(CNN) Greta Thunberg's father says he initially did not support his daughter's climate activism, calling it a "bad idea," but his view changed when he saw how happy it made her.

In an interview on BBC Radio 4's Today program Monday, Svante Thunberg revealed that by skipping school and staging Friday sit-ins outside the Swedish parliament, Greta was not only battling climate change but also the crippling depression that had plagued her for up to four years.

Greta Thunberg and her father Svante attend a press conference during the COP24 summit on climate change in Katowice, Poland on December 4, 2018.

"She stopped talking, she stopped eating, and all these things," Svante Thunberg said of Greta's period of illness, speaking on a program guest-edited by the 16-year-old activist.

"She stopped going to school. She was basically home for a year, she didn't eat for three months," the 50-year-old said of what he described as "the ultimate nightmare as a parent."

Thunberg and his wife, opera singer Malena Ernman, stopped work to look after Greta -- who at that point was only speaking to them, her sister and one of her teachers at school.

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