reduce by 50-80% of what they are on track to beAccording to the IPCC, we'd have to reduce GHG emissions by 50% to
80% of what they're on track to be in the next century to reach this level.
GHG emissions are on track to be a certain level (=what)
we reduce what (that level which is on track to be in the next century) by 50-80% --> we reach "this" level
We need a nominal relative pronoun. Which of the given option is one? "what" is.
Student or Learner