The idea that we might be able to "geoengineer" the planet to purposefully change the climate has clearly moved from the fringes into the mainstream. Momentum has been building in recent years: an essay in an academic journal by a Nobel Prize winning scientist in 2006, articles in the Wall Street Journal and Foreign Policy, a largely private gathering of researchers at Harvard.

Recently things have really broken out. In addition to multiple articles and books in the popular media, the United Kingdom's Royal Society, the authoritative national academy of science there, issued an in-depth review of geoengineering and President Obama's science advisor, John Holdren, has repeatedly stated that geoengineering must be on the table as a possible approach to addressing climate change.

Yesterday, the House of Representatives' Committee on Science and Technology held a hearing that its chairman, Bart Gordon (D-TN), said was, "the first time that a congressional committee has undertaken a serious review of proposals for climate engineering."

Gordon was quick to say that this doesn't mean he supported geoengineering, and that the consensus at the hearing seemed to be that no one should deploy geoengineering until we've done a lot more research. But the very fact of the hearing confirmed that influential people are starting to take geoengineering very seriously. It's no longer just a subject for gee-whiz fascination, with science-fiction-like scenarios such a vast parasol launched into space to shield the earth from the sun. Now scientists are formulating detailed research plans, start-ups are inventing new geoengineering technologies, and politicians and foreign policy experts are considering what all of this might mean for international relations.

So, why the sudden enthusiasm for proposals to tinker with the climate? These ideas aren't new, but until recently they've been largely kept under wraps while attention has been focused on reducing greenhouse gas emissions. There are probably three main reasons for the change. First, some view geoengineering as a cheap way to avoid costly conversions to zero-emissions technology, a potential technological fix that could help them stave off climate legislation. With geoengineering as an option, they argue, there's less of a rush. We'll just cool the planet until we can get around to switching to cleaner forms of energy.

But this could be mind-blowingly stupid. One of the most popular geoengineering approaches--shading the earth with a haze of sulfate particles in the upper atmosphere--would very likely lead to severe droughts. There are other potential side effects, but a purposeful act that causes the failure of crops for potentially hundreds of millions or billions of people could also lead to international conflict. Even geoengineering enthusiasts have admitted there's a chance of war.

The second reason why geoengineering is getting a serious hearing is that scientists are growing increasingly concerned that, even if we commit to drastically cutting emissions, we've already waited too long. By the time we actually reduce emissions, enough greenhouse gases will have accumulated to cause serious climate disasters. We may need geoengineering, then, in addition to fast cuts in emissions.

The third reason is that geoengineering is cheap, so cheap that a wealthy individual could do it. There's growing concern that unless we develop a science-based international consensus about the real dangers of geoengineering, someone will go off and do it on their own.

These last two reasons seem to have been in the back of Gordon's head during his opening remarks. "Geoengineering carries with it a tremendous range of uncertainties, ethical and political concerns, and the potential for catastrophic environmental side-effects. But we are faced with the stark reality that the climate is changing, and the onset of impacts may outpace the world's political and economic ability to avoid them," he said. "This issue is too important for us to keep our heads in the sand. We must get ahead of geoengineering before it gets ahead of us."

Not everyone is taking things seriously though. Just before the committee got underway, the ranking Republican on the committee, Ralph Hall (Texas), turned to Gordon and asked, "You can stand a little fun about that outrageous thing we're going to talk about today?" Then, during the hearing he compared geoengineering to "flying elephants."

The sequel to Freakonomics, the best-selling book that uses economics to uncover surprising facts about the world, came out today. Superfreakonomics, cowritten by Steven Levitt, a professor of economics at the University of Chicago, and Stephen Dubner, a journalist, is an attempt to outdo the original, and it does this in part by taking on a huge, controversial, and very important topic--climate change.

Unfortunately, the authors' solution to climate change, which they say is simple, cheap, and safe, is actually dangerous--a cure that could be worse than the disease. (This part of the book has already generated plenty of debate online.)

The authors set up their chapter on climate change as a challenge to global-warming orthodoxy--saying that "the movement to stop global warming has taken on the feel of a religion," putting climate-change claims in the context of past errors by scientists, and suggesting that climate models are less reliable than risk models for financial institutions that failed in the recent waves of bank closures.

So it's a little disorienting to discover that the chapter actually argues for the development of radical solutions to global warming. It argues that not enough has been done to curb greenhouse gas emissions and warns of catastrophic events like the melting of ice sheets in Greenland and Antarctica.

The solution that Levitt and Dubner put forward is geoengineering. More specifically, they advocate a scheme that would inject particles into the upper atmosphere to block a small percentage of incoming sunlight and so cool the earth--an idea that's been around since at least the 1970s. The scheme would mimic the action of big volcanic eruptions, which also inject particles into the stratosphere and have been shown to have a cooling effect.

Historically, Levitt and Dubner say, the main problem with this idea was that proposals for injecting the particles have been too expensive. They add that there might be some sort of vague environmental concerns, but label them as religious objections, not practical, science-based ones. The "moralism and angst" of these environmentalists make it hard for them to see what the authors call a "fiendishly simple" and "startlingly cheap" solution to global warming. They then describe a scheme for delivering sulfur dioxide (which will form sulfate particles) to the stratosphere and declare that it would cost $250 million for the first year and $100 million thereafter, compared to $1.2 trillion a year for reducing carbon emissions. A bargain.

Other than dismissing the potential for damage to the ozone layer, the authors don't talk about the real environmental concerns that come with sulfate injection to the stratosphere. But there are serious and specific concerns.

Scientists studying the impact of a fairly recent, large volcanic eruption--the Mount Pinatubo explosion in the Philippines in 1991--have found that not only did the layer of sulfates it produced cool the earth, it also led to a "huge change in precipitation," says Gavin Schmidt, a climate scientist at the NASA Goddard Institute for Space Studies. By decreasing direct sunlight, the event cut down on evaporation, leading to the "lowest rainfall amount over land since 1948," the earliest year that good records are available, says Kevin Trenberth, a climate scientist at the National Center for Atmospheric Research in Boulder, CO. The change in precipitation caused severe droughts that damaged crops and limited drinking water, he says. Schmidt says the potential for drought must be considered before any geoengineering is done. "What good does it do to save the Arctic if you cause the failure of the Indian monsoon on a regular basis?" he says. "That's billions of people."

The change in precipitation isn't the only known adverse affect. Shading the earth does nothing about the levels of carbon dioxide in the air. This has some benefits--plants grow better with more carbon dioxide--but it also makes the ocean more acidic, which can lead to the destruction of coral reefs around the world and prevents some shellfish and crustaceans from developing, cutting off an important source of food for fish and whales, and ultimately destroying important food sources for humans.

And then there are potential unanticipated consequences. Volcanoes inject sulfates into the stratosphere sporadically. No one knows what will happen if the sulfates become a permanent part of the stratosphere. It could very well be that major problems won't become obvious until many years or decades into a sulfate injection project. Levitt and Dubner argue that we could simply stop if problems arise. But this could be disastrous. All of the warming that's been prevented by the sulfates over the years would happen suddenly, far too fast for people to adapt.

If nothing is done to curb greenhouse gas emissions, the sulfate injection scheme will have to be kept up year after year, potentially for well over a hundred years, given the lifetime of carbon dioxide in the atmosphere. As concentrations of the gases mount, ever more sulfate will be needed to offset the warming effect, increasing costs. And the dangers of stopping the program--due to war or economic hardship or a shift in the political winds--would mount. The same holds true for another scheme the authors mention--cloud whitening, an approach that may not work and that could also lead to severely reduced precipitation over land. It is not, as they suggest, "geoengineering that the greenest green could love."

Geoengineering by shading the earth is simply not an alternative to curbing greenhouse gas emissions. In some extreme case--the impending collapse of major ice sheets, or the realization that the world is warming far faster than anticipated--it might be used to buy a little time. But even this is a risky proposition, not just because of the environmental concerns, but because of political ones, since some countries would be harmed more than others. The authors point out--in passing--that one can "imagine the wars that might break out over who controls the dials," that is, who selects how much the earth should be cooled. Oddly, they don't seem to consider this a serious objection to geoengineering.

But although the authors may be wrong in failing to point out the significant hazards of shading the earth (let alone some annoying side effects, such as obscuring the view from ground telescopes and reducing the power output from some solar power systems), they may be right that geoengineering may prove necessary. They point out that changing people's behavior is notoriously difficult, and that the uncertainty of climate predictions makes it particularly hard to set up and enforce government policies, particularly those that require international agreements. For poor countries, the uncertain cost of climate change may seem small compared to the cost of forgoing cheap electricity, at least until cheap carbon sequestration or renewable energy is available.

Donald Johnston, the former secretary general for the Organisation for Economic Co-operation and Development (OECD), has said that political realities may make strong international emissions controls impossible: "I foresee a situation about 10 years from now where the world will be warming, the new targets for greenhouse gases set [at the December 2009 United Nations climate change meeting] in Copenhagen will be ignored by many big emitters as they have in the past, and desperation will force the world to consider reducing the penetration of the sun's rays through geoengineering."

If we reach that point, we'd better have a clear idea what geoengineering might entail, so we can choose the best methods and prepare for the inevitable bad side effects. That means research must be funded to create ever more sophisticated computer models of geoengineering and to run some small- and perhaps even large-scale experiments. Also, governments need to start talking about geoengineering policy. How do you decide--and who decides--how much to cool the earth? How do you decide how to reimburse people who suffer from negative side effects? How will lawsuits be handled? What's to be done if a country decides to undertake geoengineering on its own?

This research and planning should be accompanied by continued efforts to reduce greenhouse gas emissions and, eventually, to start pulling carbon dioxide out of the atmosphere. The goal should be to shade the earth for as short a time as possible--or not at all. The only way to drive these changes is to be as clear as possible about the dangers of both global warming and geoengineering. That's going to be a lot harder with Levitt and Dubner making geoengineering sound like a panacea.

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.