Have you read the letter? It's not a call for implementing SRM, but for studying its effects (so call for funding the research). Even they admit that:
> While we fully support research into SRM approaches, this does not mean we support the use of SRM.
> SRM does not address the cause of climate change, nor all of the effects of increased greenhouse gas concentrations, it likely will never be an appropriate candidate for an open market system of credits and independent actors
Madness is focusing on methods of mitigation of one particular aspect of the "climate change". I put this in the same bucket like the shading of the sun from the space (few weeks/moths ago?). Climate change is in fact a wrong term. We should focus on all aspects of anthropocene, especially on biodiversity and restoration of ecosystems.
With functioning ecosystems and with us not flooding the earth and atmosphere with poisons (stuff that does not belong in such big quantities, like co2, methane, pfoas, plastics, pesticides/herbidices, fertilizers, etc. etc.) the nature will return to the optimum state. Let's diminish ourselves and let the nature repair itself.
Hoping that some miraculous, non-existent technology will solve our problems while ignoring obvious and easy solutions (stop doing the bad stuff) exist is madness. Promoting such efforts makes the population complacement (no need to do anything personally, it's being worked on, let's consume more). Propaganda and greed. Same old, same old.