And thanks for all the fish...

"Oh, no! Not again!" is the reaction of a bowl of petunias materialized as a side-effect of a passing spaceship powered by Infinite Improbability Drive in The Hitch-hiker's Guide to the Galaxy. The petunias find themselves high above the planet, and thus doomed to a brief lifespan of plummeting. We're left puzzling over how many times the petunias may have been put through this brief and pointless plunge.

This could well be the motto for anyone running across a global warming denier as well. I'm constantly struck by how often the exact same seemingly obscure "objections" are raised by ordinary people - regular folk, outside the hothouse of academia, who don't seem like the type to sit about reading up on galactic cosmic rays or paleoclimatic reconstruction. The selection of odd topics just keeps repeating in a most suspicious way.

I'm attending a course on the physics of radiative processes in the atmosphere (okay, so I am enfolded in the hothouse of academia). Chatting before class this morning, our prof mentioned having given a talk on global warming to grade 5 class. The kids had viewed An Inconvenient Truth and were now quite concerned.

The odd part was that one of the parents attended the talk and make it clear he is a full-out climate change naysayer. He interjected with a few rather technical questions, then buttonholed our prof for half an hour afterward. Apparently one of his issues was galactic cosmic rays (GCRs).

I don't know what sources this guy was working from, but it's fairly easy to imagine. There is no shortage of partisan websites which exist to collect new ways to argue against the reality of global warming, or at least trying to avoid human responsibility for it. They tend to engage in scientific "cherry picking," jumping on any publication or media statement by any scientist that could be either used, or twisted, in their campaign of denial.The flavour of the month is GCRs. Before that it was changing solar output as expounded by, e.g. Russian solar physicist K.I. Abdusamatov.

If you want a detailed account of what these are all about, and why they don't in any way refute the reality of human-induced global warming, I'll recommend RealClimate as a great resource. For my part, I'll just point out that in any of these "alternate explanations" of recent temperature changes that I've checked out, none have even attempted addressed the basic point that we know CO2 traps heat, and we have a good idea how strongly it affects climate. There are facts of basic physics that form the basis for the theory of the greenhouse effect and the role of CO2 as well as methane and other human additions to the atmosphere like CFCs.

Now, I'd say it's simply undeniable that humans are the cause of CO2 concentrations having risen suddenly to levels unseen for a million years or more. It's also unreasonable to suggest that we are not headed for a doubling or more over pre-industrial levels, barring a major reduction effort. The physics of the greenhouse effect tell us that this much additional CO2 will inevitably force a hotter climate.

So if you want to argue that CO2 is not already warming the planet and that higher levels in the future won't lead to more warming, it's not enough to say "I've found another outside forcing factor." Mainstream scientists using basic physics and spectroscopic measurements have worked out a figure known as the "radiative forcing" for a doubling of CO2 concentrations. The typical denial argument skips over this issue, implying that the forcing factor is zero or near zero. There is almost never any justification for that departure from the scientific literature. Richard Lindzen takes this tack, though at least he allows a non-zero value--something like 0.25 W m^-2 versus the consensus value of about 3.75 W m^-2. Lindzen at least gets points for giving his own number, but most denialists simply dismiss all this physics out of hand. The latch on to some new proposed candidate like GCRs or solar variability as if these could simply erase the existence of the greenhouse effect.

The point is that we have a clear theoretical handle on how and why CO2 absorbs infrared radiation in the range of the spectrum where the earth and atmosphere radiate strongly. This comes from hard science with precise laboratory measurements, as well as lots of ground- and space-based observation. This area of hard science provides a basis for predicting how much more heat we will trap by a given rise in CO2.

Saying there are other external forcings as well (solar variation, GCRs) says nothing to change these facts. Yet the denial movement regularly trots out these red herrings as if they somehow showed there is no greenhouse effect. The correct response to a proposed new forcing is to treat it in context of all the other forcings that have been so well covered in the IPCC assessment reports (ARs). Solar variation barely shows up on the chart; GCRs are too new a theory to have been covered in existing ARs; we'll have to wait to see how the dust settles on this new proposal.

But please be clear that this is all we have so far: a new proposed external forcing factor, which if confirmed (far from a certainty), would just have to be superimposed on all the existing forcings we have already identified and quantified. A new forcing does not wipe away all the others. Each known forcing has been assigned an error bar reflecting the remaining uncertainties over their exact values. A few are still poorly understood, but the forcings for CO2, methane and CFCs are fairly tightly constrained (unless you ask Richard Lindzen).

Next month, expect the denialist websites to latch on to some new claim, raising whatever it is to the level of a sure thing, and suggesting once again that this one means we can forget all about any greenhouse effect.

The non-sequiturs just keep rolling.

Oh, no! Not again!