Where the specialists clash is on what the evidence means for the idea that industrial civilization and the burning of fossil fuels are the main culprits in climate change. The two sides agree that carbon dioxide can block solar energy that would otherwise radiate back into space, an effect known as greenhouse warming. But they differ sharply on its strength.

Some argue that CO2 fluctuations over the Phanerozoic follow climate trends fairly well, supporting a causal relationship between high gas levels and high temperatures. “The geologic record over the past 550 million years indicates a good correlation,” said Robert A. Berner, a Yale geologist and pioneer of paleoclimate analysis. “There are other factors at work here. But in general, global warming is due to CO2. It was in the past and is now.”

Other experts say that is an oversimplification of a complex picture of natural variation. The fluctuations in the gas levels, they say, often fall out of step with the planet’s hot and cold cycles, undermining the claimed supremacy of carbon dioxide.

“It’s too simplistic to say low CO2 was the only cause of the glacial periods” on time scales of millions of years, said Robert Giegengack, a geologist at the University of Pennsylvania who studies past atmospheres. “The record violates that one-to-one correspondence.”

He and other doubters say the planet is clearly warming today, as it has repeatedly done, but insist that no one knows exactly why. Other possible causes, they say, include changes in sea currents, Sun cycles and cosmic rays that bombard the planet.

“More and more data,” Jan Veizer, an expert on Phanerozoic climates at the University of Ottawa, said, “point to the Sun and stars as the dominant driver.”

Highlighting the gap, the two sides clash on how much the Earth would warm today if carbon dioxide concentrations double from preindustrial levels, as scientists expect. Many climatologists see an increase of as much as 8 degrees Fahrenheit. The skeptics, drawing on Phanerozoic data, tend to see far less, perhaps 2 or 3 degrees.