Dehn twists, commutators, and bounded cohomology

For an element x of a group [G,G], the commutator length cl(x)
of x is defined as the minimal number n such that x can be
written as a product of n commutators. There is also the notion of
bounded cohomology of G. After introducing these two seemingly
unrelated concepts, I will discuss the relation between them
for the mapping class group of a surface.