In its most recent assessment released this fall, the Intergovernmental Panel on Climate Change said warming of the Earth’s climate system is both unequivocal and unprecedented, a conclusion that rests on multiple, independent data sources.

The authoritative IPCC study also found that it was “extremely likely” that human influence, particularly our reliance on fossil fuels, has been the dominant cause of climate change.

At the same time, international meetings — such as the Climate Change Conference just concluded in Warsaw, Poland — struggle to devise broadly acceptable agreements to reduce greenhouse-gas emissions that pose grave risks to the world’s economy, its environment and public health and well-being.

A major reason for the slow pace of global action is the posture of the United States. As the world’s leading emitter of greenhouse gases on a per-capita basis, we simply haven’t stepped up to a leadership position.

Nor, for that matter, has China, whose surging economy has pushed it to the No. 1 spot in total greenhouse-gas emissions.

What might the U.S. do to show convincingly that it is prepared to play a leading role in slowing the rate of global climate change and minimizing its effects? Passing comprehensive national climate-change legislation would be a good start.

The United States has hardly been standing still on the issue. More than half of the states and more than 1,000 U.S. cities have adopted a diversity of policies that should substantially reduce greenhouse-gas emissions. These include innovative actions on renewable fuels, energy efficiency, public transportation, building efficiency and more.

Much has happened at the federal level, too. The Obama administration has invested tens of billions of dollars in cutting-edge research on promising renewable-energy technologies.

The administration also brokered historic agreements with the auto industry that will raise fuel-economy standards . 

Equally important, the White House and the Environmental Protection Agency are developing regulations for new and existing coal-fired power plants that promise to begin shifting the nation away from use of coal to other energy resources that emit far fewer greenhouse gases.

Yet the new regulations and policy initiatives are not sufficient to tackle climate change. They also come with no national political commitment that might prod reluctant nations around the world to do their own part. And some actions, notably the EPA power-plant regulations, are certain to be challenged in court by the fossil-fuel industry.

National policy would send a different and more definitive signal to the world that the United States takes climate change seriously and that it is prepared to step out in front on the issue.

Michael E. Kraft is a professor emeritus of political science and public and environmental affairs at the University of Wisconsin-Green Bay.