Greta Thunberg suffered from depression
Greta Thunberg had "stopped talking and eating" before climate activism "saved her", according to her dad.
Svante Thunberg said he thought depression would kill his daughter before she launched her worldwide "climate crisis" campaign.
He also said he thought it was a "bad idea" for his daughter, 16, to take the "front line" in combating climate change, The Sun.
Mr Thunberg's remarks to the BBC came during a special edition of the Radio 4 Today program guest-edited by Greta.
In an interview he said of her climate change activism: "We thought it was a bad idea, just the idea of your own daughter sort of putting herself at the very front line of such a huge question like climate change.
"You wouldn't want that as a parent."
He said they spoke several times with Greta before she pursued her campaigning, explaining that she would have to do it by herself and be well prepared for questions she would face - praising her response.
Mr Thunberg also talked about the difficulties that his daughter, who has been diagnosed with Asperger's syndrome, experienced with depression.
Asked how she got better, he said: "We just took a very, very long time to spend a lot of time together and just work it out together."
He said Greta thought her parents were "huge hypocrites" because they were active advocates for refugees, noting that his daughter would ask "whose human rights are you standing up?"
Mr Thunberg said his wife stopped flying and had to "change her whole career".
He added: "To be honest, she didn't do it save the climate - she did it to save her child because she saw how much it meant to her, and then, when she did that, she saw how much she grew from that, how much energy she got from it."
He said he "became vegan" and Greta "got more and more energy" from this.
"I knew they were the right things to do because I understood the facts at that time, but I didn't do it to save the climate, I did it to save my child," he said.
'HATE?... SHE FINDS IT HILARIOUS'
On the abuse the teenager faces, Mr Thunberg said: "The hate, quite frankly, I don't know how she does it but she laughs most of the time, she finds it hilarious."
Greta, who has become a global icon for her activism, has been repeatedly trolled.
In October Piers Morgan sparked outrage on Good Morning Britain today after impersonating her.
The outspoken presenter was slamming civil disobedience group Extinction Rebellion when he launched a bizarre imitation of the 16-year-old climate activist.
"How dare you, how dare you?" Morgan said, mocking Greta, who has Asperger's.
"You have stolen my morning, you have stolen my airtime. I have no life. How dare you?"
Morgan, 54, was using some of the Swedish activist's rhetoric from an impassioned speech at the UN Climate Action Summit earlier this year, when she accused those present - including Donald Trump - of "stealing my dreams, my childhood".
But Trump, who Greta famously glared at during a UN summit, called her a "very happy young girl looking forward to a bright future".
Later he retweeted one of his supporters who branded the teen an actress.
Russian leader Vladimir Putin also took aim at the teenager branding her "poorly informed" and saying she was "telling developing countries they should keep living in poverty".
Meanwhile a private school head teacher cruelly branded Greta "a little girl with mental problems" in a newsletter sent to parents.
Rodney Lynn, the principal of Coffs Harbour Christian Community Primary School in New South Wales, Australia, has been slammed for his comments about the climate activist.
He accused the Swede of promoting "doomsday waffle talk" and labelled her a "false prophet" in the letter sent out to students and parents.
But despite her detractors, Greta has been recognised for her devotion to fighting climate change.
Earlier this month she was named Time magazine's youngest ever person of the year.
Greta, who turns 17 in January, was awarded the recognition as she stood on a box to make a speech at the COP25 climate summit in Madrid.
She accused politicians and businessmen of using "creative PR" to duck their responsibilities and "not behaving as if we are in an emergency".
She stormed: "The biggest danger is not inaction; the real danger is when politicians and CEOs are making it look like real action is happening, when in fact almost nothing is being done apart from clever accounting and creative PR."