Reconciling God of the Old and New Testament

How does one reconcile the loving God of the Old Testament  with the harsh God of the New Testament?

 When I ask this question of students, at first they are shocked,  and then most assume that I have simply misspoken, as I am  prone to do. They typically have heard the question inverted,  along these lines: "How did the mean Old Testament God  morph into a nice guy like Jesus?" I assure them that this time,  at least, I have not accidentally inverted my words. I then observe  that God in the Old Testament is consistently described  as slow to anger and abounding in steadfast love, but Jesus  speaks about hell more than anyone else in Scripture. The  word hell doesn't even show up in English translations of the  Old Testament.

—David T. Lamb, in God Behaving Badly: Is the God of the Old Testament Angry, Sexist and Racist?