Cutting down tropical evergreen forests has played a major role in worsening the climate crisis.


 The solar farm in the Arava where the measurements were conducted (photo credit: Jonathan D. Muller)
The solar farm in the Arava where the measurements were conducted
(photo credit: Jonathan D. Muller)

Environmental experts have been recommending for years that green forests should be planted around the world, largely because of photosynthesis, which removes carbon dioxide from the air, thus having a positive effect on Earth’s climate and minimizing global warming.

The cutting down of tropical evergreen forests has played a major role in worsening the climate crisis and many environmental initiatives focus on rehabilitating destroyed forests or on planting new trees. The problem is that, even if we were to cover the entire surface of the planet with trees, the resulting massive photosynthetic force would still not be enough to absorb the huge excess of carbon dioxide – the major greenhouse gas that has been pumped into the atmosphere over the past 150 years of human activity.

But it seems that there is, in fact, a speedier and more effective way to cope with the climate crisis that is neither green nor natural – at least not in the literal sense of the word.

 Measuring station in Yatir Forest – 20 years of research (credit: Jonathan D. Muller)
Measuring station in Yatir Forest – 20 years of research (credit: Jonathan D. Muller)

This artificial solution, proposed by scientists at the Weizmann Institute of Science in Rehovot, involves setting up fields of dark-colored solar panels. 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 return 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 particular 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, we may have an answer to this question based on findings from arid areas and on comprehensive measurements of the energy flow exchanged between the ground and the atmosphere – according to Dr. Rafael Stern, Dr. Jonathan Muller, and Dr. Eyal Rotenberg – from Prof. Dan Yakir’s lab at Weizmann’s Earth and Planetary Sciences department. It was co-authored by Madi Amer, also from Yakir’s lab, and by Dr. Lior Segev of Weizmann’s Physics Core Facilities department.

They have just published their study in  the journal PNAS Nexus under the title “Photovoltaic fields largely outperform afforestation efficiency in global climate change mitigation strategies.”

The first stage of the study involved comparing the impact of a forest situated on the border of a dry 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 dearth 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.

Huge solar project underway around the world

ELSEWHERE IN the world, huge solar projects are underway, including 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 that was 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. They then placed the station inside the solar panel field itself; this required overcoming operational and safety challenges resulting from the panels’ sensitivity that 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 released by solar farms to be offset by the carbon emissions that are prevented by 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 a century 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 more reduced, and the carbon capture rate by trees is higher, so the break-even point is reached within 15-18 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 explained: “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 1/100 of the size) will offset exactly the same quantity of carbon emissions.”

Forests currently absorb close to a third of humanity’s annual carbon emissions, so it’s of great importance to safeguard this capability, and prevent the kind of wide-scale deforestation that takes place in tropical regions, the team wrote.

Moreover, forests play a vital role in the global rain cycle by maintaining biodiversity and in many other environmental and social contexts.

Therefore, they concluded that the Earth’s forests must be protected and that the best solution to the climate crisis is to integrate the planting and rehabilitation of forests in humid regions with installing fields of solar panels in arid regions.