John M. Broder / New York Times News Service

WASHINGTON — For all their disputes, President Barack Obama and Mitt Romney agree the world is warming and that humans are at least partly to blame. It remains wholly unclear what either of them plans to do about it.

Even after a year of record-smashing temperatures, drought and Arctic ice melt, none of the debate moderators asked about climate change, nor did any of the candidates broach the topic — nor will they, it seems, before voters go to the polls on Nov. 6.

Throughout the campaign, Obama and Romney have seemed most intent on trying to outdo each other as lovers of coal, oil and natural gas — the very fuels most responsible for rising levels of carbon dioxide in the atmosphere.

Obama has supported broad climate change legislation, financed extensive clean energy projects and pushed new regulations to reduce global warming emissions from cars and power plants. But neither he nor Romney has laid out a legislative or regulatory program to address the fundamental questions arising from one of the most vexing economic, environmental, political and humanitarian issues to face the planet.

Should the United States cut its greenhouse gas emissions, and, if so, how far and how fast? Should fossil fuels be more heavily taxed? Should any form of clean energy be subsidized, and for how long? Should the U.S. lead international mitigation efforts? Should the nation pour billions of new dollars into basic energy research? Is the climate system so fraught with uncertainty that the rational response is to do nothing?

Many scientists and policy experts say the lack of a serious discussion of climate change in the presidential contest represents a lost opportunity to engage the public and to signal to the rest of the world U.S. intentions for dealing with what is, by definition, a global problem that requires global cooperation.

“On climate change, the political discourse here is massively out of step with the rest of the world, but also with the citizens of this country,” said Andrew Steer, the president of the World Resources Institute and a former special envoy for climate change at the World Bank. “Polls show very clearly that two-thirds of Americans think this is a real problem and needs to be addressed.”

Steer noted that climate change was no longer a partisan issue in Europe and that China, Japan, Australia and South Korea had recently taken significant steps to reduce emissions and invest heavily in clean energy technology. “The real question in this country,” said Steer, a British citizen, “is why politicians don’t see it as in their interest to discuss it.”

The list of reasons is long.

Any serious effort to address climate change will require a transformation of the nation’s system for producing and consuming energy and will, at least in the medium term, mean higher prices for fuel and electricity. Powerful incumbent industries — coal, oil, utilities — are threatened by such changes and have mounted a well-financed long-term campaign to sow doubt about climate change. The Koch brothers and others in the oil industry have underwritten advertising campaigns and grass-roots efforts to support like-minded candidates. And the Republican Party has essentially declared climate change a nonissue.

The two most studied ways of reducing global warming pollution — taxing it or regulating it — are politically toxic in a year when economic problems are paramount. After a bill died in the Senate in 2010, Obama abandoned his support for cap and trade, a market-based method to limit greenhouse gas emissions, and he has given little hint of what regulatory policies he intends to pursue if he wins a second term. Aides said he would not propose a carbon tax or other energy tax, but that he would consider supporting one as part of a larger budget and spending deal.

As governor of Massachusetts, Romney considered joining a regional cap-and-trade system, then abandoned it because of uncertainty over costs. He has opposed Obama’s steps to regulate emissions from power plants and vehicles. He has said he would reverse Obama’s air quality regulations and would renegotiate the 54.5 mpg efficiency standard that automakers agreed to by 2025.

The struggling economy has made it difficult for emerging clean energy companies to get the capital they need to reach commercial scale and compete with producers of traditional energy sources. Government programs to provide that seed money are highly controversial, as the fight over tax breaks for wind power companies and the recent failures of the solar panel maker Solyndra and the advanced battery manufacturer A123 Systems showed. The Obama administration provided $90 billion in new financing from the 2009 stimulus package for clean energy projects, but most of that money is gone.

Although there is little doubt that the burning of fossil fuels and deforestation have altered the Earth’s climate, some uncertainty remains about whether and when such changes will become unmanageable. Huge technological challenges persist in transforming the energy generation system. Both Obama and Romney refer to “clean coal,” shorthand for capturing the carbon dioxide emissions from coal-burning power plants, but the technology is still in its infancy.

International efforts to address climate change, which showed promise when Obama took office, have sputtered in recent years because of fears that limiting carbon emissions means limiting economic growth. There is also considerable resistance to any plan that would require the United States and other wealthy countries to take stronger measures than those demanded of China, India and other fast-growing economies that are responsible for the bulk of the growth in global emissions.

Romney’s chief domestic policy adviser, Oren Cass, said the United States should not take unilateral steps. He said Romney’s answer was not to tax emissions or impose new regulations or subsidize clean energy ventures, but is “technological innovation” by private industry, without the thumb of government on the scale.

Joseph Aldy, a former Obama adviser on climate and energy, said Obama’s approach to climate change was his clean energy standard, a proposal to produce as much as 80 percent of the nation’s electricity using clean sources by 2035. About 30 states now require varying amounts of power to come from wind, solar, nuclear, hydro and other nonpolluting sources. Aldy said these plans would spur innovation and provide a consistent market for renewable energy, which today is not competitive with fossil fuels. They also would avoid taxes and direct regulation, he said, although they would be easy to abandon if energy prices rose as a result and voters became disenchanted.