Towards the end of Snowpiercer, when Nam and Curtis are sitting outside Wilford's gate and Curtis is describing to Nam what went on in the tail section during the early days of the train, Nam replies by explaining that he believes the snow is melting and that life outside the train is possible. There is a moment there when Nam flashes back to the greenhouse, about to tell Curtis what he saw outside, but then says "no need to tell you that" (or something along these lines). Do we know what Nam saw in the greenhouse?
The polar bear (or some other form of ice and snow dwelling animal). The reason he withheld the fact that he saw the polar bear was because the idea that the snow could ever be melting was already such an impossible notion to the dwellers of the train who had spent years in the complete belief that the climate change was permanent, gasp that telling Curtis he saw a polar bear (or any sign of life outside the train) would be meaningless. Besides that, he's completely aware that Curtis sees him as an addict that's too hopped up on drugs to know up from down, so his findings would essentially be falling on def ears. It wouldn't have benefitted anyone under these circumstances. Besides, it made for a strong ending :)
This is very late in coming, but I've only just become familiar with this film. Nam says he knew the weather was changing because of the plane crash remnants he looks at every year at the Yakaterina bridge. His flashback of seeing the snowflake is different from the one where he looks out the greenhouse window. There is a point when we're looking out a window (I can't tell you exactly where, but it's getting toward the end) where we see a small area of blue water. IMO, he either saw the polar bear or some other animal, or water.
I believe, and I could be wrong, that he recognized the type of flake that came through the window. He mentioned something about knowing it was getting warmer because of the crystal structure of the flakes. That's what he saw in the greenhouse.
When they a having sushi he is looking out the window and sees the ships have very little snow and ice on them. As well there is tower where you with no snow on it and you can clearly see a blue stripe. Also far in the distance you can see a red and white light house clearly has no snow on it. This confirms his theory that the snow is melting.