"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," Greta said.
Michelle Obama leapt to the defence of Greta Thunberg after her husband's successor criticised a recent award to the teenage climate activist.
The 16-year-old Swede was this week recognised as Time magazine's person of the year, something US President Donald Trump called "ridiculous".
He told Thunberg to "chill" and work on her anger management problem, taken by some as a reference to her "how dare you" speech.
Obama was among those to jump in and fight Thunberg's corner as she attended the COP25 climate conference in Madrid.
"Don't let anyone dim your light," wrote Obama. "You have so much to offer us all. Ignore the doubters and know that millions of people are cheering you on."
Thunberg later used a speech at the COP25 to accuse world leaders of polishing their images rather than fighting climate change.
"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," she said.