Geoengineering might be a terrible idea, but it could also be the only option if efforts to slow carbon dioxide emissions continue to fail, according to a new report by the Royal Society in London.

Broadly defined, geoengineering is any attempt to counteract climate change on a massive scale. It includes two main approaches: pulling carbon dioxide out of the atmosphere (such as by increasing the growth of algae that take up carbon dioxide) or somehow decreasing the rate at which the sun heats the earth (such as by shading the planet or increasing the reflectivity of clouds).

In both cases, scientists don't know what might go wrong with the proposed schemes--their scale is unprecedented. According to John Shepherd, who chaired the Royal Society's study, "used irresponsibly or without regard for possible side effects, geoengineering could have catastrophic consequences similar to those of climate change itself."

Yet if climate change starts to get out of control, we may be left with no alternatives. "Geoengineering and its consequences are the price we may have to pay for failure to act on climate change," Shepherd adds.

The study analyzed the proposed geoengineering alternatives to decide which are most likely to work without disastrous consequences. Notably, one approach advocated publicly by U.S. Energy Secretary Steven Chu--painting roofs white to reflect sunlight--didn't come out well.

Here's a summary of what the Royal Society study found.

Regarding carbon capture methods:

· CO2 capture from ambient air: This would be the preferred method of geoengineering, as it effectively reverses the cause of climate change. At this stage no cost-effective methods have yet been demonstrated and much more research and development is needed.

· Enhanced weathering: This technique, which utilizes naturally occurring reactions of CO2 from the air with rocks and minerals, was identified as a prospective longer-term option. However, more research is needed to find cost-effective methods and to understand the wider environmental implications.

· Land use and afforestation: The report found that land-use management could and should play a small but significant role in reducing the growth of atmospheric CO2 concentrations. However, the scope for applying this technique would be limited by land-use conflicts, and all the competing demands for land must be considered when assessing the potential for afforestation and reforestation.

Should temperatures rise to such a level that more rapid action needs to be taken, the following solar radiation management techniques were considered to have most potential:

· Stratospheric aerosols: These were found to be feasible, and previous volcanic eruptions have effectively provided short-term preliminary case studies of the potential effectiveness of this method. The cost was assessed as likely to be relatively low and the timescale of action short. However, there are some serious questions over adverse effects, particularly the depletion of stratospheric ozone.

· Space-based methods: These were considered to have potential for long-term use, if the major problems of implementation and maintenance could be solved. At present the techniques remain prohibitively expensive and complex and would be slow to implement.

· Cloud albedo approaches (e.g., cloud ships): The effects would be localized and the impacts on regional weather patterns and ocean currents are of considerable concern but are not well understood. The feasibility and effectiveness of the technique is uncertain. A great deal more research would be needed before this technique could be seriously considered.

The following techniques were considered to have lower potential:

· Biochar (carbon dioxide reduction technique): The report identified significant doubts relating to the potential scope, effectiveness, and safety of this technique and recommended that substantial research would be required before it could be considered for eligibility for U.N. carbon credits.

· Ocean fertilization (carbon dioxide reduction technique): The report found that this technique had not been proved to be effective and had high potential for unintended and undesirable ecological side effects.

· Surface albedo approaches (solar radiation management technique, including white-roof methods, reflective crops, and desert reflectors): These were found to be ineffective, expensive, and, in some cases, likely to have serious impacts on local and regional weather patterns.