Solar Radiation Management is the deployment of technology to reflect more light away from the earth for the purpose of controlling global temperature.

I'll count it as "serious" if enough materials are being deployed that they're expected to have a combined potency of negating 30% of the temperature increase from 1990 levels. (This is likely to be some years after the decision to deploy is made, as artificially cooling too rapidly would likely create more problems than it solves.)

Closing date will be extended until we know.
Consensus among climate engineers that it cannot or should not be done would be a closing condition.