The world is warming, this much we know. But exactly how much it will warm in the coming decades, and the exact effects that warming will have is still uncertain.
Equally as uncertain is humanity's ability and desire to undo what we have done.
Lately, efforts to stop the warming, or at least slow it down by reducing the amount of greenhouse gases pumped into the atmosphere are stalling, and so attention from everyone from climate scientists to Bill Gates has increasingly turned toward developing ways to counteract the effects of global warming, with the worry that it might already be too late to stop them.
These proposals at geoengineering — the intentional manipulation of the Earth's climate — range in scope from sucking carbon dioxide from the air and burying it deep in the ocean to building a space-based sunshield that would block some of the sun's radiation from warming up the Earth.
But most scientists are cautious about putting too much emphasis on geoengineering in lieu of mitigation efforts. Many are also uncertain about how well these strategies would actually work, and the potential harmful side effects that they could cause. Yet another worry is that if one group or nation decides to move ahead on geoengineering, it could cause tensions with the rest of the world.
"There's 18 reasons why it might be a bad idea; the solution to global warming is mitigation, it's not geoengineering," said Alan Robock, a climate scientist at Rutgers University in New Brunswick, N.J. "If anybody thinks this is a solution to global warming, it will take away what push there is now toward mitigation."
But others, such as James Lovelock, founder of the Gaia hypothesis — the idea of looking at the Earth as a whole instead of a set of separate systems — don’t think humanity is dedicated enough to curtailing emissions and stopping global warming and so think that geoengineering is our best bet for saving the planet and ourselves.
"I think we are almost certainly past any point of no return, and that global warming is irreversible, almost regardless of what we do in the conventional things, like following the Kyoto Protocol," told LiveScience previously.
The bottom line: Can we really afford to conduct even more experiments on the Earth given the ramifications of the biggest, albeit unintentional, experiment that we've run to date? And just who gets to make that decision?
"The trick is how do we explore what the capabilities of this technology are without: 1) taking too many risks with the climate system itself, so poking it and finding out that we don't know what we're doing; 2) without making too many political tensions;" and 3) without falling into the basic moral hazard that could develop if "people think they have a patch" for global warming that leads them not to mitigate against it, said Jason Blackstock, a physicist and expert in international relations with the International Institute for Applied Systems Analysis.
The ideas to geoengineer Earth's climate can be grouped by their lines of attack, which fall into two camps: removing carbon dioxide already emitted from the atmosphere, and trying to cool the planet by blocking solar radiation.
Some ideas proposed to get carbon dioxide out of the atmosphere include building artificial trees to scrub carbon from the air and store it; injecting carbon dioxide into wet, porous rocks deep underground to store it there for thousands of years, a process known as carbon sequestration; and dumping the nutrient iron into the ocean to stimulate the growth of algae, in the hopes that the resulting blooms of these tiny marine plants will eat up excess carbon dioxide from the atmosphere and store it in the ocean once they die and sink to the sea's depths.
Even Lovelock has proposed a geoengineering plan: He suggests helping the Earth to "cure itself" by artificially ramping up ocean mixing with pipes, which would also stimulate the growth of carbon-munching algae.
The other line of approach to the problem aims to essentially put a dimmer switch on the sun — less solar radiation hitting the Earth means less warming.
One idea is to construct a giant "sun shade" by creating an artificial ring of small particles or mirrored spacecraft that would block some of the sun's rays from hitting the Earth, thereby reducing heating. Another, which has been particularly talked about lately because it would be relatively cheap and fast to implement, is shooting tiny particles, or aerosols, of sulfur compounds into the air to reflect incoming sunlight back to space (this happens naturally after a volcanic eruption, which spews aerosols into the atmosphere in huge quantities). This approach has been championed as an emergency strategy by chemist Paul Crutzen, who won a Noble prize for his research on the ozone hole.
But the research on these plans and the technologies needed to implement them is still in its infancy. And scientists are worried about both the potential side effects that these strategies could have and that society may come to see geoengineering as a replacement for reducing greenhouse gas emissions instead of an emergency contingency plan.
The need for research
Many scientists stress that geoengineering strategies — especially aerosol injection — may not be the solution to climate change.
"The only reasonable way ever to use it would be like in the event of a climate emergency, if things were running away," Robock told LiveScience.
But despite the unease that scientists have with geoengineering strategies, they still call for more research into them, so that if the climate situation does become especially dire, humanity has a backup plan.
"We better not throw anything off the table right now," said climatologist Stephen Schneider of Stanford University. "You can't pull the plug entirely on things you may need one day."
In particular, modeling studies and small-scale lab experiments need to be done, especially in the case of aerosol injections.
"We need to understand the utility and limits of these sorts of technologies," Blackstock said.
Of course, models and labs aren't the real world: there are factors that climate models don't take into account and a degree of uncertainty included in their projections, particularly at smaller, regional levels.
"So as a result of that, there's always the possibility of a side effect," Schneider said.
Pros and cons
Each geoengineering strategy has its own set of potential benefits and risks.
If the technologies can be mustered, carbon sequestration holds the promise of taking out some of the excess carbon dioxide in the atmosphere, as well as preventing more from being emitted. But those technologies don't yet exist in any practical form. There are also worries that buried carbon dioxide could eventually leak back out from its underground tomb and once again have a warming effect.
With ocean iron fertilization, there are concerns over harming ocean ecosystems by changing the distribution of nutrients and the balance of species, and uncertainty over how much carbon dioxide such an effort would actually remove.
"That's not [carbon dioxide] removal directly, that involves messing up an ecosystem," Schneider said.
A space sun shield would be able to cool the planet, but would have an enormous cost associated with it. There the added problem that once it's in place, it's pretty much there for good. So if mitigation efforts work and carbon dioxide concentrations are reduced, such a shield could then cool the planet more than intended.
"Mirrors in space in my opinion are an absolute, must be prohibited ‘no,’" Schneider said. "You can't shut 'em off once they're up there."
Aerosol injection is one of the most discussed options at the moment, and has the advantage of being relatively cheap and easy to implement. Its cooling effects would also be nearly immediate,
But aerosol injection comes with several complications: the need to continually replace the injected particles; ozone depletion and acid rain; and the risk of causing negative climate reactions in some places.
"You can do it whenever you want, but there will be negative consequences," Robock said.
If sulfate particles are injected into the atmosphere, they won't stay there forever — eventually they fall out of the air, lasting only about a year or two. Once the particles are gone, so is the cooling effect they cause.
This effect can be seen with very large volcanic eruptions, Earth's natural form of aerosol injection. For example, the eruption of Mount Pinatubo in the Philippines in 1991 spewed 20 million tons of sulfur dioxide into the atmosphere. Aerosols that made it to the higher layers of the Earth's atmosphere caused almost 1 degree Fahrenheit (0.5 degree Celsius) of cooling over the globe during the following years. But that cooling effect went away once the aerosols settled out after about three years.
Mount Pinatubo's aerosols also contributed to ozone depletion at the Earth's poles, another big concern about attempts at artificial injection. Sulfate aerosols can also contribute to acid rain, a problem that plagued industrial areas for decades until pollution reductions began to take effect towards the end of the last century.
And while using aerosol injection as a climate manipulation would likely offset global average heating, it could have other unintended effects.
"That's the global average temperature; climate is a lot more than global average temperature — it's weather patterns, precipitation patterns," and much more, Blackstock said.
And the uncertainties of geoengineering strategies, particularly aerosol injection, are compounded by the fact that "we have one subject to test it on — we have the world," Blackstock added.
One scenario in which aerosol injection could be used would be in the case that the effects of global warming end up on the worse end of current projections, in which case we may need a quick solution to stop at least some of the effects. In this case, aerosol injection might be a temporary solution while humanity works at developing carbon removal technologies, Schneider said.
Part of the problem with considering any geoengineering solutions is the ease with which one group of people could decide to start large-scale experiments that could have a global impact.
To make sure that any geoengineering strategies and their potential impacts are well-understood, "scientists are aware that we need norms and ethics and best practices for how to do this research," Blackstock said.
But understanding the science isn't enough.
"At the same time, we need to be building that same sort of discussion among the political, policy, decision-making crowd," Blackstock added.
While current modeling efforts and small-scale research aren't likely to cause international tensions, later larger-scale efforts could. For example, a true effort at aerosol injection could have impacts not just in the country where the aerosol is released, but in other regions of the world — for example, some models suggest that aerosol injections would cause drought conditions in parts of Africa — those affected countries could perceive such tests as a threat.
"My biggest worry about geoengineering is less the side effects than it is what happens when nations perceive this as a hostile act," Schneider said.
Recent attempts by private companies to experiment with iron fertilization have already caused tension with other countries and environmental groups. Part of the problem being that there are no international treaties or regulations governing anything like a geoengineering experiment.
"One country could do it without asking anybody else, and there's no really clear international law on that or enforcement mechanism," Robock said.
Exactly how the world should oversee geoengineering research and its potential implementation is something that nations have yet to really tackle.
"What is essential to me is that we have a first-use treaty," Schneider said. Such a treaty would stipulate that "no country, no group of countries can practice large-scale geoengineering on their own."
But others aren't sure how international agreements will work out, given humanity's mixed record: While the Montreal Protocol was largely successful in reducing the use of ozone-destroying chemicals, the Kyoto Protocol and its successors have had little impact on greenhouse gas emissions.
"This is a challenge that we don't have a good answer to right now," Blackstock said. "The existing mechanisms aren't all working for the challenges that we're facing right now."
Lack of understanding
Another worry is that public perception won't reflect the current scientific understanding on geoengineering. This underscores the need to have discussions about geoengineering in the public sphere, with scientists and policy makers communicating developments to the public.
"It all needs to be very transparent and public, including the technologies that are developed," Blackstock said.
When scientific understanding isn't well communicated to the public, it can lead to backlash, as has been seen with such things as the ban of foods from genetically modified crops in Europe. If large-scale testing of geoengineering begins before the public has even heard much about the various ideas, "it can raise unwarranted concerns," Blackstock said. "Once those concerns exist, once there's a certain perception about these issues, it may become very hard to shake."
For the time being though, no geoengineering strategy is ready for the big time, and scientists and policy makers are becoming more aware of the need to inform themselves on these strategies and discuss them in a more international setting.
The U.S. House of Representatives and the British Parliament have both held hearings on geoengineering in recent months, with experts testifying on the merits and risks of geoengineering. Scientists and policy makers are also meeting in Asilomar, California in March to discuss the merits of geoengineering and how to build international cooperation on the matter.
Meanwhile, research into geoengineering continues, which will also give humanity more information to make the decision on whether or not any of these strategies is warranted, and if so, which ones should be used. For now, the future direction that climate action will take is anybody's guess: If we begin reducing emissions, we could avoid some of the worst predictions, but then again, we might be too late.
"I think in the next five or 10 years there will be a lot of action [on mitigation], the question is, 20 years from now, in spite of what we do in the next five or 10 years, will there still be too much climate change and will we need to do geoengineering for a decade or so while we continue to solve the problem. And we don't know yet what the probability of that is," Robock said.
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Andrea Thompson is an associate editor at Scientific American, where she covers sustainability, energy and the environment. Prior to that, she was a senior writer covering climate science at Climate Central and a reporter and editor at Live Science, where she primarily covered Earth science and the environment. She holds a graduate degree in science health and environmental reporting from New York University, as well as a bachelor of science and and masters of science in atmospheric chemistry from the Georgia Institute of Technology.