and Rotenberg. They began by placing this measuring station close to the solar panel field to measure the flux of energy between the ground and the atmosphere — as it occurs in an arid area without solar panels. Then they placed the station inside the solar panel field itself; this required overcoming operational and safety challenges stemming from the sensitivity of the panels, which had interfered with such measurements in the past. At both locations, the experiments were repeated during different seasons of the year. Finally, to compare their results to the similar process occurring in a forest, the scientists relied on data that Yakir and Rotenberg had collected over the past 20 years in Yatir Forest — the largest of the forests planted in Israel by the Jewish National Fund — on the northern edge of the arid Negev Desert.

The researchers discovered that the albedo effect of both of these “forests” was similar, but the absorption or prevention of carbon emissions was very different, favoring the solar forest. To complete the comparison, they calculated the equilibrium points at which the opposing effects on the Earth’s climate — heating from both forests’ dark color and cooling from reduced atmospheric carbon dioxide — balance out one another, ultimately lowering the concentration of greenhouse gases in the atmosphere as a result of the natural forest’s photosynthesis or the solar forest’s reduced electricity-production emissions. It turns out that it takes two and a half years for the heat emitted by solar farms to be offset by the carbon emissions that are averted thanks to the energy they generate. This even takes into account the carbon emissions from the manufacture, transportation and operation of the panels, as well as of batteries used for electricity storage. In the case of a forest of similar size, it would take more than 100 years of photosynthesis to offset its heating effect.

The researchers also wanted to establish how the heating and cooling ratio changed in other climates. Using data from similar measurements collected from satellites and databases, they found that in more humid environments such as the tropics or in temperate grassland regions like Europe, the heating effect of planting large numbers of trees is smaller. This is because the ground there is darker to begin with, which means that the albedo-related effect is smaller, and the carbon capture rate by trees is higher, so the break-even point is reached within 15 to 18 years. With that, they note, it must be kept in mind that less open space is available in these areas for planting new forests.

Stern and Muller explain: “Our study unequivocally shows that in arid environments, where most of the open land reserves exist, building solar farms is far more effective than planting forests when it comes to dealing with the climate crisis. In this environment, erecting solar panels on areas that are far smaller than forests (up to one hundredth of the size) will offset exactly the same quantity of carbon emissions. Having said that, forests currently absorb close to one-third of humanity’s annual carbon emissions, so it’s of paramount importance to safeguard this capability and prevent the kind of widescale deforestation that takes place in tropical regions. Moreover, forests play a vital role in the global rain cycle, in maintaining biodiversity and in many other environmental and social contexts. Therefore, the conclusion from our study is that we must protect the Earth’s forests, and that the most appropriate solution to the climate crisis is to combine the planting and rehabilitation of forests in humid regions with erecting fields of solar panels in arid regions.”

Prof. Dan Yakir’s research is supported by the Helen Kimmel Center for Planetary Science and the Schwartz Reisman Collaborative Science Program.

Prof. Yakir is the incumbent of the Hilda and Cecil Lewis Professorial Chair.

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