Global warming has become perhaps the most complicated issue facing world leaders. Warnings from the scientific community are becoming louder, as an increasing body of science points to rising dangers from the ongoing buildup of human-related greenhouse gases — produced mainly by the burning of fossil fuels and forests.

Global emissions of carbon dioxide jumped by the largest amount on record in 2010, upending the notion that the brief decline during the recession might persist through the recovery. Emissions rose 5.9 percent in 2010, according to the Global Carbon Project, an international collaboration of scientists. The increase solidified a trend of ever-rising emissions that scientists fear will make it difficult, if not impossible, to forestall severe climate change in coming decades.

However, the technological, economic and political issues that have to be resolved before a concerted worldwide effort to reduce emissions can begin have gotten no simpler, particularly in the face of a global economic slowdown.

For almost two decades, the United Nations has sponsored annual global talks, the United Nations Framework Convention on Climate Change, an international treaty signed by 194 countries to cooperatively discuss global climate change and its impact. The conferences operate on the principle of consensus, meaning that any of the participating nations can hold up an agreement.

The conflicts and controversies discussed are monotonously familiar: the differing obligations of industrialized and developing nations, the question of who will pay to help poor nations adapt, the urgency of protecting tropical forests and the need to rapidly develop and deploy clean energy technology.

But the meetings have often ended in disillusionment, with incremental political progress but little real impact on the climate. The negotiating process itself has come under fire from some quarters, including the poorest nations who believe their needs are being neglected in the fight among the major economic powers. Criticism has also come from a small but vocal band of climate-change skeptics, many of them members of the United States Congress, who doubt the existence of human influence on the climate and ridicule international efforts to deal with it.

A Non-Issue in Campaign

During the 2012 presidential campaign, neither President Obama nor Mitt Romney, his Republican challenger, spoke much about climate change, despite the fact that both men agree that the world is warming and that humans are at least partly to blame. None of the moderators of the four general-election debates asked about climate change, nor did any of the candidates broach the topic.

Throughout the campaign, Mr. Obama and Mr. 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.

Mr. 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 Mr. Romney has laid out during the campaign 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