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Geoengineering is the intentional global-scale modification of Earth’s climate system, in order to reduce global warming. There are two general categories of geoengineering schemes — CO2 removal and insolation reduction.

Insolation Reduction

These projects seek to reduce the insolation (incoming solar radiation) by deflecting sunlight, or by increasing the albedo (reflectivity) of the atmosphere. They do not reduce greenhouse gas concentrations in the atmosphere, and thus do not seek to address problems such as ocean acidification caused by these gases. Solar radiation 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:

  • 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. However, it does hold promise for small-scale projects such as cities.
  • Making the atmosphere more reflective through the injection of sulfur aerosols (this mimics what volcanoes do) or other particles such as aluminum oxide dust into the stratosphere, which will make the Earth more reflective. We know that this works because of the cooling that follows large, explosive volcanic eruptions that inject tiny aerosols (particles) of sulfate into the stratosphere. Based on the volcanic eruptions, we can estimate how much sulfur is needed to counteract a doubling of CO2 — about 5 Tg of S per year (one Tg or teragram is 10^12 g), which is about half the amount that injected into the atmosphere by the eruption of Mt. Pinatubo. The estimated cost of this would be on the order of $50 billion per year (consider that the US military expenditures are about $700 billion). These particles have a limited residence time (1-2 years) in the stratosphere, so this would require continual injection via airplanes, large naval guns, or balloons. The costs of doing this are surprisingly small, but it would have to be maintained.
  • Reducing insolation with space-based mirrors or other structures. The most promising proposal here involves the placement of roughly 16 trillion small disks at a more or less stable position 1.5 million km above the Earth. Each disk would have a diameter of 60 cm and would weigh just one gram. They would not be true mirrors but would scatter enough sunlight to reduce the insolation by 2%, which might be sufficient. Getting these disks into place and then keeping them there would be a challenge, and it is estimated that it would take 10 years to put them into place using a special type of gun that could transport up to 10 million of them at a time. The total cost could be 5 trillion dollars every 50 years (the lifetime of the disks). This sounds a bit like science fiction, but it has been developed by a group of prominent astronomers and physicists.

Carbon Dioxide Removal

Carbon dioxide removal projects address the root cause of warming by removing greenhouse gases from the atmosphere. These projects are generally slower and more expensive than some of the insolation reduction schemes. There are a variety of ways that this could be done, including:

  • Carbon capture and sequestration, which typically involve the removal of CO2 from power plants or other big sources, and then the injection of the liquefied CO2 into deep aquifers; a number of pilot projects of this type have already begun. This could double the cost of coal-generated electricity, making this an expensive proposition, but that in itself would encourage developing cleaner energy sources. Other means of carbon capture have been proposed, including the promotion of natural chemical weathering reactions of some rocks in which atmospheric CO2 is consumed. There is a wide range of proposals to enhance the biological fixation and long-term storage of carbon.
  • Iron fertilization of the surface waters in the southern oceans to promote plankton blooms, which use CO2 from the surface oceans, thus enabling them to absorb more from the atmosphere. It turns out that in many parts of the oceans, iron is a limiting nutrient for photosynthesizing plankton, so adding iron can help the plankton achieve their full potential in terms of CO2 consumption. A few small-scale experiments have been conducted and they appear to work in the short-term, but scaling this up would be challenging and the iron would have to be continuously applied, just as fertilizer is continuously applied to crops.

Problems and Concerns

Although these geoengineering schemes may be attractive in the sense of providing a solution to the problem without having to get all the countries of the world to make a dramatic reduction in CO2 emissions, some of them clearly have potentially harmful consequences.

As a general rule, when people try to control natural systems, they find that the natural systems are more complex than they realized, with many unintended consequences. This is likely to be the case with geoengineering as well. With many of these schemes, there may be winners and losers — cooling the climate in one region (or the globe as a whole) may lead to devastating droughts, floods, or damaging cold in other regions. Adding sulfur to the atmosphere will lead to acid rain, and it may also deplete the ozone layer, exposing us to more harmful ultraviolet radiation from the sun.

The aerosol injection schemes are so inexpensive that one country could decide to take action unilaterally, and this would likely lead to serious problems in international relations. All of the insolation management schemes have the problem of not addressing the CO2 emissions, which would allow the CO2 concentration in the atmosphere to rise to very high levels; then if the geoengineering scheme failed for some reason, the climate would warm much faster than anything we have experienced so far.


Most scientists agree that geoengineering schemes should not be seen as the silver bullet that will solve our global warming problems quickly and painlessly. These schemes are like treating the symptoms of a disease rather than the root cause, a course of action that is never a real solution. But until we find a way to treat the root cause (reduce emissions of CO2), some of these schemes may buy us more time while helping us avoid serious climate damages. While many climate scientists believe that there may be an important role for these schemes in our future, we do not yet fully understand the potentially harmful side effects of these projects, so continued research is very important.