“Climate change.”

It’s a popular term these days, cropping up in the news and online discussions, in books and movies.  The ubiquity of the topic reflects the importance of the issue, and many of the proposed solutions to climate change are technological, ranging from practical to science fictional.

Climate engineering, or geoengineering, is the catchall term for these global-scale interventions in Earth’s climate, intended to mitigate or eliminate the effects of global warming. Most geoengineering efforts fall into one of two categories: greenhouse gas removal or solar radiation management. There’s a lot of active research happening in both these areas with engineers often at the forefront, designing and building the tech to test—and possibly eventually implement—these methods.

But is altering the Earth’s climate on such a large scale a good idea? Is it even feasible? Engineers and scientists across the world are designing new technologies, running simulations and testing theories to try and answer these questions.

Three geoengineering proposals that look promising are: direct air capture to remove carbon dioxide from the atmosphere, releasing reflective stratospheric aerosol particles into the upper atmosphere, and obstructing solar radiation with a space-based mirror or sunshade. All three methods appear promising as a way to manage Earth’s climate, but also may come with significant detrimental side effects.

Carbon Dioxide Direct Air Capture

Direct air capture (DAC) processes remove carbon dioxide from the ambient air in order to sequester and store it, and at a large enough scale this technology is believed to significantly reduce the amount of CO2 in Earth’s atmosphere. This would help mitigate the effects of global warming and the greenhouse effect, to which CO2 is a main contributor.

While there are already projects involving air scrubbing technology that removes CO2 at the source—that is, in factories, power generation stations or gas fields—these efforts only remove CO2 before it enters the atmosphere, and do not lower the existing atmospheric CO2 levels.

Existing CO2 scrubbing at the single-source level uses chemical reactions, usually an alkaline substance that can absorb CO2, to take the CO2 out of the air and sequester it, after which it can be removed and stored or turned into fuel. Similar chemical scrubbing techniques can be incorporated into DAC systems, making these one of the more immediately achievable climate engineering proposals.

In fact, there is already a DAC plant operating commercially in Switzerland, which opened in May 2017.  The plant looks like a wall of giant fans that sucks air in through filters which absorb the CO2. In this instance, the removed CO2 is used as fertilizer in nearby greenhouses, but other options for using the CO2 as fuel are also possible.

Pros & Cons

Some of the advantages to DAC systems include their scope, their mobility, and their feasibility with current technology.

Scope and Reach

DAC systems have an advantage over scrubbers incorporated directly into a power generation or factory facility because DAC can effectively mitigate CO2 emissions from multiple sources, such as vehicles, homes, unscrubbed facilities and other sources that are difficult or impossible to use single-source level CO2 removal.

Mobility

Since DAC systems target removing CO2 from ambient air rather than being attached to a single source of CO2, they can be built and deployed anywhere in the world and remain effective. This means there are opportunities to build these facilities in areas where the environment is otherwise unsuitable for agriculture, human habitation or other natural CO2 mitigation efforts such as reforestation and afforestation.

Technologically Feasible and Accessible

As seen in the Switzerland DAC plant, these systems are technologically feasible with the knowledge and equipment available today. With an investment of money and effort, these plants could be built and operating in much less time than other, more prospective solutions.

There are, of course, some disadvantages requiring consideration, including build and operation costs, storage concerns and energy use.

Build and Operation Costs

DAC systems are large-scale infrastructure projects requiring design and construction of roads, building facilities, equipment and machinery in order to make these systems a reality. This would require dedicated investment as well as international and intergovernmental collaboration, both of which can be hard to come by.

Storage and Post-Processing

One of the main concerns of installing and operating DAC systems at a global scale is what to do with the CO2 after it has been removed from the air. Underground storage brings concerns over possible leaks and other environmental damage or contamination, as well as questions of responsibility for the long-term operation and monitoring of storage sites.

Overall Energy Use

Any type of facility requires energy input to operate, and one of the main concerns of DAC systems is that they will be so energy-intensive that the CO2 produced by power generation plants to run the DAC system will still outpace any CO2 removal the DAC system performs.

Reflective Stratospheric Sulfate Aerosols

The idea for this solar radiation management (SRM) technique didn’t materialize out of nowhere—rather, it’s inspired by a natural process that occurs during volcanic eruptions.

Natural aerosols are made up of tiny particles less than a millionth of a meter in size and are suspended in the air—in this case, Earth’s atmosphere. A certain amount of these naturally occurring aerosols are already present in the atmosphere and they serve to scatter and reflect a certain amount of sunlight back into space, rather than letting it through to the planet’s surface. This results in a cooling effect that helps regulate Earth’s climate.

Significant geoengineering research is currently underway in this area, pursuing the idea of artificially injecting particulate aerosols into the atmosphere in order to mimic the natural processes of volcanic eruptions. The commonly cited natural example is the 1991 Mount Pinatubo eruption that released several million tons of aerosols and dust into the atmosphere, which noticeably affected how much sunlight reached the Earth’s surface and resulted in a 0.4C (0.7F) decrease in the overall global temperature over the following year.

The geoengineering proposal is to inject an increased amount of aerosol particles into the stratosphere, which will reflect a greater amount of sunlight away from the planet, maintaining a cooling effect over the long term that will offset global temperature increases.

While this wouldn’t necessarily be a permanent solution—other carbon-emission-cutting and CO2 removal measures would still need to be undertaken—using stratospheric aerosols could be enough to give humans a grace period of several decades before climate change reaches the irreversible point, allowing time for these carbon reduction methods to be in place and take effect.

Most proposals suggest dispersal methods including releasing the compounds from high-altitude aircraft or balloons, or even fired from high-powered artillery.

Pros & Cons

The proposed use of atmospheric aerosols has the potential for a much greater effect on Earth’s climate, which brings with it more significant advantages.

Imitates a Natural Process

Much of the popularity of this method of solar radiation management comes from the fact that it mimics a natural process. This makes aerosol-based proposals feel less risky than some other techniques, as there is a large body of scientific knowledge and observational data on the effects of volcanic eruptions that can be used to develop and manage the artificial injection of aerosols.

Accessibility and Low Cost

Where many other SRM and CO2 removal proposals require costly technology to be developed or have high operating costs, aerosol dispersal is possible with technology we already have available, such as aircraft, high-altitude weather balloons, artillery, and the chemical manufacturing capabilities to produce the necessary compounds.

Effective and Reversible

Thanks to the observational data on natural aerosol cooling, it’s believed that the aerosol-based SRM solution is a good choice because it’s likely to be very effective at cooling the climate. The effects from natural processes are also temporary, usually dispersing within a few years. Proponents of this SRM solution cite that the effects of artificial aerosol injection are likely to also be temporary. Also, if the attempts are ineffective or show detrimental outcomes in the early stages, it’s believed the compounds will work their way out of the atmosphere within one to four years of halting the process.

With the greater potential effectiveness, however, there are also some significant potentially detrimental effects to the aerosol injection method.

Depletion of the Ozone Layer

Sulphur aerosols are known to contribute to the depletion of Earth’s ozone layer, combining in the atmosphere to produce sulfuric acid. Ozone depletion means lower absorption of the sun’s ultraviolet radiation, which among other effects will increase the risk of skin cancers and eye damage in humans.

Disruption of Natural Processes and Weather

One of the primary concerns is the effect that adding aerosols to the atmosphere will have on natural processes such as the hydrological cycle, atmospheric air circulation, and local weather events. These predictions have been borne out by computer models and simulations, with results including extreme weather events such as droughts, temperature fluctuations, storms and flooding increasing in both frequency and intensity.

Side Effects of Stopping the Process

While some researchers consider this process to be reversible and therefore safer to attempt, there is other research to support the idea that stopping the use of aerosols after time spent deploying them could result in a reactionary rise in climate temperatures faster and higher than before the attempted geoengineering. Effectively, this means there is the possibility that once we start this process, we may not be able to stop without causing even greater damage to the climate and local environments.

Space-Based Solar Shield

Possibly the most technical and least feasible option for SRM, the idea of a solar shield in space that blocks out sunlight remains a popular proposal. It’s also the most science-fictional proposal, since at the current time we don’t have advanced enough technology and space capabilities to build, install and operate something such as this.

The idea of a solar shield designed to divert or block sunlight away from the planet was first proposed in 1989 by James Early, and involved plans for a large occulting disk to sit in space at the Earth-Sun L1 Lagrangian point.

Technology has advanced beyond Early’s initial design, but the basic premise remains the same—as does the general level of unfeasibility. Rather than a large occulting Fresnel lens, newer versions of a solar shield propose the use of clouds or fleets of small spacecraft outfitted with mirrors or transparent lenses that will deflect sunlight away from Earth, or the use of a thin wire mesh to act as a diffraction grating between the Sun’s light and Earth. Other proposals suggest creating a cloud of lunar dust between the Sun and Earth, which would offer similar sunlight-deflecting capabilities.

Most simulation models indicate that even a 2 percent decrease in the amount of sunlight to reach Earth would potentially be enough to offset the effects of atmospheric CO2-induced warming.

Pros & Cons

However, a solar shield is an extreme solution. While it does offer some benefits, it also introduces some significant drawbacks.

The advantages to this method of SRM include how quickly and effectively we’ll see results once the shield is in place, as well as the fact that a solar shield could offset almost any degree of temperature rise.

Quick to See Results

Because the solar shield is something of an all-or-nothing solution, there would be an almost immediate and noticeable change to Earth’s climate once the shield was in place, with some simulations estimating a significant reduction in the Earth’s average atmospheric temperature within just a few years.

No Upward Limit to Temperature Reduction

Depending on the specific solar shade technology used, there is theoretically no upward limit to how much global temperature rise that could be counteracted with this method. For example, by adding more tiny, mirrored spacecraft to the orbiting fleet, or by increasing the amount of lunar dust distributed around the planet, any increase in climate temperature could be offset.

The effectiveness and immediacy of a solar shield also means the potential detrimental effects are significantly more concerning.

Astronomical Costs

Most of the technology needed for any form of space-based solar shield would require an astronomically high monetary investment from multiple countries and governments in order to become a reality—think in the realm of tens of billions, if not trillions, of dollars—just to develop, build and launch a project of this scope. Ongoing costs for operation and maintenance of the shield would equal these costs, or even exceed them.

Not Timely

Since the technology isn’t available at this point, there is a significant and unavoidable delay in any efforts to deploy a solar shade. With the state of tech today, it would take decades to develop the necessary launch and space assembly vehicles, as well as the shield itself—not to mention the time needed to complete the launch and in-space installation.

Detrimental Environmental and Climate Effects

Perhaps the most concerning of the drawbacks to a solar shield system are the potential knock-on effects that will change the climate from the global scale right down to local weather. As with the other methods discussed, while the intention is to create beneficial changes, there is a high likelihood that a geoengineering solution as significant as a solar shield will cause wide-ranging problems related to extreme weather, droughts and floods, damage to agricultural production, desertification and more—to the point that any beneficial effects are counteracted by worse and more frequent problems.

Research and Testing Is Worth Pursuing

These are only three of the more popular climate geoengineering proposals to be pitched over the past few decades, but they serve as a good example of the challenges any large-scale geoengineering project will face. Every method has its proponents and detractors, as well as pros and cons, which is why it is essential for engineers and scientists to pursue in-depth research now in order to determine how we might put these geoengineering processes into effect, and what the best-case and worst-case scenarios might be.

A common concern to all methods of climate geoengineering is that people may take these solutions to be final, rather than just a stop-gap measure to control the rate of climate change to give the world time to curtail the production of greenhouse gas emissions and pursue reforestation efforts that can halt human-caused climate change entirely. 

"There's no way around reducing emissions," states Stefan Schaefer, climate engineering program leader at the Institute for Advanced Sustainability Studies in Potsdam, Germany. "Without that, none of these techniques can do anything useful. None of them can be a silver bullet, but investing some money to research these approaches makes sense."