20 Then Noah built an altar to the LORD and took some of every clean animal and some of every clean bird and offered burnt offerings on the altar.
21 And when the LORD smelled the pleasing aroma, the LORD said in his heart, "I will never again curse the ground because of man, for the intention of man's heart is evil from his youth. Neither will I ever again strike down every living creature as I have done.
22 While the earth remains, seedtime and harvest, cold and heat, summer and winter, day and night, shall not cease."
After the waters of the flood had dried up, Noah comes out of the ark and sacrifices to God. This is a sacrifice of thanks and as a symbol of atonement where the sin of mankind (or those remaining) is symbolically covered by the sacrifice.
While God has not changed the nature of man, noting that "the intention of man's heart is evil from his youth", he makes the promise not to wipe out the entire earth in this way again anyway.
Remembering that Noah was called righteous by God, but that every man is evil including Noah, it is important to remember that righteousness comes by faith and not good deeds. For those who follow God, and seek God's glory, God will make them righteous through the Christ to come who will bruise the serpent's head and completely defeat our sinfulness (see DVP007 for more on this)