Where does the politics end and the science begin? A few basic questions may help clear some of the smog.
Who came up with this greenhouse gas concept and how seriously is it taken?
According to physicist and historian Spencer Weart, the idea can be traced to French mathematician Joseph Fourier. Back in the 1820s, Fourier did some calculations to show that a rock like Earth orbiting at 93 million miles from the sun should be a big snowball. He suggested our balmy temperatures could be attributed to our atmosphere, which might hold in heat - sort of like a greenhouse.
There wasn't much experimental evidence to back this up until the 1850s, when British scientist John Tyndall started shining infrared light through various gases.
Tyndall knew the Earth absorbs solar radiation and emits infrared. If the greenhouse theory was right, gases that make up the atmosphere would absorb some of this infrared, thus raising the temperature.
At first he almost disproved the greenhouse effect by showing that infrared passes straight through oxygen and nitrogen - the main components of our atmosphere. But before he quit, Tyndall tried a few other gases, including CO2, and found it was a powerful absorber of infrared. Water vapor had the same property.
That suggested that while carbon dioxide makes up less than 0.001 percent of our atmosphere, it's working along with water vapor to prevent infrared radiation from escaping to space. These gases also emit radiation, some of which is directed back toward the ground. "As a dam built across a river causes a local deepening of the stream, so our atmosphere . . . produces a local heightening of temperature at Earth's surface," Tyndall wrote.
But people liked the greenhouse analogy and that one stuck, even though a greenhouse heats through a different mechanism.
Did Al Gore invent the idea of human-generated global warming?
No. It was the lesser known Arvid Hogbom who first suggested that humans might alter the climate by burning coal.
It was the dawn of the 20th century and scientists of the time were competing to figure out what caused the ice ages. It had become well accepted that carbon dioxide in the atmosphere exerted a powerful warming effect - and so it followed that a drop in carbon dioxide might lead to cooling.
One theory was that carbon dioxide waxed and waned with volcanic activity - and this drove the cycle of ice ages.
Hogbom, perhaps out of curiosity, decided to calculate how much additional carbon dioxide was being sent into the air from coal burning. He found to the shock of many that it was comparable to volcanoes.
That was enough to persuade Swedish chemist Svante Arrhenius to look into the matter. He calculated that if coal burning doubled atmospheric CO2, the global average temperature would rise by about 5 degrees Celsius - a little more than many climate scientists predict today, but in the ballpark.
Did people freak out when they heard this?
It wasn't a big concern at first, said Weart, author of the 2003 book The Discovery of Global Warming. For one thing, the population was fewer than a billion people and per-capita energy consumption was a quarter what it is today. At the rate they were going, it looked as if it would take a thousand years to double the world's CO2.
Also, these guys were in Sweden, so warmer weather didn't sound so bad.
But by the 1930s, a few scientists started to warn that with population and energy use rising, the CO2 component of the atmosphere would probably double sometime in the 21st century. They suspected that the rise would start to have a noticable effect by around 2000.
How do we know carbon dioxide is building up in the atmosphere?
This was an important scientific question in the 1950s. Some argued that all the CO2 we added from coal burning would dissolve in the oceans.
Others decided to see for themselves. American geophysicist Charles David Keeling did some of the most meticulous work, said Weart, traveling to Mauna Loa and the South Pole to get pure, representative samples of air.
He found that even year to year, the CO2 was rising - going from 311 to 314 parts per million from 1957 to 1958.
To see whether this represented a longer-term trend, a French-Soviet team took core samples from Antarctic ice and found a way to measure the CO2 in the bubbles. That showed the preindustrial carbon dioxide at about 290 ppm. (It's now at 389 ppm.)
The ice-core work also showed that carbon dioxide levels rose and fell in sync with temperature. While the cycle of ice ages appears to be driven by an interplay between the Earth's distance from the sun and the tilt of its axis, CO2 may have amplified those changes.
Is the Earth really getting warmer or were measurements faked?
Estimates of global temperatures have been the subject of a recent flap dubbed "climategate." A group of hacked e-mails cast doubt over one influential group in England.
The most serious accusations seem to relate to data and analysis from the 1980s that were destroyed or lost.
Meanwhile, other groups tracking temperatures have noted a rise over the 20th century.
But these may not even represent the most critical evidence, said Peter Pilewskie, an atmospheric physicist at the University of Colorado. "Thermodynamically, global average temperature has almost no meaning," he said. If the extra carbon from human activity is absorbing more energy and adding kinetic energy to the atmosphere, this could manifest itself with complicated regional effects, he said.
"If you want to see climate change, look no further than the Arctic," he said, where sea ice is melting even faster than predicted by some of the most dire forecasts. "That can't be denied."
How do we know humans are to blame?
"It's a process of elimination," said Pilewskie, who studies changes in the sun. Nothing unusual has been going on with the sun in the last 30 years, he said. Others have noted that nothing unusual is occurring with volcanoes. The one thing we know has changed is the CO2 component of the atmosphere - it's risen about 30 percent since the start of the industrial revolution.
"What cannot be denied," he said, "is that there has been an increase in CO2 and this increases the infrared opacity of the atmosphere and therefore increases the atmospheric greenhouse effect."
Do carbon dioxide and other greenhouse gases cause warming on other planets?
Yes, said David Grinspoon, a planetary scientist and curator of astrobiology at the Denver Museum of Nature & Science. The greenhouse effect is essential for understanding Venus and Mars.
"On the most basic level, planetary science shows that the greenhouse effect is real and that global change is real," he said. "If we were wrong about the basic physics of carbon dioxide greenhouse warming, we would not be able to predict surface temperatures of Venus and Mars."
Both of our neighboring planets, he said, have gone through catastrophic climate shifts thanks to changes in the greenhouse effect.
Various lines of evidence suggest Venus and Mars were more temperate in the past, but Mars lost most of its atmosphere, while Venus' atmosphere thickened through what's called a runaway greenhouse effect. Today Venus' surface is hot enough to melt lead.
In the late 1970s, long before Al Gore popularized climate change, astronomer Carl Sagan was talking about the greenhouse effect and making connections between our planet and our neighbors.
Why should we obsess about carbon dioxide when water vapor is the dominant greenhouse gas?
Because carbon dioxide is the driving force while water passively follows it, said Grinspoon. If we had no carbon dioxide, our planet would be frozen solid.
Water condenses and freezes at a higher temperature than carbon dioxide, he said, and the vapor in the atmosphere couldn't sustain itself without the carbon dioxide. Without our carbon dioxide, we'd freeze.
How hot will it get?
This is where experts remain uncertain. Without some drastic changes, carbon dioxide will almost certainly double before the end of the 21st century. Most estimates for the resulting temperature rise range from 2 to 4.5 degrees C.
Much of the wiggle room today centers on feedback loops, said Gerry North, a climate scientist at Texas A&M University.
"Most people in the field believe as you warm the climate, water vapor in the atmosphere will increase," he said, leading to more warming in a positive feedback loop.
But the models are clouded by clouds. Warming can increase clouds and they can cause either cooling or warming depending on a bunch of other complex factors.
"Climate modeling is hard," said Grinspoon. "I don't believe everything a climate modeler says . . . but I believe global warming is a very serious concern."
Contact staff writer Faye Flam at 215-854-4977 or firstname.lastname@example.org.