In order to restore the earth's energy balance and counteract global warming, it is theoretically possible to act not only on the root cause (reducing human emissions to zero) but also on the other factors that determine this balance. This type of intervention is called “climate engineering” and is divided into two main categories: the removal of carbon dioxide from the atmosphere (the CDR), and the futuristic and somewhat controversial Solar Radiation Management (SRM) that we will look at in this article. SRM techniques are very different from CDR both in terms of maturity (they are still in the research phase and for now the actions under discussion are purely hypothetical) and in terms of costs and risk profiles. In fact, the unwanted impacts of SRM are little known or completely unknown and it is precisely these uncertainties that mean its possible use in the future is still the subject of much debate. To understand the reason for so many doubts, it is enough to describe what SRM involves. Put simply, SRM aims to counteract global warming with interventions that increase the amount of solar radiation reflected from the surface and the atmosphere, preventing its absorption and transformation into heat by the earth system.
The possibilities of these technologies
SRM can be grouped into six categories with fascinating names reminiscent of the science fiction vision of a Jules Verne novel. The first is known as “stratospheric aerosol injection” and is inspired by what happens during volcanic eruptions: clouds of ash come out of the volcano and rise into the atmosphere (the famous plume) releasing large quantities of sulfur dioxide which, when combined with water vapor, form sulfuric acid aerosols capable of reflecting sunlight. In order to achieve a similar effect, some climate scientists have proposed using planes or balloons at high altitudes to introduce artificially special aerosols into the atmosphere. However, it is very difficult today to assess what the overall impact of their use would be (which aerosols will be used? Perhaps sulfur dioxide with the risk of creating acid rain?) and consequently, the degree of uncertainty over the final effect remains high. The second technique is called “marine cloud brightening.” This technology would involve the use of special systems installed on ships to spray seawater (salt spray) toward the clouds above the sea. The salt spray would facilitate the condensation of the vapor held in the clouds into water droplets, causing the clouds to enlarge and making them brighter, meaning that they would better reflect the sunlight. In this case, too, there is a lot of concern this technique may influence other mechanisms of the climate system, generating negative impacts.
Another proposal under discussion is “high albedo crops and buildings”. In simple terms, it is a question of making the roofs and facades of buildings more reflective, by painting them white, for example, as was done in the past in many of the sunniest coastal villages (nothing new under the sun, you might say). The same strategy could be applied to crops, introducing genetic modifications to give the leaves of plants a particular shine capable of reflecting the sun's rays. To achieve the same result on large ocean surfaces, scientists have come up with an intervention known as “ocean mirrors”: ships equipped with special systems would sail around, creating millions of tiny micro bubbles on the surface of the ocean, using special chemical additives to make them last for longer. This would create large white trails (like those formed when a ship passes by) capable of reflecting ten-times more sunlight than normal. One of the potentially negative aspects is that less sunlight would penetrate deep into the water, reducing photosynthesis and the growth rate of marine vegetation, with negative impacts transmitted along the entire food chain. Furthermore, creating so many micro bubbles on the surface of the ocean and above all keeping them “alive” for several days or weeks would require large amounts of energy.
The next technique sounds like a sort of facelift surgery: "Cirrus cloud thinning". Cirrus clouds are high-altitude white clouds made up of dust-like ice crystals. Whilst it is true that cirrus clouds reflect part of the incoming solar energy, the overriding effect is that they prevent the thermal radiation reflected by the earth from escaping into space, resulting in a climate impact similar to that caused by greenhouse gases. Proponents of this approach believe that if we remove or thin out the cirrus clouds that form in the sky, we could partially offset the greenhouse effect caused by the increased concentration of carbon dioxide in the atmosphere. How could this be done? With aerial vehicles (drones) that inject into the cirrus clouds special solid particles (aerosols) that encourage the formation of larger ice crystals with a shorter life. This would not only make the cirrus clouds disappear more quickly from the sky but they would also have a greater capacity to dissipate heat from the earth into space. The final SRM technology discussed by scientists involves sending fleets of “space sunshades” into orbit that reflect part of the incoming sunlight. Some scientists believe that a two percent reduction in sunlight would be enough to offset the warming caused by the doubling of CO2 concentrations compared to pre-industrial levels. The main problem is that putting one or more space mirrors into orbit is a huge technological challenge with prohibitive costs.
The discussion on solar radiation management technologies remains open and we do not yet know if they will ever be used in the future. The high degree of uncertainty over their effectiveness and especially over the negative collateral impacts fuel the mistrust of a large part of the scientific community, the political decision-makers and society in general, who instead consider essential an approach based on prevention (immediately reducing net greenhouse gas emissions, without compensating for them with other futuristic interventions of little known or unknown effects).