Geoengineering techniques can be divided into two major classes: those that act directly on the cause of global warming by modifying solar radiation, and those that do so by removing excess CO2 from the atmosphere.

Managing solar radiation means screening the sun’s rays at all levels of light frequency that hit the earth’s surface: ultraviolet, visible and near infrared. That may involve acting on the earth’s surface, the troposphere, the upper atmosphere, or directly on the space between our planet and the sun.

On the earth’s surface, we can first of all try to increase the planet’s albedo – the ability of an object to reflect light instead of absorbing it in the form of heat. Asphalted roads reflect just 3-20% of the solar rays that hit them, absorbing the rest of the light that hits them and making our cities boiling hot. But roads, houses or industrial buildings that are painted white reflect more than half – and sometimes up to 90% – of the radiation that hits them, keeping them relatively cool, as it proves the white painted roads experiment in Los Angeles. The effect of brick, concrete and tiles is halfway between the two extremes.

There’s no need to go around painting everything white, though. Coloured pigments have been created that reflect most of the sun’s radiation while allowing us to live in a world of glorious colour. At the laboratories at the University of California in Berkeley they’ve developed a range of pigments, including a ruby-red one, that reflect light effectively. But this isn’t a particularly new idea. The Egyptians were already painting their amphorae blue 5,000 years ago. They’d found out that colour, which was the first synthetic pigment in the world, reflected light as infrared radiation, keeping the painted container or house.

To increase the whole planet’s albedo, people have suggested using the oceans, covering them with highly reflective, floating substances that are also stable and harmless to flora and fauna. The best solution might just be tiny bubbles. It would be possible to build facilities – fixed or floating on ships – that suck up water, mix it with air and inject microbubbles back into the sea. These in turn reflect the sun’s light more efficiently, reducing the warming effect. However, to make the microbubbles stable enough, we’d have to introduce surfactants into the sea that would compromise the delicate balance of the marine ecosystem. In any case, it’s possible the microbubbles would deprive phytoplankton of a great deal of the light they need to survive and keep providing the basis for the ocean food chain. The concentration of dissolved oxygen would also increase in tandem with aerobic organisms that live on the water’s surface, to the detriment of other organisms and the balance between species. Another line of research has looked at using agricultural fields. The idea is to genetically modify or find suitable varieties of staple crops, or simply change our diet and start intensively growing crops that have a lighter colour. This would be relatively easy to do and reduce surface temperature by as much as 1 °C. However, it would result in not inconsiderable dietary and social problems.

Research is being done into creating artificial clouds in the troposphere that reflect light before it hits the water below. For the upper stratosphere there’s another strategy in mind, also inspired by a natural phenomenon. The US National Center for Atmospheric Research has studied how the volcano Mount Pinatubo affected the world’s climate for at least a couple of years by emitting 20 million tonnes of sulphur dioxide into the atmosphere. This provided the idea of launching sulphide dust into the atmosphere with rockets or planes, to create clouds that will, according to estimates, eliminate the warming effect of thousands of tonnes of CO2 for every kilogram of sulphides they contain. But this also comes with a lot of drawbacks. The dominant convection currents tend to move aerosols towards the earth’s tropics and cool them down, at the expense of its poles, where the melting of glaciers and ice caps melt could continue. Furthermore, screening the layers below could lead to a change of direction in the currents. If the currents at the top have to transport these aerosols to the poles, they will damage the ozone layer that protects us from ultraviolet rays. The sulphide gradually falling into the sea and onto the land would have repercussions on forests, crops and animals.

Moving ever closer to the earth’s heat source, geoengineers have looked at partly blocking the sun’s rays as they travel to earth by setting up a vast umbrellathat reflects or absorbs incident radiation, stopping them from heating our atmosphere. This would not leave us in the dark of course. Reducing by just 1.7% the solar radiation that hits our planet, would be enough to block temperature rises by 2 °C.
The place for this special umbrella is what the astronomers call Lagrangian point L1. It lies 1.5 million kilometres from the earth and 148 million kilometres from the sun, right between the two. The gravitational forces imposed on this point by the two bodies is perfectly balanced with the centripetal force on them.
To avoid disrupting the day-night cycle of all living beings, we would have to resort to masses of small umbrellas, which would at least be easier to transport! NASA has already created a prototype. It’s a disc around 60 centimetres in diameter and 5 micrometres thick, and weighs about a gram. The real problem is that we’d need 16 million billion of them, weighing around 20 million tonnes in total. To put it another way, in order to create a cloud of 3.8 million square kilometres of discs blocking about 2% of the solar radiation that hits the earth, we’d have to launch a rocket with 100 tonnes on board every day for 20 years. Nasa says this would cost 130,000 billion dollars, or 18,500 dollars per person on the planet.
Another solution is placing a giant Fresnel lens in L1, 1,000 kilometres in diameter and a few millimetres thick. This could reduce 1% of solar rays in space, once we have come up with a way to send the materials for building it into orbit. The costs would be 20,000 billion dollars.
Meanwhile, at the University of Strathclyde they’ve come up with the idea of diverting an asteroid from its orbit while travelling near earth and placing it in L1. Calculations show that the asteroid would attract enough cosmic dust to screen a significant amount of sunlight – provided it doesn’t escape in the process!

The second major class of geoengineering strategies works on the effects of climate change. It involves removing excess carbon dioxide directly from the atmosphere. The main problem with directly capturing CO2 from air is that it’s extremely diluted, to 0.04% to be precise. Even if we could develop filters for extracting it efficiently, we’d have to process 2,500 litres of air to gather just one litre of carbon dioxide. The energy for doing so would have to be produced with renewable sources with a low carbon impact.

A more promising technique is what is known as bioenergy with carbon capture and storage (BECCS). It’s based on trusting nature, letting trees and crops trap CO2 from the atmosphere through photosynthesis and use it to grow. These plants are then harvested and burnt in a controlled environment, and the carbon dioxide that escapes is recovered. A method then has to be found to lock it up permanently. For example, it can be injected into spent oil fields. The question is how to find large areas for planting that won’t interfere with crop farming for feeding humans and animals.

This involves carrying out pyrolysis on biomass from farm animal waste, vegetable waste or microorganisms grown for the purpose. That means burning it at a very high temperature in an environment with little oxygen, to produce a type of charcoal named biochar. But if it’s then used as “bio-coal” to feed power plants, the CO2 nature has taken so much effort to store is lost along with all the environmental benefits of the process. A much better idea is to fertilise extremely acidic land with it. As a porous material it can absorb fertilisers and release them in a controlled way, improving crop yield. CO2 can therefore be trapped and blocked efficiently, and kept for thousands of years in soil with a high carbon content. Such earth goes by the Portuguese name terra preta, because it resembles the earth fertilised with charcoal by the pre-Columbian inhabitants of the Amazon.

Yet another alternative to the methods above uses the sea to capture CO2. Ocean fertilisation involves spreading substances over the water’s surface that stimulate phytoplankton, whose growth is inhibited by a lack of iron. So as iron is spread finely over the ocean and the phytoplankton multiply, they consume more CO2 from the atmosphere. Based on the concentration of nutrients in the different oceans, the iron can be supplemented with a mix of others, like phosphorus and nitrogen.
Volcanic ash has the ideal composition and is an attractive alternative.

Unlike technology for reducing climate change that is applied at a specific location, geoengineering techniques can only work if applied at a worldwide level. Very localised experiments are currently being carried out, with fairly conflicting results. We should consider that the earth is a unique ecosystem that tends to compensate for imbalances (due to experiments, which can also be influenced), hence, an attempt to alter the planet’s dense network of checks and balances can be rather risky.
While the scientific world continues to look for possible avenues to explore, from ambitious solutions to local mitigation techniques, we must all do our part. Walk or cycle if you need to go somewhere nearby. Don’t use your air conditioner and heating too much. Buy energy-efficient appliances. Or even just turn off your screen after reading this article.