Two gray machines sit inside a pair of utilitarian buildings here, sniffing the fresh breezes that blow across thousands of miles of ocean.
They make no noise. But once an hour, they spit out a number, and for decades, it has been rising relentlessly.
The first machine of this type was installed on Mauna Loa in the 1950s at the behest of Charles David Keeling, a scientist from San Diego. His resulting discovery, of the increasing level of carbon dioxide in the atmosphere, transformed the scientific understanding of humanity's relationship with the earth. A graph of his findings is inscribed on a wall in Washington as one of the great achievements of modern science.
Yet, five years after Dr. Keeling's death, his discovery is a focus not of celebration but of conflict. It has become the touchstone of a worldwide political debate over global warming.
The greatest question in climate science is: What will that do to the temperature of the earth?
Scientists have long known that carbon dioxide traps heat at the surface of the planet. They cite growing evidence that the inexorable rise of the gas is altering the climate in ways that threaten human welfare.
Fossil fuel emissions, they say, are like a runaway train, hurtling the world's citizens toward a stone wall - a carbon dioxide level that, over time, will cause profound changes.
The risks include melting ice sheets, rising seas, more droughts and heat waves, more flash floods, worse storms, extinction of many plants and animals, depletion of sea life and - perhaps most important - difficulty in producing an adequate supply of food. Many of these changes are taking place at a modest level already, the scientists say, but are expected to intensify.
Near the end of his enormous story, Gillis finally raised the viewpoints of "contrarians" to global-warming dogma, with a slightly dubious tone.
However, the contrarians who have most influenced Congress are a handful of men trained in atmospheric physics. They generally accept the rising carbon dioxide numbers, they recognize that the increase is caused by human activity, and they acknowledge that the earth is warming in response.
But they doubt that it will warm nearly as much as mainstream scientists say, arguing that the increase is likely to be less than two degrees Fahrenheit, a change they characterize as manageable.
Among the most prominent of these contrarians is Richard Lindzen of the Massachusetts Institute of Technology, who contends that as the earth initially warms, cloud patterns will shift in a way that should help to limit the heat buildup. Most climate scientists contend that little evidence supports this view, but Dr. Lindzen is regularly consulted on Capitol Hill.
Gillis concluded with a melodrama return to Mauna Loa.
Late at night, as the delegates were wrapping up in Mexico, the machines atop the volcano in the middle of the Pacific Ocean issued their own silent verdict on the world's efforts.
At midnight Mauna Loa time, the carbon dioxide level hit 390 - and rising.
Gillis provided warning of the danger of overly evenhanded climate coverage (something the Times is in little danger of committing) in a post on the paper's "Green " blog.
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