Large volcanic eruptions are able to send gases high in the atmosphere. These gases (mostly sulfur) form particles called aerosols that can travel worldwide, reducing the amount of sunlight that gets to the Earth’s surface. In June 1991, this happened when Mt Pinatubo erupted. The climate cooled about 0.3℃ for a couple of years. Researchers are looking at ways to use engineering to have a similar effect - shading the planet and helping to cool our climate.
Blocking some solar radiation from getting to Earth could involve sending gases or particles into the atmosphere, like volcanoes. It could also include methods like making clouds or the Earth’s surface brighter so that they reflect sunlight back out to space. Methods like these could help slow climate change.
The amount of incoming sunlight that is reflected back out to space is called albedo. On planets where most of the energy from the Sun is reflected back out to space, albedo is close to 1.0 (which means that all of the sunlight is reflected). On planets where most of the energy from the Sun is absorbed, albedo is low, close to zero. Earth’s albedo is 0.3, which means that about a third of the energy that gets to Earth from the Sun is reflected back out to space. The largest effect on the Earth’s albedo are clouds. Today, albedo is changing in some locations. For example, Arctic sea ice typically reflects sunlight because it is light in color, but climate change is causing the ice to melt, and pollution settling out of the air darkens the ice’s surface. This lowers albedo, causing more sunlight to be absorbed, warming the Earth system.
Researchers are looking at ways to increase the albedo, which means reflecting more solar energy away from Earth. If less solar energy is absorbed by the planet, then the surface of the Earth would cool. This is called Solar Radiation Management. Examples include:
- Creating Brighter Clouds: Some types of clouds, particularly cumulus clouds over the oceans, do a good job at shading the Earth. They are able to reflect sunlight back out to space. Researchers are developing ways to make these clouds whiter and brighter, and increase the time they stay in the sky. They are focusing on clouds over the ocean where albedo is particularly low in hopes of increasing the albedo, either in a region of the ocean or worldwide. This could be a way to shade coral reefs, which are in decline due to the warming climate.
- Adding Aerosols to the Stratosphere: Adding small particles (aerosols) to the stratosphere will help scatter incoming solar radiation, cooling the planet in much the same way that volcanoes do. One way to do this is to inject sulfur dioxide into the atmosphere, which turns into aerosols.
While these methods have not been developed yet, they are thought to be inexpensive compared to other ways to cool the climate, and they could quickly cool the climate. However, changing the albedo of our planet could be risky. For example, we could cool the climate too much. In the past when large volcanic eruptions cooled the climate, like during the Year Without a Summer, crops died and people went hungry. Modifying clouds and adding sulfur dioxide to the stratosphere could also have unintended consequences, which are still unknown. Both could cause changes in rainfall and weather patterns. Adding aerosols to the stratosphere could also affect the ozone layer, which protects Earth from harmful ultraviolet radiation. If there was a way to monitor these techniques and the effects they have, we would know if they are causing too much cooling or having other effects.
Because of the unknown risks and lack of monitoring, a report by the National Academies concluded that “albedo modification at scales sufficient to alter climate should not be deployed at this time.” Researchers are working to learn more about these methods and how they could be used safely, but at this time, we don’t know enough about whether they will work well.
Also, because Solar Radiation Management methods are temporary, we’d have to repeat them over and over again until we reduce greenhouse gases in the atmosphere significantly. If we don’t reduce greenhouse gases and we stopped Solar Radiation Management, then the Earth would heat up again.