Global land-ocean temperature index showing annual and five-year mean temperature anomalies since 1880. Source: NASA/GISS

Climate change caused by the release of greenhouse gases is a major issue today. According to NASA, sixteen of the warmest seventeen years in the 136-year temperature record have occurred since 2001. 2016 was the warmest year on record with an annual mean temperature anomaly of nearly 1° C. Arctic sea ice is declining at a rate of 13.3 percent per decade and sea level is rising by an average of 3.4 millimeters per year.

If no action is taken, temperatures will continue to rise leading to an increase in extreme weather and climate events such as droughts, floods, hurricanes and wildfires. According to the Intergovernmental Panel on Climate Change’s (IPCC) Fifth Assessment Report, global mean surface temperatures are projected to rise by 2.6 to 4.8° C by the end of the century if greenhouse gas emissions continue unmitigated.

Rising sea levels, stronger storms and extreme heat waves do not have to be an inevitable future outcome. There are steps humanity can take to limit climate change risks. One option is to significantly reduce greenhouse gas emissions from sources such as fossil fuel combustion and industrial processes.

Another option is climate engineering. Also known as geoengineering, it involves intentionally manipulating the climate to combat the harmful effects of climate change. Climate engineering encompasses a broad range of approaches that can be classified amongst two categories: greenhouse gas removal and solar radiation management.

Greenhouse Gas Removal

Greenhouse gas removal involves removing greenhouse gases like carbon dioxide from the atmosphere and storing them in natural or artificial reservoirs such as oceans, timber, soil or commercial materials. Biological carbon sequestration is one approach that accomplishes this goal by taking advantage of the natural photosynthetic process in which plants use carbon dioxide and water to produce nutrients for growth. Afforestation exploits the photosynthetic process by inducing forest growth in areas with previously little or no vegetation, removing carbon from the atmosphere and incorporating it into the trees’ structure. Other biological sequestration techniques involve the oceans. With ocean iron fertilization, iron is spread into the ocean in an attempt to stimulate phytoplankton blooms that remove carbon dioxide from the atmosphere. When the blooms die out, the algae sinks, trapping the carbon at the bottom of the ocean.

Phytoplankton bloom in the South Atlantic Ocean. Fertilizing the ocean with iron could stimulate blooms that remove carbon dioxide from the atmosphere. Source: NASAOcean alkalinization involves releasing alkaline compounds such as calcium carbonate into the ocean to increase its alkalinity and promote additional uptake of carbon dioxide. In addition to removing CO₂ from the atmosphere, this approach could also mitigate damage to coral reefs caused by rising ocean acidity that occurs as CO₂ reacts with water to form carbonic acid.

Another greenhouse gas removal technique is ocean upwelling. This method involves pumping cold nutrient rich water from the ocean’s depths to its surface using long pipes. This would lower ocean surface temperature and enhance carbon dioxide uptake as cooler water more readily absorbs CO₂. It would also fertilize plant life that converts CO₂ into organic matter, some of which sinks to the bottom of the ocean.

Finally, direct air capture is a technique that separates carbon dioxide from air using chemicals that bind to CO₂ in the atmosphere. The CO₂ can then be sequestered in geologic formations underground or in materials like plastic or cement. It can also be recycled as a pure gaseous product or converted for use in a fuel.

Regardless of the specific approach taken, greenhouse gas removal techniques would need to be deployed at large scales and over long time periods to have a significant effect on CO₂ concentrations. They are not expected to have an immediate impact on global temperatures. There is a delay between the introduction of removal methods and a reduction in greenhouse gas concentrations, as well as a time lag before lower temperatures are realized due to the ocean’s capacity to store heat.

Solar Radiation Management

Solar radiation management, the second category of climate engineering techniques, could have potentially more immediate effects on global temperatures compared to greenhouse gas removal. Solar radiation management involves reflecting sunlight away from or blocking it from reaching Earth.

Water easily condenses on aerosol particles, creating dense, bright white clouds that reflect sunlight. Source: NASA Increasing the reflectivity of the surface of the planet could be accomplished by a variety of means, including painting surfaces white, creating a layer of microbubbles in the ocean, planting crops with a high albedo or covering deserts with reflective plastic material.

Injecting particles such as sulfates into the upper atmosphere could create a barrier reflecting sunlight back into space. Furthermore, the natural reflectivity of clouds could be enhanced by seeding them with droplets of aerosolized ocean water, increasing the amount of sunlight reflected into space.

Solar radiation management could also be achieved by placing large reflectors in the stratosphere, in orbit or at the gravitationally stable L1 Lagrange point to shade the planet from sunlight. Deployment of a sufficient quantity of mirrors to block a significant amount of sunlight, however, could be prohibitively expensive. And even if solar radiation management could provide rapid cooling in comparison to greenhouse gas removal, it would not prevent the negative effects of higher CO₂ concentrations unrelated to warming such as ocean acidification.

Implementation Concerns

There are a variety of implementation difficulties, risks and concerns associated with climate engineering techniques. A lack of research, engineering development and experimental evidence makes it difficult to predict the feasibility and effectiveness of climate engineering.

Climate engineering techniques deployed at scales deemed practical might not be effective without also reducing greenhouse gas emissions. A study published in Nature Communications in 2014 found that a variety of climate engineering methods were relatively ineffective. When applied individually, less than 8 percent warming reductions were realized, or they had potentially severe side effects and could not be stopped without causing rapid climate change.

Unintended side effects are a major concern with climate engineering. Deployment of greenhouse gas removal and solar radiation management techniques could have unwelcome consequences at local, regional and global scales. Negative outcomes could include changes to ecosystems and biodiversity, such as the proliferation of invasive species, or alterations to water cycles and weather patterns, such as varying rainfall amounts or increased storm surges. Unwanted ozone depletion could be the result of injecting particles into the atmosphere to reflect sunlight into space, and afforestation could reduce the salt content of coastal waters, negatively impacting marine life.

Resources allocated to climate engineering might be better spent on efforts to reduce greenhouse gas emissions. But the massive scale of the climate change problem could necessitate a multi-pronged solution combining several of the greenhouse gas removal and solar radiation management techniques mentioned above in addition to emissions reductions.