These projects seek to reduce the insolation (incoming solar radiation) — the energy input for our climate — absorbed by the Earth. They do not reduce greenhouse gas concentrations in the atmosphere, and thus do not address problems such as ocean acidification caused by the excess CO2. Insolation management projects appear to have the advantage of speed, and in some cases, costs. There are a variety of ways that we might achieve a reduction in insolation and thus cool the planet.
Ocean acidification is one of the more serious consequences of emitting carbon dioxide into the atmosphere. The atmosphere and the oceans exchange gases like oxygen and carbon dioxide to achieve a kind of equilibrium or balance in terms of concentration. So, if we put more CO2 into the atmosphere, the oceans will try to absorb a lot of that CO2. Indeed, we now know that the oceans absorb something like 25% of the CO2 we have emitted through the burning of fossil fuels. This is a good thing in terms of moderating the greenhouse gas forcing of our climate, but it has the result that the oceans become more acidic — just as carbonated water is more acidic than tap water. The problem with this is that the phytoplankton that make up the base of the food chain in the oceans cannot tolerate acidic conditions. The oceans are in fact becoming more acidic, and while we humans would never sense this change, the far more sensitive phytoplankton definitely feel it. If further acidification occurs, the phytoplankton will decline and because they are the base of the food chain, most other life in the oceans will also decline, putting the whole ocean ecosystem in peril. This is yet another reason why we need to stop emitting CO2 and even reduce the amount of CO2 in the atmosphere. If we pull CO2 out of the atmosphere, the oceans will release some of its CO2 into the atmosphere, reducing the acidity of the oceans.
Changing the Surface Albedo
Directly changing the albedo of the surface through the use of light colored or reflective materials on buildings, glaciers, etc. For buildings, this has the added benefit of reducing the cooling costs, but it is not likely to be as effective on a global scale as some of the other schemes. Buildings in cities represent something like 0.1% of the Earth’s surface, so by changing the albedo of the building tops, we cannot make much of a change in the global albedo and thus the global temperature. To calculate this, we need to do some very simple math. As a whole, our planet has an albedo of 0.3, so if we change the albedo of 0.1% (which, as a fraction, is 0.001) of the whole planet to an albedo of 0.9 (very reflective), then we get the new albedo by adding 0.3*0.999 + 0.9*0.001 — this give us the new albedo of 0.3006. This would lower the temperature by 0.06°C, which is clearly not going to be enough! However, this approach does hold promise for individual cities, which suffer from a phenomenon called the “urban heat island effect” — they are hotter than the surrounding countryside where there are more plants. Plants cool an environment by releasing water in the form of vapor — this is called transpiration, and just like evaporation, it cools the surface. So, it is a good idea to whiten up building tops, but this is not going to solve our global warming problem. There are few, if any, risks associated with these kinds of operations. But, the cost of doing this could be as high as \$300 billion per year based on a study by the Royal Society — a lot of money for a small reduction in temperature!