A few years back I bought a big jug of Titebond2 glue, figuring it'd last me a decade fulfilling my occasional need to make some wood stick together better than nails or pocket holes. I store it on my back porch, which is indoors but only gets climate control from the blower return in the room, so the room hovers between about 50F (10C) and 90F (32C) depending on the season.

I keep it capped when not pouring it into a smaller bottle that's easier to manage.

I didn't use it last year.

This morning I had a need, so I opened the cabinet and the attached picture is what I see. The glue has gone from an off-white to this translucent reddish-brown, and it won't flow out of the bottle.

I was able to squish a decent amount onto a piece of scrap wood, and smear glue into the joints, then wipe off the excess - but it looks like I need to go buy more glue. I'm not happy about it, but I'll own that I messed up and should have bought a smaller bottle, it's on me.

Can anyone tell me what might have happened? I tried to be careful with it, but clearly something happened that I didn't plan for. What can I do to prevent this from happening in the future?

Thanks!