Many scientists believe that global warming caused by carbon dioxide buildup in the atmosphere is for all practical purposes irreversible. That’s because the current concentration of carbon dioxide in the atmosphere will keep the engine of global warming running for thousands of years. Consequently, the common goal of keeping our planet from heating up more than two degrees Celsius before the end of this century, as agreed upon by the stakeholders in the 2016 Paris Agreement, will be extremely difficult to achieve, unless we go “carbon negative” by mid- to late twenty first century.(Carbon negative, or negative emissions,means removing more carbon dioxide from the atmosphere than adding it.) Scientistshave, therefore, come to the conclusion that the only way we can tackle global warmingis via geoengineering, also called climate engineering.

Geoengineering is defined as “human’s planned measures” to reverse or forestall some of the adverse effects of climate change. It encompasses two different classes of approaches using a variety of cutting-edge technologies to undo the effects of two centuries of anthropogenic greenhouse gas emissions.They are:

1. Removal and sequestration of carbon dioxide to lower its concentration in the atmosphere;

2. Offsetting global warming by blocking some of the Sun’s rays from ever reaching the Earth’s surface via a program called solar radiation management.

Carbon Removal: The idea of removing carbon dioxide from the air was first proposed in 2008 at a workshop sponsored by the US National Science Foundation. The idea is simple. Emulate what trees and plants do already. They take in water and carbon dioxide from the atmosphere and through photosynthesis convert them into oxygen and organic compounds. Accordingly, there are twocarbon dioxide removal (CDR) approaches‒ Direct Air Capture (DAC) and Bioenergy with Carbon Capture and Sequestration (BECCS).

A pilot project for DAC was started in 2009 by aVancouver-based company called Carbon Engineering. A giant network of fans is used to suck ambient air into a “gas-liquid contactor,” where a strong hydroxide solution reacts with carbon dioxide in the air to form carbonate pellets. The pellets are processed and converted back into carbon dioxide and water vapor, which could be used as synthetic fuel, or stored in porous rocks or depleted oil wells for later use.

A Swiss company called Climeworks compresses the captured carbon dioxide and uses it as fertilizer, or manufacture fuel and carbonated soft drinks. The long-term goal of the company is to capture at least one percent of the global annual carbon dioxide emissions by 2025. Industry experts estimate that in order to meet this goal, approximately 250,000 DAC plants would have to be built.

The other CDR approach, Bioenergy with Carbon Capture and Sequestration, was proposed in 2001 by a doctoral student from Sweden. At its most basic, BECCS involves growing crops, burning them to generate electricity, capturing the carbon dioxide emitted during combustion and storing it deep down into the Earth’s crust.

In little more than a decade, BECCS had gone from being a highly theoretical proposal to being one of the most viable and cost-effective negative emissions technologies. According to the International Energy Agency, there are currently about two dozen BECCSpilot projectsoperatedby big companies like Shell, Chevron and ArcherDaniels Midlandthat capture and store around 0.1 percent ofthe total global emissions of carbon dioxide. The amount, albeit miniscule, is nonetheless an indication that the technology is a promising one.

Solar Radiation Management: A range of options for solar radiation management (SRM) has been proposed. They include reflecting sunlight away from the Earth by an occulting disk or space-based mirrors to seeding clouds so that they become more reflective to simulating the effects of volcanic eruptionsor asteroid impact to building homes with white roofs.

Occultation: Perhaps the most practical but challenging SRM concept to reduce solar insolation wasadvanced in the early 2000s by space scientists at the Lawrence Livermore National Laboratory in California. They proposed deployinga large occulting disk that would act as a sunshade at a gravitationally stable point, called the Lagrange point, between the Earth and the Sun. Calculations indicate thata disk roughly the size of Greenland would be able to block one to two percent of the sunlight ‒ enough to reduce substantially the amount of solar radiation reaching the Earth.

Another suggestion involves deploying an array of reflecting mirrors covering the equivalent shading area of the occulting disk. By incorporating the effect of mirrorsto different climate models, researchers at the Max Planck Institute for Meteorology in Hamburgobtained an overly optimistic result ‒ the average global temperature could be lowered to preindustrial levels, although “unevenly.” Sea levels would still rise because they respond slowly to changes in Earth’s temperature.

As an aside, NASA is currently working on solar sail propulsion system that would use large mirrors to harness solar radiation and redirect it towards Mars in order to initiate greenhouse effect on the planet. These mirrors could also be used to reflect sunlight away from the Earth.

Cloud Seeding: The fraction of incident solar radiationthat is reflected back into space by a non-luminous body is determined by a quantity called albedo. The albedo of Earth is thirty percent, of which clouds account fortwenty percent, air for six percent and surface(land and water) for four percent. Sincethe albedo of clouds is high, theyhave greater cooling effects locally as well as globally.

Albedo can readily be changed by human action. Hence, it should be possible to increase the albedo of low-level cloudsby spraying seawater into the airwhere they would evaporate to form sea salt which wouldseed the clouds above the oceans. The best part of this plan is that it involves spraying seawater instead of harmful chemicals into the air.

At the University of Washington, an international research collaboration of atmospheric scientists called The Marine Cloud Brightening Project (formerly Silver Lining) developed a machine that can suck up ten tons of seawater per second, turn them into tiny particles and shoot them up over a kilometer into the air. They are currently conducting limited area field experiments with the spray technology to “provide new understanding of the interactions between aerosols and clouds.”

The technology has a long way to go before it can significantly lower the Earth’s temperature. A recent study showed that about 2,000 ships equipped with the spray machines would have to ply the seas and oceans just to stop the temperature from rising.

Another proposed cloud-based approach involves thinning the high-altitude Cirrus clouds by seeding them with heterogeneous ice nuclei. While this method is not technically an example of SRM, thinner clouds wouldallow moretemperature-raising trapped infrared radiation to escape to space, and thus, potentially cool the Earth’s climate.

Simulating the Effects of Volcanic Eruption: Gaseous and minute solid particles, such as sulfate aerosols, injected into the atmosphere after a massive volcanic eruption or a series of lesser intensity eruptions could linger as long as three to four years in the stratosphere ‒ a layer of the atmosphere extending to about 50 km above the Earth’s surface. As a result, the Earth-shrouding gases and aerosols could impact global climate by increasing the Earth’s albedo. As an example, the abnormally cold summer of 1783, both in Europe and in the US, is attributed to the enormous eruptions of a chain of volcanoes in Iceland that lasted for eight months.

Indeed, inspiredby the 1783 eruptions and the eruption of Mount Pinatubo in the Philippines in 1991 and thesubsequent cooling effect of their sunlight-blocking plume of sulfate particles, scientists are considering injecting sulfate aerosols into the stratosphere.In the year following the Pinatubo eruption, global temperatures did cool by approximately 0.5 degree Celsius.

Asteroid Impact: Approximately 65 million years ago, an asteroid impact cooled the Earth by probably four or five degrees. Since the possibility of an asteroid slamming into Earth during our lifetime, or in the next 100 years or so, is very remote, space scientists at the University of Strathclyde in Scotland suggest an out-of-this-world solution ‒ blasting off a few near-Earth asteroids, so that the resulting dust cloud in space would protectthe Earth from excessive solar radiation.

Under the auspices of Asteroid Redirect Mission, NASA is developing a robotic mission to collect a large boulder from an asteroid’s surface and redirect it into a stable orbit around the moon. If the mission succeeds, the technique could be used to generate a dust cloud by steering asteroids into a collision course with each other.

Surface-Based Option: A technology with a proven record of success in mitigating global warming is white roofed houses. Since light-colored surfaces such as white have a high albedo, theseroofs keep the Earth cool by reflecting more sunlight back to space. In fact,according to the article “Economic comparison of white, green, and black flat roofs in the United States” published in the March 2014 volume of Energy and Buildings, researchers at the Lawrence Berkeley National Laboratory in California noted that a simple white roof reflects three times the sunlight as a green rooftop garden. Theyalso presented evidence that by absorbing less sunlight, a 100 square meter area of rooftop painted white has about the same one-time impact on global warming as cutting ten tons of carbon dioxide emissions.

Time Frame of the Programs’ Effectiveness: Once geoengineering program starts, the completion time of the CDR and SRM approaches in tackling global warming would be different. Most of the SRM approaches would act fast, producing a detectable climate response within months, scientists conclude. By contrast, the CDR approacheswould be slow to reduce climate risks, requiring decades to make an appreciable impact on atmospheric concentrations of carbon dioxide.

Nevertheless, scientists almost unanimously agree that if SRM and CDR programsare implemented in tandem, then even a reduction of two percent of solar radiation could offset the effects of two degrees Celsius increase in temperature. Otherwise, we may not be able to achieve the desired result.

Challenges and Environmental Implications: Out of the many approaches, DAC and BECCSare perhaps the most benign form of geoengineering. However, as BECCSuses a relatively clean source of fuel (energy-generating biomass)to produce negative emissions, it is a clear-cut winner over DAC and other technologies designed to combat global warming.

Achallenge for DAC is that the atmosphere blanketing the Earth is very big, and carbon dioxide is a relatively small part of it ‒ about 0.04 percent. That’s why the technology will work effectively only in the vicinity of power plants where the gas is emitted in large concentrations.

Another area of concern with DAC is energy efficiency. Carbon dioxide itself is not a very reactive molecule, so extracting it is both energy and resource intensive. Based on a 2011 report prepared by the American Physical Society, it is estimated that to extract a billion ton of carbon dioxide, a figure viewed by many experts as climatically significant, maximally efficient DAC systems would require about 10 GW of power. This is equal to about three times the capacity of the Palo Verde Generating Station ‒ largest nuclear power plant in the US.

If BECCS is to succeed on a wide scale, its demand for land could be massive, especially if only exclusive crops, such as corn or soybean,are used as raw materials. In that case, BECCS plants will start eating into our food supplies. This problem can be alleviated to some extent if the plants use waste products in agriculture, animal farms and forestry, too.

In order for bothCDRtechnologies to be feasible, it is crucial that the amount of carbon dioxide removed is appreciably greater than the amount of carbon dioxide emitted in the removal process.In addition, theCDRschemes could find themselves in a continuous game of catch-up with the voluminous output of greenhouse gases, unless we rein in on their emissions.  

Despite the many benefits, some of the SRM approaches have limitations and they are also a route that could adverselyaffect the environment. While the magnitude of the consequences is generally proportional to the scale of deployment of the technologies, several issues ‒ scientific, environmental, ethical, economic and political ‒ are yet to be resolved.

In the case of occultation, the potential forunintended consequences, such as drought, arehigh. In particular, studies show that a reduction of 1.7 percent insolation could bring about important changes to regional climates, with warming at high latitudes while cooling below necessary level in sub-tropical regions. These residual changes to regional climates may cause important damage to the local ecosystems and economies, too. The prohibitively large cost of space transportation and the high number of launches required to deploy the occulting disk(s) and mirrors are also seen as shortcomings of this program.

Seeding low-level clouds with seawater could produce changes in regional rainfall amounts and patterns, as well as changes in ocean currents. A decrease in rainfall, which is a conclusion of several studies, would significantly reduce agricultural yield. On the other hand, targeting Cirrus clouds could result in changes to the precipitation even in regions far away from geoengineered regions, underscoring the risks of remote side effects.

Spraying the stratosphere with sulfate aerosols could also have “catastrophic effects in parts of the world already battered by natural disasters,” as noted in a research article published in Nature Communications in 2016.The authors caution that it may increase the frequency of cyclones and droughts in some parts of the world.The aerosols could also deplete the ozone layer that protects us from the harmful ultraviolet radiation. Afterthe Mt. Pinatubo eruption, there was a three percent reduction in the amount of ozone in the atmosphere and a measurable decrease in rainfall in some parts of the world.

As for the dust cloud produced by breaking up asteroids,the particles in the cloud run the risk of getting dispersed over time by solar radiation and the gravitational pull of the Sun, moon, satellites and nearby planets.They could also interfere with the operation of communication satellites orbiting the Earth.

A fundamental problem with SRM approachesis that they would require continual refreshing. Additionally,once they are put into operation, they have to be continued indefinitelyin order to counterbalance the forcing associated with greenhouse gas emissions. SinceSRM only offsets warming, once the programsare stopped following their deployment, temperature changes caused by greenhouse gases would manifest themselves suddenly and could rise beyond the level they otherwise would have by 2100.  Risks and Concerns: Although the prospect of cooling the Earth seems real, unease surrounds the questions pertaining to ethics, costs, limitations, feasibility, benefits and risks of geoengineering. To the critics of geoengineering, the wisdom of the program remainshighly controversial because of underlying scientific and technological uncertainties.They are particularly concerned about the SRM approaches which are still at the level of relatively simple analyses, small-scale laboratory experiments and preliminary computer simulations. They are also apprehensive that the governance and financial challengeshave not yet been fully studied. One of their strongest fears though is that geoengineering technologies may divert resources and momentum away from already waning efforts to reduce emissions of carbon dioxide.

Moreover, critics are troubledat the thought thatour attempt to control the Earth’s climate system by deliberate, large-scale manipulation ispossiblya matter of hubris rather than a desirable evolution.Besides, before shifting the gear to overdrive, they would like to know whether the advantages of geoengineering outweigh the risks of climate change and how it would alter humanity’s delicaterelationship to Nature.

More importantly, critics believe that within social and political context, deployment of space-based technologies has the risk of “reckless pursuit of self-interest by powerful actors” on the world stage. In other words, the specter of incompetent, negligent, or even malicious uses of the yet-to-be fully developed SRM technologies by rogue leaders alarms the critics.

Proponents of geoengineering hold that because the Earth’s atmospheric system is large and complex, it is impossible to anticipate fully all the consequences, detrimental and beneficial, of intentionalintervention in advance.They, however, believe that through dedicated research and critical discussions with the global community, it would be possible to develop an understanding of the technical, political, social, cultural,environmental and ethical issues pertaining to geoengineering. At the same time, they are worried that research on geoengineering could be hijacked by climate change deniers, such as the US administration led by a self-proclaimed “very stable genius.”

Furthermore, if the past is any indication of human apathy towards Nature, it is exceedingly likely that once we design a system that would reverse global warming, we may no longer feel any incentive to change our carbon-emitting lifestyles. Eventually the problem will just build up and we will once again be back at square one.

In conclusion, we are at a crossroad. We have to decide whether to allow the Paris Accord run its course while our planet heats up threatening our very existence. Or should we resort to geoengineering to manage the climate the way we have managed so many other things successfully.

The writer is a Professor of Physics at Fordham University, New York