Thunberg, 18, whose activism has inspired a global movement, testified virtually to a House of Representatives panel on the day President Joe Biden kicked off a virtual two-day Earth Day summit pledging to slash U.S. greenhouse gas emissions in half by 2030.Thunberg, whose activism began at age 15 when she started skipping school on Fridays to protest outside the Swedish parliament for climate change, admitted she was pessimistic."I don't believe for a second that you will actually do this," she lectured the lawmakers of the House Oversight Committee's environmental subcommittee."You still have time to do the right thing and to save your legacies, but that window of time is not going to last for long," Thunberg said. "We the young people are the ones who are going to write about you in the history books ...So my advice for you is to choose wisely."The chairman of the subcommittee, Representative Ro Khanna, is pressuring his fellow Democrat Biden to end fossil fuel subsidies as part of a plan to rebuild U.S. infrastructure.