Can We Limit the Amount of Sunlight to Stop Climate Change?

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. This happened in June of 1991 when Mt Pinatubo erupted. The climate cooled about 0.3℃ (0.5°F) for a couple of years. Researchers are looking at ways to engineer 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 do. It could also include methods like making clouds or the Earth’s surface brighter so that they reflect more sunlight back out into 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 into space, albedo is close to 1.0 (1.0 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 (zero means that none of the sunlight is reflected). 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. Clouds have the largest effect on the Earth’s albedo. Today, albedo is changing in some locations. For example, Arctic sea ice typically reflects sunlight because it is white, but climate change is causing the ice to melt, and pollution settling out of the air is darkening the ice’s surface. This lowers its albedo, causing more sunlight to be absorbed and warming the Earth system.

Researchers are looking for ways to increase the albedo of Earth, because If less solar energy is absorbed by the planet, then the surface of the Earth will cool. This concept is called solar radiation management. Two examples include:

• Creating brighter clouds: Some types of clouds, particularly cumulus clouds over the oceans, do a good job of shading the Earth because they reflect a lot of sunlight back out into 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 also be a way to shade coral reefs, which are in decline due to the warming climate.
• Adding aerosols to the stratosphere: Adding aerosols (small particles) to the stratosphere would 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 do it quickly. However, changing the albedo of our planet could be risky. For instance, 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 test these techniques and monitor the effects, we would know if they cause too much cooling or have other unanticipated effects. 

Because of the unknown risks of solar radiation management, 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 them or whether they will work the way we want them to.

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 by other means. If we didn't reduce greenhouse gases and we stopped solar radiation management, then the Earth would heat up again and that effort would be wasted.