Stratospheric barrier to curb warming

Published: 6th Dec, 2018

According to a study by U.S. scientists, spraying sun-dimming chemicals high above the earth to slow global warming could be “remarkably inexpensive”, costing about $2.25 billion a year over a 15-year period.
The geo-engineering technique known as stratospheric aerosol injection (SAI) could limit rising temperatures that are causing climate change.

Context

According to a study by U.S. scientists, spraying sun-dimming chemicals high above the earth to slow global warming could be “remarkably inexpensive”, costing about $2.25 billion a year over a 15-year period.
The geo-engineering technique known as stratospheric aerosol injection (SAI) could limit rising temperatures that are causing climate change.
Aim: To cut the rate of temperature change by half.
Discounting other methods of deployment because of cost and feasibility, the research assumes a special aircraft can be designed to fly at an altitude of about 20 km and carry a load of 25 tonnes.

About

What is Stratospheric Aerosol Injection (SAI)?

Delivery of precursor sulfide gases such as sulfuric acid, hydrogen sulfide (H2S) or sulfur dioxide (SO2) into the stratosphere.
How: It would involve the use of huge hoses, cannons or specially designed aircraft to spray large quantities of Sulphate particles into the upper layer of the atmosphere to act as a reflective barrier against sunlight.
Less sunlight will help in decreasing the temperature of the Earth.

What are Stratospheric Sulphur Aerosols?

Stratospheric sulfur aerosols are sulfur-rich particles which exist in the stratosphere region of the Earth’s atmosphere. The layer of the atmosphere in which they exist is known as the Junge layer, or simply the stratospheric aerosol layer.
These particles consist of a mixture of sulfuric acid and water. They are created naturally, such as by photochemical decomposition of sulfur-containing gases, e.g. carbonyl sulfide.

Sulfur aerosols are common in the troposphere as a result of pollution with sulfur dioxide from burning coal, and from natural processes.
Volcanoes are a major source of particles in the stratosphere as the force of the volcanic eruption propels sulfur-containing gases into the stratosphere.

Example:

In 1991, Mount Pinatubo erupted in the Philippines. It was the second largest eruption of the 20th century, according to the United States Geological Survey (USGS).
In total, the eruption injected 20 million tons of sulfur dioxide aerosols into the stratosphere.
USGS said the Earth's lower atmosphere temperature dropped by approximately 1-degree Fahrenheit.
The effect only lasted a couple of years because the sulfates eventually fell to Earth.

What is aerosol formation?

Primary aerosol formation, also known as homogeneous aerosol formation, results when gaseous SO2 combines with oxygen and water to form aqueous sulfuric acid (H2SO4).
This acidic liquid solution is in the form of a vapor and condenses onto particles of solid matter, either meteoritic in origin or from dust carried from the surface to the stratosphere.
Secondary or heterogeneous aerosol formation occurs when H2SO4 vapor condenses onto existing aerosol particles.
Existing aerosol particles or droplets also run into each other, creating larger particles or droplets in a process known as coagulation.
Warmer atmospheric temperatures also lead to larger particles.
These larger particles would be less effective at scattering sunlight because the peak light scattering is achieved by particles with a diameter of 0.3 μm.

Arguments in favour of this technique:

Risks associated:

Scientists have said SAI could cause droughts or extreme weather in other parts of the world.
SAI could harm crop yields as well as potential public health and governance issues.
It also does not address the issue of rising carbon dioxide emissions, the main greenhouse gas blamed for global warming.
Lifespan of aerosols: Tropospheric sulfur aerosols are short lived.

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