Who has the moral high ground, Greta or Donald?

Neither. Greta doesn't know enough to know how things work and don't work and isn't old enough to come up with any solutions that nobody else hasn't already thought of but found to be unrealistic. EG. stop all fossil fuels immedietly (just turn off heating for everybody, sure) She's being an alarmist because she's likely been coached to believe what she says which is irresponsible and i'd say possibly immoral if you're scaring kids into believing the worlds going to end (its not).

Trump says he's going to plant a trillion trees which is good but I don't think he's particularly concerned anyway so no morality points to him either.