(10/1) You could be forgiven for not knowing this: Most atmospheric scientists who are counted as global warming skeptics agree that human activity is causing Earth’s climate to warm. Global warming, of course, is what causes all the phenomena covered by the broader term "climate change"—hurricanes, fires, floods, hotter summers, milder winters and so on.

And, to repeat the point, the climate change controversy that you may encounter is not happening in the scientific community. It’s the product of pundits and politicians who, frankly, don’t know much science, not even facts so well established that they are in textbooks.

Research on climate change started in the 1890s. That’s when a Swedish chemist named Svante Arrhenius got curious about why ice ages happened. He knew that sunlight is the main determinant of Earth’s surface temperature. So he studied various components of the atmosphere and wondered whether they fluctuated in the past.

Arrhenius found that most of the gases that make up our air are transparent to sunlight. Solar radiation comes straight through to the ground. But about half the heat we experience does not come directly from the sun. Once sunlight strikes surfaces (land, water, vegetation, sunbathers, etc.), some of it is reflected, allowing us to see these things. But some is absorbed, which turns it into heat. That’s why objects in direct sunlight become warm to the touch. Then much of that heat is radiated back up as infrared radiation, warming the air still further.

Arrhenius found that when ground heat is radiated up, specific molecules in the air did something unusual. They captured the infrared rays and then quickly re-emitted them. Two molecules did this, water vapor (H2O) and carbon dioxide (CO2). A molecule of either one would absorb the heat coming up from the ground, warm up briefly, and then spit it out again. Some of the re-radiated heat goes back down to the ground and adds to the heat on Earth’s surface. This is all established physics.

On average, the amount of heat absorbed at the surface is balanced by the amount radiating into space. On any given day, however, the balance can shift. We notice this most on cloudy nights when the air does not cool off as much as on clear nights. Those clouds are catching the heat coming up from the ground and sending some of it back down.

Those two gases, water vapor and carbon dioxide, are called greenhouse gases. (That’s not how greenhouses work, but we’re stuck with the metaphor.) The amount of water vapor varies hugely from day to day. Water molecules soon fall back to Earth. But carbon dioxide stays in the atmosphere for decades, even centuries. That’s why this is the gas of greatest concern for climate. All of this was well-established in the 19th century.

Arrhenius speculated that a reduction in carbon dioxide could have cooled the climate and brought on an ice age. But he also knew that human activity was pumping CO2 into the air, and he speculated that this could warm the planet beyond levels current in his day.

His scientific paper, published in 1896, was controversial at first. Many scientists could not accept that a trace gas — carbon dioxide constitutes just 0.04 percent of our atmosphere — could have such a profound effect. Those scientists were, in today’s parlance, global-warming skeptics. And properly so, given the science of the day.

But from the 1930s through the 1950s, the theory and the evidence underlying it gained credibility. In 1958, scientists started collecting the first proof that carbon dioxide concentrations were, in fact, growing. When precise measurements began in 1958, Earth’s air contained 315 parts per million of carbon dioxide. As of last spring, the concentration had grown to 419 ppm, a 33 percent increase.

No reputable scientist questions that increasing carbon dioxide in the atmosphere warms the air. Even one of the most scientifically respected global-warming skeptics, Richard Lindzen, professor emeritus of atmospheric physics at MIT, accepts this.

In a scientific paper he delivered in 2014 at a conference in France, Lindzen laid out four statements that he and, he said, most skeptical climate scientists agree with. They are: 1) Climate changes, 2) Carbon dioxide is a greenhouse gas; 3) Adding greenhouse gas to the atmosphere causes warming; and, 4) Human activity increases greenhouse gases.

In other words, most global-warming "skeptics" agree that human activity is causing the climate to warm.

So what is the supposed controversy about?

First, let’s put aside the loudest skeptics — the politicians and pundits. Their opinions are just that; they are not facts, nor are they educated interpretations of facts. The real issue among climate experts is how fast the warming will progress. Lindzen and the very small group of scientific greenhouse skeptics argue that it is happening so slowly that the world can adapt without much crisis or calamity. A much larger group — the overwhelming majority of climate scientists — fear otherwise. But fear is an emotion, and we are focusing on facts.

Interestingly, one prominent skeptic, Richard A. Muller, professor of physics at the University of California, Berkeley, has changed his mind. Muller, once a darling of global-warming deniers, had written articles doubting the main findings of global-warming projections and given speeches to that effect. But in 2012 he published an op-ed in The New York Times that began this way: "Call me a converted skeptic."

As part of his analysis, Muller sought to get rid of potentially misleading sources of evidence. For example, skeptics often challenge the evidence for rising temperature by noting that many official thermometers are in urban areas, which have grown and gotten warmer simply because they are surrounded by more buildings and pavement, the well-known urban heat island effect. So Muller used only data from rural weather stations. Those still showed warming.

Muller concluded that over the last 250 years the planet had warmed by 2.5 degrees Fahrenheit and that 1.5 of those degrees came in the most recent 50 years. Tellingly, he found that the temperature curve paralleled the rising carbon dioxide curve.

"I was flabbergasted," Muller says in a video on the website of his research group, BerkeleyEarth.org. He notes that his research discredits such alternative explanations for the warming such as solar variability and the recent absence of major volcanic eruptions. "We can rule out every scientific theory other than the greenhouse gas theory."

Which brings us back to Svante Arrhenius. His 1895 paper predicted that warming would proceed fastest in the north polar region because as the ice and snow cover declined, there would be less of that white surface to reflect sunlight back into space. The surface would be darker and, therefore, would absorb more light, converting it to heat. That’s exactly what has been happening. Average temperatures in both polar regions are rising twice as fast as elsewhere.

It is this global warming that drives a wide array of phenomena covered by the term climate change. Again, none of this is controversial within science. Outside of science, that’s another story. Facts and logic don’t matter there.

Read past editions of Science Matters