WASHINGTON — The United States needs to enact a major climate change law, such as a tax on carbon pollution, by the end of this decade to stave off the most catastrophic impacts of global warming, according to the authors of a report released this week by the United Nations Intergovernmental Panel on Climate Change.

But aggressive efforts to tackle climate change have repeatedly collided with political reality in Washington, where some Republicans question the underlying science of global warming and lawmakers’ ties to the fossil fuel industry have made them resistant to change. The rise of the Tea Party in recent years has also made a tax increase unlikely.

This week’s report makes clear, however, that the window is rapidly narrowing to forge new policies that will protect the globe from a future of serious food and water shortages, a drastic sea level rise, increased poverty and disease and other profound risks.

“What would be required is a nationwide carbon pricing policy,” said Robert Stavins, director of Harvard’s environmental economics program and a lead author of the report. “And that would not be possible without action from Congress.”