In a second speech, Thunberg said: "Our house is still on fire. Your inaction is fuelling the flames by the hour. And we are telling you to act as if you loved your children above all else."
Trump made his second appearance at Davos this year. While Thunberg and Trump did not mention each other directly in their speeches, their remarks represented a head-on collision of worldviews.
Trump said he was a "big believer in the environment" but did not single out climate change and lashed out at "alarmists." He said the United States will participate in a plan to add one trillion trees worldwide - a plan that climate activists argued would not even begin to scratch the surface of the problem.
During her afternoon panel, Thunberg said, "Planting trees is good, of course, but it's nowhere near enough of what is needed."
Her words echoed a common sentiment among climate activists, whose anger at world leaders and big businesses has gained renewed momentum amid the devastating bushfires that have wreaked havoc in Australia in recent weeks.
Thunberg is helping drive that conversation. "Not even catastrophes like these seem to bring any political action. How is this possible?" she tweeted last month, drawing the enmity of Australian Prime Minister Scott Morrison, who has continued to back Australia's influential coal industry, despite research connecting it to climate change. Scientists say rising temperatures have increased the ferocity of this year's fire season.
With Morrison absent from the Davos conference, much of the ire among climate activists was centered on Trump.