Lesson 11 Summary

In this lesson, we investigated the concept of geoengineering, and investigated various of the specific geoengineering schemes that have been proposed as potential approaches to mitigating climate change. We found that:

• Geoengineering schemes that have been proposed fall within one of two basic categories, either seeking to prevent CO₂ from building up in the atmosphere, or offsetting greenhouse warming through schemes aimed at reducing the short wave, solar radiation received by Earth's surface.
• The least invasive scheme (so much so, that it is often not even considered geoengineering at all) is carbon capture and sequestration (CCS), aimed at capturing and burying CO₂ at the point of emission. CCS is only useful, however, for large, concentrated emission sources such as coal-fired power plants. Moreover, it cannot lower CO₂ ambient levels, but simply decrease the rate at which they are increasing.
• An alternative scheme, known as air capture, actively filters CO₂ out of the atmosphere. It has the advantage of that it can be deployed widely (i.e., it isn't necessary to place it at or near the source of emissions), and because it acts on ambient CO₂ levels, it could in principle be used to lower CO₂ concentrations as well as stabilize them. It has the disadvantage of being far less efficient (and more expensive) than methods such as CCS which act on concentrated streams of CO₂ emission.
• A number of other geoengineering schemes that fall under the category of solar radiation management instead attempt to decrease the incoming shortwave radiation by placing reflective sulfate aerosols, or reflecting mirrors, in the lower stratosphere, or by decreasing the albedo of Earth's surface.
• Stratospheric sulfate aerosol injection might be a relatively inexpensive option for offsetting greenhouse warming, but suffers a number of potential undesirable side-effects, including inhomogeneous temperature changes that could actually lead to even more warming in certain regions like the Arctic, possible decreases in continental precipitation, worsening of ozone depletion, and other potentially harmful chemical side-effects. Other schemes for radiation management avoid some of these complications, but are likely to be highly expensive or impractical.
• Another scheme known as oceanic iron fertilization seeks to stem the buildup of CO₂ in the atmosphere by adding iron to regions where marine phytoplankton productivity is primarily limited by this nutrient, thus potentially speeding the uptake of atmospheric CO₂ by the ocean. Initial experiments have shown no clear evidence however of increased deep ocean carbon burial, calling into question the efficacy of the scheme. Possible unintended consequences include increasing the competitive advantage of harmful plankton, such as those which cause red tides.
• In general, geoengineering schemes appear to be fraught with potential uncertainties, harmful known and perhaps unknown side effects. The law of unintended consequences would appear to apply to many, if not all schemes that have been proposed.

Reminder - Complete all of the lesson tasks!

You have finished Lesson 11. Double-check the list of requirements on the first page of this lesson to make sure you have completed all of the activities listed there before beginning the next lesson.