President Trump, for his part, took the opposite approach. In a Twitter message on the eve of Earth Day, he promised help for the fossil fuel sector, saying he had instructed administration officials to ensure that federal funds be made available to the oil and gas sector, “so that these very important companies and jobs will be secured long into the future.”

His administration has rolled back a raft of environmental protections and announced its exit from the Paris climate accord, which is designed to slow down the rate of global temperature rise and avert the worst effects of climate change.

The coronavirus pandemic, which has claimed around 180,000 lives, came at a time when attention to climate change had risen globally. Children and teenagers had launched worldwide protests, demanding that presidents and prime ministers take climate action. The titans of business and finance had unveiled pledges to reduce their carbon footprints, and BlackRock, the world’s largest asset manager, with $7 trillion in global investments, announced that it would start pulling its money from ventures with high levels of climate risk. For the first time, climate change policies had begun to figure prominently in the United States presidential election campaign.

Then came the pandemic, which, as Greta Thunberg, the 17-year-old climate activist, put it, “turned everything upside down.”

“Whether we like it or not, the world has changed,” she said Wednesday from the Nobel Museum in Stockholm, her home city. “It looks completely different from how it did a few months ago and it will probably not look the same again and we are going to have to choose a new way forward.”

Khristen Hamilton, 18, an organizer with a youth activist group called Zero Hour, said her organization had been forced to drop plans for an ambitious bus tour and in-person voter registration drive. Now, the group is running the registration effort online and hosting weekly webinars on what it calls the roots of the climate crisis.