If you didn't know anything at all about the shape of the earth, and you just looked around you, how could you tell that the earth isn't flat? You can't see the curvature of the earth, so it's natural to think of the earth as a flat circle supporting the sky, as ancient peoples did. (It was the Greek natural philosophers who figured out that the earth is in fact a sphere.)

Similarly, it's natural to think that the earth's climate is stable. It may be colder one year and warmer the next year, but it seems implausible to imagine that it's going to get steadily warmer.

One of the Canadian think tanks I follow is the Institute for Research on Public Policy. I was surprised to see a recent blog post arguing that the scientific debate on man-made global warming isn't over yet. John Robson:

So apparently the scientific debate ended with the discovery that a warming process that has been going on for over 300 years was not man-made for the first 250 but has been since, and a cycle of warming and cooling going on for 3000 years was not man-made for the first 2950 but has been since. And don’t even get me started on that 800,000 year cycle, or what happened in the Jurassic, or the pattern since the “Cambrian explosion”. See, there’s no debate, nothing to see, just this odd business where the physical processes driving climate change for over 500 million years were suddenly replaced with a different set 50 years ago for reasons no one can explain.

Robson's sarcasm aside, let's take the question seriously: when did the scientific debate on global warming end?

In science, you always want to keep Cromwell's rule in mind: "I beseech you, in the bowels of Christ, think it possible that you may be mistaken." Scientific theories are always provisional: you can never prove a scientific hypothesis, you can only disprove it. Even when a theory is generally accepted, new evidence can overturn it (Newton's laws turn out not to apply at quantum scales or relativistic speeds). But when you're looking at the steady stream of new scientific discoveries, you want to distinguish between those that overturn major theories (which happen extremely rarely) and those that can easily be fitted into them, perhaps with some refinements.

Also note that even eminent scientists may refuse to accept new developments-- Einstein famously rejected the randomness of quantum mechanics, saying that "God does not play dice with the universe."

So, bearing in mind that there will always be a few skeptics, when did scientists understand that burning fossil fuels was going to cause major changes to the climate?

The logic isn't hard to understand:

• Atmospheric CO2 traps heat. We can measure this directly.

• We can directly measure the level of CO2 in the atmosphere and see that it's steadily increasing.

• Thus we can expect the amount of heat trapped by the atmosphere to increase over time.

There's a phenomenon called the illusion of explanatory depth, where we think that because we're familiar with something, we understand it--until we try to sit down and explain it step by step. My question to global-warming skeptics is always: Can you explain (step-by-step) how increasing CO2 is not going to result in global warming?

Returning to Robson's question, the key discoveries (described in detail in Spencer Weart's "The Discovery of Global Warming") were:

• CO2 traps heat--Gilbert Plass, 1956.

• Measurement of atmospheric CO2--Charles Keeling, starting in 1958.

  

• Temperature and atmospheric measurements for the last 400,000 years, extracted from Antarctic ice cores--published in 1985. These show that atmospheric CO2 had cycled between 180 parts per million (during the coldest periods) and 280 ppm (during the warmest periods). In the 1980s, CO2 had reached 350 ppm, and was still rising. It was clear that we're putting the atmosphere through changes that previously only happened over geological timescales.

  

So the scientific debate was over by the late 1980s. After that, there were still skeptics within the scientific community (Richard Lindzen, for example), but as evidence piled up that warming was occurring, and as paleoclimatologists continued to reconstruct past climate records, they became increasingly rare.

You would have to go back at least 15 million years to find carbon dioxide levels on Earth as high as they are today, a UCLA scientist and colleagues report [October 8, 2011] in the online edition of the journal Science.

"The last time carbon dioxide levels were apparently as high as they are today — and were sustained at those levels — global temperatures were 5 to 10 degrees Fahrenheit higher than they are today, the sea level was approximately 75 to 120 feet higher than today, there was no permanent sea ice cap in the Arctic and very little ice on Antarctica and Greenland," said the paper's lead author, Aradhna Tripati, a UCLA assistant professor in the department of Earth and space sciences and the department of atmospheric and oceanic sciences.

Robson also asks about how the climate changed in the past, before we started burning fossil fuels. The answer is, Milankovitch cycles. Skeptical Science is a good resource for anyone interested in patient dissection of skeptical arguments:

Scientists have shown that CO2 and climate moved in lock-step throughout the Pleistocene ice ages. The ice ages were actually many pulses of cold glacial phases interspersed with warmer interglacials. These pulses had a distinct regularity caused by wobbles in Earth’s orbit around the Sun (Milankovitch cycles). When Earth’s orbit reduced the intensity of sunlight in the northern hemisphere, the Earth went into a glacial phase. When the orbital cycle increased the intensity of insolation in the northern hemisphere, ice sheets melted and we went into a warm interglacial. Because warmer oceans can dissolve less CO2, the CO2 levels see-sawed extremely closely with Earth’s temperature. It was a slow pace of change, taking tens to hundreds of thousands of years, and yes..., in the last million years the biggest orbit-induced cycles were every 100,000 years.

But we know these orbital changes are not behind today's global warming. In fact our orbit dictates we should be cooling now, not warming.

The Earth was indeed cooling over the last 6,000 years due to Earth's orbit, heading into the next glacial phase scheduled for about the year 3500 AD. But all that changed when we got to the industrial era. Global temperatures departed from that cooling trend, and instead rose parallel with our greenhouse gas emissions.

The question of what policy we should follow--what should we do about increasing greenhouse gases?--is of course a completely separate question. Fossil fuels are awesome (Mark Jaccard describes them as "concentrated sunlight"), which is why they're so hard to give up.

Joseph Heath describes this as a collective action problem:

"I think everyone can understand free rider problems, but almost no one bothers to think of the world in that way."

Sad but true. One of the things I’m constantly amazed by in discussions over climate change is how elusive the basic concept of a collective action problem remains, and how unintuitive it is for many people (whether to grasp, or just to apply, as James suggests). I know that I certainly didn’t “get it” right away. I had been told the story of the Prisoner’s Dilemma several times before I realized that it was not just a little puzzle, but in fact a very big deal. (Probably reading Russell Hardin’s book, Collective Action, is what caused the heavens to open for me. Or perhaps David Gauthier’s Morals by Agreement.)

Anyhow, since that time, seeing the world in terms of collective action problems has become such second nature to me that I have increasing difficulty imagining what it would look like in any other terms, and thus, I have difficulty believing that anyone still fails to see it in those terms. I teach the Prisoner’s Dilemma, the Tragedy of the Commons, and all the basic stuff about collective action problems, every year in my classes. And yet I feel intensely self-conscious every time I do, figuring that what I’m saying is so obvious that I’m boring most of the students. (I usually preface my little lecture with an apology to all those who have heard the basic line before.)

And yet, the other day I was reading this little book by Naomi Oreskes and Erik M. Conway, The Collapse of Western Civilization, and they totally don’t get it. The book is all about climate change, and yet the concept of it being a “collective action problem” just doesn’t show up. Thus they express complete bewilderment over the fact that we might all know that outcome x is undesirable, and yet fail to act to avoid outcome x. So they wind up getting stuck on the dilemma that so many environmentalists wind up stuck on, when it comes to explaining our inaction: either 1. it must be the fault of scientists, for somehow failing to communicate effectively how bad x is going to be, or 2. there must be some “ideology” that holds us prisoner, preventing us from acting.

If you don't understand collective action problems, you get this kind of dynamic:

“Outcome x is really bad, and our doing y is making it worse. We should all stop.”

Time passes

“Nothing happened. People are still doing y, and we’re still getting outcome x. How could that be?”

“It must be that they don’t understand how bad outcome x is going to be. Guys! Outcome x is going to be REALLY BAD, don’t you get it?

Time passes

“Nothing happened. People are still doing y, and we’re still getting outcome x. How could that be?”

“It must be that they don’t understand how bad outcome x is going to be. Guys! Outcome x is going to be a FUCKING CATASTROPHE, don’t you get it?

Time passes

“Nothing happened. People are still doing y, and we’re still getting outcome x. How could that be?”

“It must be that they don’t understand how bad outcome x is going to be. Guys! Outcome x is going to be the END OF WESTERN CIVILIZATION, don’t you get it?"

Time passes

“Nothing happened…”

Repeat ad infinitum….

Another article by Joseph Heath which goes into more detail: It's Not Easy Being Green.