。
It seems to me that each year gradually gets warmer.
Literal
Every-year gradually becomes-warm-come [like-ように] is-seemed.
だんだん ('gradually, bit by bit') is a reduplicated adverb — Japanese is rich in these. 暖かくなってくる uses the inchoative ~くなる ('become warm') with the direction ~てくる ('come to'), expressing a gradual change approaching from the past toward the present. 思われる is the passive/spontaneous of 思う — here spontaneous 'it occurs to one,' 'it seems.'