In a 2002 essay, Dutch atmospheric chemist Paul Crutzen popularized the name “Anthropocene” for the current geologic time period. Though unofficial, it is widely accepted by scientists because of how drastically humans have altered the Earth. We have cleared forests, built cities, diverted waterways, replaced wild animals with domestic ones, created billions of tons of plastic waste, and perhaps most significantly, changed the climate.
The average global temperature is increasing faster now than at any time in the last 2 million years. The 10 hottest years on record are all in the 21st century, and it is predicted that future years will be even hotter. This has fueled record-breaking droughts, heat waves, and wildfires. As the air and oceans warm and the water cycle accelerates, weather patterns have intensified, causing more extreme and damaging hurricanes and rainfall.
Climate change is driven by the greenhouse effect: greenhouse gases in the atmosphere act like a blanket and prevent heat from escaping the Earth, increasing the average temperature. Carbon dioxide (CO2) is the primary greenhouse gas emitted by humans when we burn fossil fuels like coal, oil, and natural gas to produce energy.
Around 80% of energy in the United States and the world is currently produced by burning fossil fuels. The rate of increase of CO2 in the atmosphere will slow if we switch to energy sources that don’t release greenhouse gases like solar, wind, water, and nuclear energy. However, because CO2 can persist in the atmosphere for hundreds of years, the CO2 already in the atmosphere will continue to exert warming effects for centuries.
Figure 1. The Greenhouse Effect: Incoming sunlight is partially reflected but mostly absorbed by the Earth. Some of the heat absorbed by the Earth is radiated out to space, but greenhouse gases act like a blanket and trap the heat, increasing the planet’s average temperature. Carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), and water vapor are all greenhouse gases. CO2 is a stable atmospheric gas, and its warming effects persist for decades.
We have changed the Earth’s climate so dramatically that parts of the Earth are becoming uninhabitable. Can we take steps towards reversing these effects?
Geoengineering is the large-scale modification of Earth’s climate. The goal is to reduce some effects of climate change, such as rising temperatures and extreme weather. Although the best way to slow climate change is to address the root cause and reduce greenhouse emissions, geoengineering is worth discussing and researching because countries have been cutting their emissions too slowly to make any near-term impact. This post will discuss some of the proposed geoengineering solutions.
Giving Earth Some Shade
Greenhouse gases prevent sunlight from bouncing back out to space, heating the Earth. To counter this effect, several methods have been proposed to cool the Earth by reflecting more light away. These methods are collectively called solar geoengineering.
Reflecting Sunlight with Sulfate Aerosols
One solar geoengineering method involves spraying sulfate aerosols into the atmosphere to reflect sunlight. Sulfate aerosols are tiny particles that are released from volcanoes and desert dust. They reflect more sunlight than they absorb, which helps cool the Earth. Sulfate aerosols are also produced by burning fossil fuels and offset a portion of global warming caused by greenhouse gas products of fossil fuel combustion.
When sulfate aerosols are released at ground level during fossil fuel combustion, they cause dangerous levels of air pollution. So scientists are experimenting with releasing sulfate aerosols into the stratosphere, where they are too high to cause significant pollution damage. Aerosols could be sprayed from airplanes or tethered hot air balloons.
The main advantages of this method are speed and reversibility. Additionally, sulfate aerosols are relatively well studied because they exist in the atmosphere already. However, the stratosphere contains a layer of ozone, a gas that absorbs the most harmful types of ultraviolet radiation from the sun, and it’s possible that sulfate aerosols could initiate ozone-destroying reactions, allowing more ultraviolet rays to reach the Earth.
To avoid the potentially negative chemistry of sulfate aerosols, scientists are looking into using aerosols made of other substances like calcium carbonate (limestone). In addition to reflecting sunlight, calcium carbonate may be able to neutralize ozone-destroying reactions. But unlike sulfate, calcium carbonate doesn’t exist naturally in the atmosphere, so scientists need to further research its effects.
Reflecting Sunlight with Clouds
Another cooling method involves brightening clouds above the oceans. In general, darker objects absorb light while brighter objects reflect it. Oceans are dark colored, so they absorb sunlight. Brightening clouds above oceans would cause the clouds to reflect more light away before it hits the oceans below, causing less sunlight to be absorbed.
Figure 2. Atmospheric Layers: The Earth’s atmosphere is comprised of five distinct layers. The troposphere, the closest layer to the Earth’s surface, is where weather occurs. Because clouds are found in the troposphere, cloud engineering occurs here too. The next layer is the stratosphere, which contains the ozone layer and is where commercial airplanes fly. Light-reflecting sulfate aerosols can be sprayed into the stratosphere to avoid polluting the tropospheric air that we breathe.
A cloud’s brightness depends on the size of the water droplets that comprise it (smaller droplets have more surface area per unit volume than larger ones, so they scatter more light and appear brighter). To brighten clouds, tiny seawater aerosols could be sprayed over the oceans, so that small water droplets form around them. The smaller the aerosols, the smaller the droplets that stick to them.
However, changing the droplet size could also affect how long the clouds last and how much water they can hold. Because clouds form in the lowest level of the atmosphere, called the troposphere, cloud brightening has a greater risk of affecting weather patterns than spraying sulfate aerosols high into the stratosphere.
The primary knowledge gap of solar geoengineering is in how rapid sunlight changes will affect other aspects of climate besides temperature. This can be studied in climate models, but it is difficult to test in the real world. There is also concern that the drop in sunlight may decrease plant growth, thereby increasing the amount of atmospheric CO2 and reducing crop yields.
Solar geoengineering aims to cool the Earth by reflecting sunlight, but it does not actually remove CO2 from the atmosphere. It does not address other issues caused by excess atmospheric CO2 like ocean acidification. From this perspective, the following geoengineering methods are attractive because they address the root cause of climate change.
Engineering the Oceans
Another major problem of too much CO2 in the atmosphere is ocean acidification, which occurs when CO2 dissolves into the ocean and makes carbonic acid. Acids dissolve alkaline (i.e. basic) materials, including the hard shells of many ocean animals like corals. Without their shells, these animals die. Additionally, many animals without shells are also sensitive to changes in acidity and can’t survive outside a narrow range.
A high concentration of CO2 in the oceans causes even more CO2 to accumulate in the atmosphere. Oceans currently absorb 25% of the CO2 that humans release into the atmosphere, but the amount of CO2 that oceans can absorb falls as its concentration increases.
Ocean geoengineering consists of two main methods: fertilization and alkalinization. They both ultimately reduce the amount of CO2 in the atmosphere, so they have the potential to reduce both ocean acidification and global warming.
Ocean fertilization involves supporting the growth of phytoplankton, which convert CO2 into oxygen through photosynthesis. Photosynthesis is the chemical reaction that plants use to convert carbon dioxide into sugar, which they use for energy. In this process, CO2 is removed from the atmosphere and oxygen is released. Microscopic phytoplankton perform around 50% of the world’s photosynthesis.
Just like fertilizer can be added to gardens to help plants grow faster, different ocean fertilizers can help phytoplankton grow faster and consume more CO2. When the phytoplankton die (if they aren’t eaten), they sink to the ocean floor, where the carbon in them gets trapped and can not contribute to the greenhouse effect. Iron is the main ocean fertilizer under consideration, and this process would be much cheaper and faster than planting more trees on land to remove CO2.
However, there are potential unintended consequences of this method. Overgrowth of phytoplankton could cause algae blooms that deplete oxygen from water, thereby harming marine animals. Additionally, although phytoplankton are crucial at the bottom of the marine food chain, a sudden increase in their population may shift the balance of different algal species and destabilize the marine ecosystem.
Certain chemical reactions between CO2 and solid minerals on land can naturally remove CO2 from the atmosphere. Scientists are studying how to take advantage of these processes to make oceans more alkaline (i.e. less acidic). First, CO2 in the air reacts with water to produce carbonic acid (the same reaction of CO2 and seawater). Next, the carbonic acid reacts with minerals from rocks on land to form bicarbonate.
Due to water run-off from land, the bicarbonate will eventually make its way to the oceans. Because bicarbonate is alkaline, it makes the oceans LESS acidic and is also taken up by organisms to make their shells. This removes CO2 from the atmosphere, reduces ocean acidity, and helps rebuild coral shells that were dissolved by acidic seawater.
Minerals that react best with CO2 on land are calcium carbonate and sand-like minerals called silicates. The reaction naturally occurs when rocks are eroded by wind or water, which produces fine mineral particles. Small mineral particles react much faster than large particles with CO2. Humans can enhance this process by physically breaking rocks into small pieces and spreading the pieces on land.
The biggest issue surrounding ocean alkalinization is how it might affect the oceans in other ways. There is some indication that increasing the alkalinity of seawater in nutrient-poor parts of the ocean might inhibit phytoplankton growth. This would remove less CO2 from the atmosphere and harm ocean ecosystems. The mineral + carbonic acid reactions can produce some mineral side products, and it is possible that a sudden increase in these side products will negatively affect the oceans.
Both ocean fertilization and alkalinization work slower than solar geoengineering methods to cool the Earth, but they directly address the root problem of too much CO2 in the atmosphere.
Direct Carbon Capture
Direct carbon capture is an umbrella term for chemical reactions that filter CO2 from the air. The technologies can store CO2 underground or funnel it to be used to make consumer goods. Carbon capture has fewer risks than the other methods, and it addresses the root problem of excess atmospheric CO2.
Carbon capture methods are also interesting because they facilitate growth of a market for carbon trading. The carbon trading market is a cap-and-trade system: governments set limits on how much CO2 companies can emit, and companies reduce their emissions or buy carbon capture technologies to offset CO2 release. If their net output is less than the emission cap, they can sell the difference to other companies. The Acid Rain Program used the same method in the 1990s and early 2000s to cut sulfur dioxide emissions.
Based on the success of the Acid Rain Program, the international carbon market was established in the 1997 Kyoto Protocol. However, it ended up failing because of loopholes, corruption, and poor standards for technologies. It was revamped in later years, and the 2021 Climate Change Conference in Glasgow (known as COP26) provided details on how countries can implement carbon markets. Such cap-and-trade systems for carbon are already in place in the European Union and California.
Due to financial incentives, the carbon market will likely spur development of more efficient and cheaper processes to remove CO2 from the atmosphere. Several companies, such as Carbon Engineering, Global Thermostat, CarbonCure, and Climeworks, have been working to commercialize their technologies and are gaining traction.
How Much Will It Cost?
Solar geoengineering works faster to reverse climate change than methods that remove CO2 from the atmosphere. Stratospheric sulfate aerosols are the fastest and least expensive to deploy . The direct risks of the method are best understood, and it is quickly reversible (aerosols will fall out of the atmosphere in weeks to months).
Cloud brightening requires more investment up front to build the machines necessary to take in seawater, convert it to tiny droplets, and spray them into the air. The machines would likely be carried on boats to move to different parts of the ocean, but they must be able to withstand strong ocean currents and weather conditions. 
Ocean geoengineering methods are more expensive, less efficient, and will take longer to set up than aerosol methods. The cost depends on the types of materials (iron, calcium carbonate, etc.) added to the ocean and the dispersal machinery. Ocean geoengineering is risky and incites a lot of controversy. Many scientists therefore believe that it may only work on a local scale where it can be monitored well. 
The cost of direct carbon capture depends on the chemical reactions and deployment systems. Carbon capture is slower than reflecting sunlight, and it requires development of efficient chemical reactions to remove CO2 from the air. But it is low-risk, and the carbon market has caused greater investment and interest in capture methods. The costs of developing these technologies will most likely fall to the private sector, whereas public investment will be in the form of monitoring and regulation.
Figure 3. Summary of Geoengineering Methods: Many other geoengineering methods are being researched and explored, but the primary ones are shown here. Geoengineering is divided into solar geoengineering (reflecting sunlight to cool the planet) and carbon capture (removing CO2 from the atmosphere to slow greenhouse effect-induced warming).
Geoengineering For the World
The primary challenges of geoengineering are 1) conducting field experiments to accurately assess the consequences, and 2) developing international agreements to safely deploy and monitor geoengineering technologies. If geoengineering were adopted, a combination of techniques would be used depending on cost, regional conditions, and the climate’s response. Different methods may have local or global effects, so regulatory policies need to be agreed upon by the international community. Many scientists have called for the creation of regulatory agencies to advise the United Nations and lay out plans for how geoengineering methods should be prioritized.
Climate change has caused rising sea levels, more frequent severe weather, and food and water scarcity, forcing large-scale human migrations. By releasing vast quantities of greenhouse gases, we have set climate change in motion, and it cannot be reversed by cutting emissions alone. Geoengineering could help us reverse climate change in a more controlled manner, buying us time to make our society more sustainable.
For More Information:
- How to Cool the Planet by journalist Jeff Goodell
- Article in Nature about the lack of geoengineering research and its challenges.
- Atmospheric chemist Ken Caldeira, who studies geoengineering and energy science.
- Harvard Solar Geoengineering Research Program, headed by David Keith.
- Development of controlled experiments in the stratosphere by Frank Keutsch.
- Article in Nature Scitable explaining ocean acidification and buffering.
- Other geoengineering technologies not discussed in this post.
- Journalist Elizabeth Kolbert's writings on global warming
- IPCC 2021 Full Report
Thank you to Jovana Andrejevic, a fifth-year Applied Physics Ph.D. student in the School of Engineering and Applied Sciences at Harvard University, for making the figures for this post!