More than 600 self-professed climate skeptics are meeting in a Times Square hotel this week to challenge what has become a broad scientific and political consensus: that without big changes in energy choices, humans will dangerously heat up the planet.

The three-day International Conference on Climate Change  organized by the Heartland Institute, a nonprofit group seeking deregulation and unfettered markets  brings together political figures, conservative campaigners, scientists, an Apollo astronaut and the president of the Czech Republic, Vaclav Klaus.

Organizers say the discussions, which began Sunday, are intended to counter the Obama administration and Democratic lawmakers, who have vowed to tackle global warming with legislation requiring cuts in the greenhouse gases that scientists have linked to rising temperatures.

But two years after the United Nations Intergovernmental Panel on Climate Change concluded with near certainty that most of the recent warming was a result of human influences, global warming’s skeptics are showing signs of internal rifts and weakening support.