What is the evidence that CO2 is causing global warming?

As you can imagine, the How to Talk to a Climate Sceptic guide attracts a lot of comment from people who are less than inclined to agree with the general thrust of the material. Most can be easily answered with a pointer to another article or a rephrasing or expansion on one of the points in the post above it. (I’m not trying to claim that usually satisfies my skeptical visitors, but I don’t often go to more trouble than that. I try not to bang my head too hard against any brick walls that come my way!).

But I had one recent commenter who asked a very straightforward question that seemed to deserve a substantive answer, so I decided to make a new post rather than just add another comment. The question and answer are a bit off topic for that particular thread anyway.

It is a simple question, which I will prune of its snooty tone and quote here:

“What is the evidence that CO2 is causing global warming?”

The simplest thing to say to that, is “go read the IPCC report”. It is very thorough and very meticulous. (See the latest one here, but I encourage beginners to use the more convenient HTML format of the 2001 report here (even though it is out of date on many details). But because my visitor did specify “in my own words” (pop quiz!) and it is a good question when sincerely posed, I will try to lay it out below.

The very first thing to note about a response to a CO2 rise, is that an increase in the temperature of the global climate is completely expected.

We are all familiar with that basic scientific process where we examine the known properties of a system, observe or surmise a change of some sort, and then formulate an expectation based on an hypothesis, right? In the case at hand (and using a very broad brush), the system is the earth/ocean/atmosphere; the known properties are those of radiative gases, thermodynamics and electromagnetic radiation; and the change to the system is a slow and inexolerable increase in the amount of CO2 in the air. The next step is usually to perform an experiment and thus confirm or deny your hypothesis when your expecations are or are not met. Unfortunately, there is only one planet and one timeline to move it along, so that is out.

Fortunately, we have gone ahead and run that experiment on this one and only home planet of ours during our one and only chance to make our one and only human history! (Did I just say “fortunately”??).

(image courtesy of Global Warming Art)

So, because we know that CO2 is a radiatively active gas that allows the shortwave (visible) radiation from the sun into the climate system and slows that same energy down on its way out as longwave (infrared) radiation, we quite clearly expect that adding more CO2 will raise the average temperature of the earth’s surface. This has been expected for over 100 years! So, just like the internet, Al Gore did not invent Global Warming. (Also like the internet, Al Gore didn’t really claim to, but that’s a different story…)

In 1896, Svante Arrhenius wrote a paper on this very idea. You can follow the long, slow evolution of Anthropogenic Global Warming theory that followed at Spencer Weart’s most excellent “History of Global Warming”. Scientists have been improving our knowledge and watching closely for generations now.

So, it makes sense that it should happen. Is it?

(image source)

Yes, it is. And there are other indicators besides direct measurements of surface temperature.

But let’s not stop there, because expected things can happen for unexpected reasons, and correlation is not causation and all that. We need to eliminate other potential causes. Maybe it’s the sun? Maybe it’s natural causes [hand wave]? Maybe it’s volcanoes? Maybe it’s geothermal? Maybe it’s galactic cosmic rays? Well, the sun has not changed its output significantly since the fifities, or enough overall to explain the degree of warming. Saying “natural causes” is really <a href="http://scienceblogs.com/illconsidered/2006/02/this-is-just-natural-cycle.php&quot;just a cop out: what natural cause?? Blaming volcanoes or geothermal is silly and sillier. Cosmic rays is a pretty far fetched grasp at straws. The connection is only plausible, far from demonstrable, it has been looked for and not found, and it requires that some major foundations of current climate theory be completely wrong. That’s never impossible, but that possiblilty becomes more vanishingly unlikely all the time, having all but vanished decades ago. (I’m talking basics here, like CO2’s radiative properties, the basic dynamics of heat transfer in and between the atmosphere and ocean, accurate observation of basic climate properties).

The fact of the matter is, the IPCC reports spend alot of time on attribution studies[PDF]. It was never just taken it for granted that because we expect it and it has happened we therefore understand it. But in all of this hard researching, no other primary candidate cause has emerged that can explain the observations.

(image courtesy of Global Warming Art)

Just to pile on, here are some rather key specific observations beyond the rise in seasonally averaged global temperature that fit in well with an enhanced greenhouse effect (the relevant effect of increasing CO2 concentrations). These observations do not fit with other potential forcings.

  • Temperatures have risen more at night than during the day. This really defeats the notion of a solar powered climate change on its face.
  • The stratosphere is cooling. Models that predict the warming we are seeing also predict this particular feature of the current climate change.
  • An increasingly enhanced greenhouse effect should cause an energy imbalance between incoming sunlight and outgoing infrared radiation. This has been detected.

So to summarize: we know anthropogenic climate change is real because there is no other likely candidate cause, the CO2 rise is unquestionably the result of our activities, the particulars of the warming signature are consistent with an enhanced greenhouse effect and the whole phenomenon is entirely consistent with very long standing theories and expectations.

If it looks like a duck, and it quacks like a duck, why on earth would you think it is a galactic cosmic ray?

351 thoughts on “What is the evidence that CO2 is causing global warming?

  1. Since we have nothing left to consider, or we refuse to consider anything else, we must assume it’s a duck. No, not really. Only if you want to claim yourself right.


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