You choose whether the Human’s base power and toughness change as the ability resolves, not as the ability is put onto the stack.

If Exuberant Wolfbear leaves the battlefield while its triggered ability is on the stack, use its power and toughness as it last existed on the battlefield to determine the Human’s new base power and toughness.

Any effects that modify the Human’s power and/or toughness without setting them to a specific number or value will apply after its base power and toughness are set, regardless of the order in which those effects were created. The same is true of counters that modify its power and toughness.

Exuberant Wolfbear’s power and toughness are unaffected when the target Human takes on those values.

The Human’s new base power and toughness are set to Exuberant Wolfbear’s actual power and toughness, not just its base power and toughness. If Exuberant Wolfbear’s power or toughness changes later in the turn, the Human isn’t affected.