Been there.
I planned to be gone for six months, maybe a year. Several years ago.
I left two electrics, a mandolin, and a classical in storage. In my parent's basement. Unlike you, I didn't even think about taking any precautions. I left them tuned and in their cases.
When I was home last (two years ago, making it 5 years that they'd been in storage), there was absolutely no change to any of them. No problems at all.
Changes in climate and humidity aren't that bad for an instrument, as long as they're gradual and consistent. If you've got the chance to set the temperature and humidity, I'd go for whatever the median is for where you're living.