What does Genesis 8:21 mean?

The previous verse recorded Noah's first act after leaving the ark: to build an altar and offer animal sacrifices to God. Now God responds to this humble act of worship. We're told that God smells the pleasing or soothing aroma of the sacrifice and, apparently greatly pleased, makes a new commitment. This is the only time Scripture shows God explicitly smelling the aroma from a sacrifice, though that is the direct intention of many sacrifices described later in the Bible. This is not meant to be read as if God is literally inhaling smoke. Rather, the reference to smoke, and its scent, is a common Scriptural metaphor involving prayer, and how our sacrifices are received by God.

God's commitment is to never again curse the ground or the earth as He has done through the flood. This should not be read as God lifting the original curse on the ground in response to Adam's sin. The curse of weeds and frustrating toil and the work required to bring crops from the ground remains to this day. Instead, God's commitment here should be seen as a decision not to annihilate life on the ground as He did with the flood. The flood brought destruction on the whole earth, on all the ground. God is declaring that He won't do that again.

God seems to make this commitment while acknowledging that human nature has not been changed by the flood. Human beings will continue to harbor evil intentions from youth and throughout their lives. God knows this and decides not to respond to human sinfulness in the same way again by cursing the earth with a flood. In addition, God promises to never again strike down every living thing. He will not wipe out humanity and animal-kind with a global and fully life-ending catastrophe as He has done with the flood.

We are meant to be comforted by these promises and to be intrigued about how God might respond to human sinfulness, instead.