Global warming is the increase in the average temperature of the Earth’s surface and atmosphere due to the accumulation of greenhouse gases, such as carbon dioxide, methane, and nitrous oxide, in the air. These gases trap heat and prevent it from escaping to space, causing the planet to warm up. Human activities, especially the burning of fossil fuels, are the main source of greenhouse gas emissions.
To limit global warming and its harmful consequences, such as sea level rise, extreme weather events, biodiversity loss, and food insecurity, we need to reduce our greenhouse gas emissions and switch to a low-carbon economy. However, some experts argue that this may not be enough to keep global warming below 1.5°C or 2°C above pre-industrial levels, the goals of the Paris Agreement. They suggest that we also need to consider geoengineering, or the deliberate manipulation of the Earth’s climate system to counteract global warming.
What is Geoengineering?
Geoengineering is a broad term that covers a range of methods and technologies that aim to alter the Earth’s natural cycles and processes to cool down the planet. Geoengineering can be divided into two main categories: carbon dioxide removal (CDR) and solar geoengineering (SG).
CDR methods aim to remove excess carbon dioxide from the atmosphere and store it in natural or artificial reservoirs, such as forests, soils, oceans, or underground formations. Examples of CDR methods include afforestation, biochar, enhanced weathering, ocean fertilization, and direct air capture.
SG methods aim to reflect or block some of the incoming solar radiation and reduce the amount of heat that reaches the Earth’s surface. Examples of potential SG methods include stratospheric aerosol injection, marine cloud brightening, cirrus cloud thinning, space mirrors, and surface albedo modification.
How do Aerosols Affect Global Warming?
Aerosols are tiny particles or droplets that are suspended in the air. They can come from natural sources, such as dust, sea salt, or volcanic eruptions, or from human activities, such as burning fossil fuels, agriculture, or industry. Aerosols have a complex and uncertain role in global warming. Depending on their size, shape, colour, and composition, aerosols can either cool or warm the planet by scattering or absorbing solar radiation or by influencing cloud formation and properties.
The percentage contribution of aerosols to global warming is not easy to quantify, as different aerosols have different effects and lifetimes in the atmosphere. However, some estimates suggest that aerosols have offset about 50 percent of the warming that would have occurred due to greenhouse gases. This means that without aerosols, the Earth would be about 1 °C (1.8 °F) hotter.
What is the Effect of Shipping on Global Warming?
Shipping is one of the major sources of human-made aerosols. Ships emit sulfur dioxide and other pollutants that form sulfate aerosols in the atmosphere. These aerosols can seed and brighten low-lying clouds that follow in the wake of ships. These clouds reflect more sunlight back to space and cool the planet. This phenomenon is known as ship tracks.
In 2020, regulations imposed by the United Nations’s International Maritime Organization (IMO) cut ships’ sulfur pollution by more than 80% and improved air quality worldwide. The reduction has also lessened the effect of sulfate particles in seeding and brightening ship tracks. The 2020 IMO rule has created a natural experiment that shows what happens when aerosols, in this case, caused by SO2 emissions, are reduced.
By reducing the number of ship tracks, the planet has warmed up more than it would have. That trend is more visible where maritime traffic is particularly dense. In the shipping corridors, the increased light represents a 50% boost to the warming effect of human carbon emissions.
The IMO rules have created a natural experiment that provides climate scientists with a rare opportunity to study a geoengineering scheme in action, albeit one that is working in the wrong direction.