Posted on 10 July 2010 by John Cook

The Skeptics Handbook asks a central question: "What evidence is there that more CO2 forces temperatures up further?" A Scientific Guide to the 'Skeptics Handbook' answers this question by looking at various independent lines of evidence that our CO2 emissions are causing warming. In her response to the Scientific Guide, creator of the Skeptics Handbook Jo Nova agrees that "Sure there is evidence that man-made emissions might contribute to as much as 1.2°C and no more.".

Now I must confess, I was a little staggered by this admission. When I wrote A Scientific Guide, the target audience was not specifically Jo Nova but anyone willing to openly examine the full body of evidence for CO2 warming. So I was more than a little taken aback that Jo Nova herself would adopt a position undermining her own Handbook.

The Handbook's central tenet is that there's no evidence that more CO2 forces temperatures up further. She uses the missing tropospheric hot spot to conclude that warming "wasn't caused by greenhouse gases". She uses the CO2 lag to conclude "CO2 is trivial" and therefore "something else is causing the warming". She claims saturation of the CO2 effect means that "doubling CO2 won't make much difference". She goes so far as to say "the effect is so small, it's unmeasurable". That's right, the Skeptics Handbook states that the effect of rising CO2 is unmeasureable.

So A Scientific Guide presents the independent lines of evidence that the warming effect from CO2 is measureable and in fact has been measured. Direct observations closely match climate simulations. In the light of this weight of evidence, Jo Nova now agrees with the IPCC and James Hansen's assertions that the heat trapped by doubled CO2 would cause direct warming of 1.2°C. So you can imagine my jaw dropping when I read her response. She just debunked her own handbook!

That's not to say Jo waved a white flag in her response. On the contrary, she comes out kicking and punching in her trademark combative style. A lot of her critiques are due to one simple fact - when writing A Scientific Guide, I made the decision to keep it as short and simple as possible. While every instinct in me yearned to cram as much science as possible into the booklet, I disciplined myself to keep things brief and accessible to a broad audience. I could detect some frustration in Skeptical Science readers when they first read the Guide, with comments suggesting I include this or that factoid. So to satisfy both Jo and Skeptical Science readers, I've now added an Extended Scientific Guide to the 'Skeptics Handbook'. This webpage (HTML only, not a PDF) features links to 4 subpages that considerably flesh out the bare bones explanations in the PDF. The page also includes links to all translations of the Guide.

So to recap: the Skeptics Handbook claimed rising CO2 has an unmeasurable effect. A Scientific Guide to the 'Skeptics Handbook' shows how the CO2 effect has been measured. I often mention there's only one thing better than direct measurements made out in the real world. That's multiple sets of direct measurements made out in the real world, all pointing to the same result. When the full body of evidence all points to a "single, coherent answer", the conclusion is unavoidable. Well, there is one way to avoid it. Change the subject.

Jo Nova's response is essentially a big "yes but". In other words, "sure, CO2 causes warming but it's total feedback that matters". I've encountered this approach in many climate discussions. When you have someone pinned under the weight of the full body of evidence, they squirm away with a "yes but" and change to another topic. What Jo's doing here is the equivalent of Bill and Ted yelling "look, the Goodyear Blimp!" to distract the approaching cowboys.

But let's take a look at that Goodyear Blimp. The climate debate really needs to move on from whether CO2 traps heat, which is one of the more established and well understood areas of climate science, and onto more interesting questions like climate feedback. The reason A Scientific Guide didn't tackle feedback was because the focus of the Skeptics Handbook was on the CO2 effect. But as we now all seem to agree that CO2 traps heat and even agree on the degree of direct warming from CO2, we can move onto how much climate feedbacks will amplify or reduce the CO2 warming.

How do we find out what the total feedback is? The same as before. By considering the full body of evidence. There have been many studies into determining climate sensitivity, the measure of total feedback. These studies use a variety of empirical measurements to work out our climate's response to a change in energy imbalance. This includes the instrumental record, ocean warming, satellite measurements of outgoing radiation and paleoclimate reconstructions of various periods of Earth's past. To work out whether our climate has net positive or negative feedback, you need to consider all this evidence - not just a few isolated studies. All the different lines of evidence point to a climate sensitivity between 2 to 4°C for doubled CO2, with a most likely value around 3°C. This indicates net positive feedback.

But let's not get too distracted by the Goodyear Blimp. Climate sensitivity is an important subject and in subsequent posts, we'll be going into more detail about the various lines of evidence indicating positive feedback. Nevertheless, the topic at hand was the Skeptics Handbook and it's assertion that the CO2 effect "is so small, it's unmeasurable". We know this is a false statement - from multiple lines of empirical evidence published in peer-reviewed studies and from Jo Nova's own words.