Environmental activists have staged protests of thousands at the western German forest, and some have also continued to live there in treehouses after police dismantled a protest camp set up five years ago to block a coal-mining project.
The protesters are trying to prevent utility RWE from clearing the forest that it bought decades ago to expand mining in the area.
Most of the forest has already been chopped down.
RWE earlier this year gave assurances it would not touch until late 2020 the Hambach forest near Cologne, a recent focus of anti-RWE and coal protests, which it had hoped to clear for lignite mining.
Protesters say this is not enough, and that the country is not moving quickly enough to ditch fossil fuels.
Germany aims to raise wind and solar power's share of energy generation from a third now to 65 percent by 2030 to help to cut carbon dioxide emissions and achieve its climate commitments.
Thunberg also spoke about getting ready for her upcoming trip when she crosses the Atlantic in a racing boat.
To avoid travelling by air, Thunberg will cross to New York for a UN climate summit on board a 60-ft racing yacht, the Malizia II, fitted with solar panels and underwater turbines that produce electricity onboard, with the aim of making the journey zero-carbon.