From Wikipedia,

Solar radiation management (SRM) proposals are a type of climate engineering which would seek to reflect sunlight and thus reduce global warming. Proposed methods include increasing the planetary albedo, for example using stratospheric sulfate aerosols. Restorative methods have been proposed regarding the protection of natural heat reflectors like sea ice, snow and glaciers with engineering projects. [...]

Solar radiation management has certain advantages relative to emissions cuts, adaptation, and carbon dioxide removal. Its effect of counteracting climate change would be experienced very rapidly, on the order of months after implementation, whereas the effects of emissions cuts and carbon dioxide removal are delayed because the climate change that they prevent is itself delayed. Some proposed solar radiation management techniques are expected to have very low direct financial costs of implementation, relative to the expected costs of both unabated climate change and aggressive mitigation.

There remain risks, however. The most commonly cited risk is that people may be less likely support reducing carbon emissions if they knew temperatures were being adequately managed via other means. Since carbon emissions still cause ocean acidification, among other effects, we may prefer to reduce emissions instead. Another commonly cited reason for not using solar radiation management is that the effects are difficult to predict, though this claim is disputed. There is also a risk of a "termination shock" whereupon the discontinuation of solar radiation management, the Earth rapidly resumes its previous climate path, which could be hazardous.

There are many proposed types of solar radiation management: statospheric aerosol injection, marine cloud brightening, ocean sulfur cycle enhancement, literally painting surfaces with white colors and developing space mirrors to deflect solar radiation.

Will large scale solar radiation management be used to mitigate the effects of climate chage in the 21st century?

Given the multitude of approaches, an exact operationalization for large scale solar radiation management is difficult. While I could simply write a long disjunction of the above approaches, I have instead opted for this definition:

Large scale solar radiation management is said to be used to mitigate the effects of climate change in the 21st century if yearly average atmospheric carbon dioxide levels are above 600 parts per million at the start of 2101, and yet the Earth's mean surface temperatures are less than 1.5 degrees Celsius above the pre-industrial baseline (as defined and reported by a reliable institution). This question resolves ambiguously in case there is some significant natural event that reduced mean surface temperatures, such as an unexpected reduction in solar radiation. Metaculus moderates use their discretion when resolving ambiguously.