The side effects of geoengineering could kill EVERYTHING.
The side effect of not geoengineering will kill everything.
Geoengineering is a skill we will need to master sooner or later. One day, whether we're the cause or not, this planet will not be inhabitable. We have three options: 1) direct our planet towards a consistently inhabitable state, 2) create an inhabitable world elsewhere, 3) die.
I don't really consider (3) to be much of an option, and (2) is so far beyond our current capabilities even experimentation is not a consideration. That leaves option (1).
Personally I'd rather we start our apprenticeship now by correcting our own effects on the environment rather than waiting until the planet makes it an unavoidable necessity regardless.
There should be at least some care taken before any major operation is undertaken, with that in mind.
It's sad that you think this might not be the case. We've spilled far worse into the oceans than iron, so try not to be offended when people that know what they're doing dismiss out of hand this hysteria over small scale experimentation.
I first heard about the electron migration problem as a reason for not overclocking back in the 386 days.
And yet my Celeron 300A has been running stable (first under OS/2, then XP) at 450Mhz since I bought it, 10 years ago.
Record uptime was ~650 days under XP, before a disk failure got it. And that disk was essentially "DOA" (visible bad sectors) but rather than RMA it I decided to see how long it lasted (obviously it's not a very important box).
PowerMax and NTFS/chkdsk recovered the initial damage and marked about 400k worth of visible bad sectors. It then survived another 4 years before SMART started reporting errors again, and it took another 6 months before it finally actually crashed.
SeaTools reset 102 problem sectors, NTFS/chkdsk reports no new visible bad sectors and recovered minor inconsistencies due to the crash. System is back online once again.
So it would seem modern tech is a little more resilient than we give it credit for, especially if the software doesn't shit itself as a result (this experiment would have failed miserably on ReiserFS).
If you can't learn to do it well, learn to enjoy doing it badly.