Some thoughts on the subject.
Rather than give an opinion based on a personal prejudice (either way), I’ve had another look at the AR4 and come up with some stuff in support of the argument that AGW is exaggerated. See what you think.
Wherever possible, I’ve kept the numbers within the known bounds of probability and the IPCC’s own error bars.
First; it is really hard to argue against the case that CO2 warms the atmosphere, but by how much? The lower estimate of the IPCC is <+1.50 W/M2.
It is also hard to argue that there hasn’t been recent warming, so it has to be coming from somewhere.
Top estimates of solar variation put it at 0.3W/m2, though some reputable people have suggested that this is a conservative estimate: for the sake of argument, let’s say that Stott et. al. (a ‘legit.’ paper, unlike Solanki or some others) are correct. The actual figure ‘should’ be ~0.5W/M2.
What other factors come into play? There has been no recent change in volcanicity, or a major eruption/slowdown in annual outgassing, so the forcing (either way) from volcanoes is still around 0.
What about the Milankovitch cycles?: on the timescales we are dealing with here, no marked change outside normal variations is likely to be measurable, so they don’t account for the changes, in the short term.
Aerosols; an area of ‘low’ understanding and large error bars. Let’s say that the net direct and indirect (cloud albedo) aerosol effect is at the lower end (not so negative) of the estimates: net forcing here would be as low as ~-0.5W/M2.
Positive forcing from Ozone and water vapour are also limited: total of around +0.2w/M2
There are a few other to play with too. What comes out? Even if the forcing from CO2 is 1.5W/M2, the net anthropogenic forcing (positive effects minus negative effects) could be as low as 0.6W/M2 – this is the bottom end of the IPCC estimate. Messing around with the variables allows us to permit the possibility that CO2 forcing is not as high as the best guess, that the negative forcings have had less of a damping effect than has been assumed, that the sun has had a bit more of an effect, and cloud albedo and water vapour numbers, being as they are highly uncertain, favour the ‘not as bad as they are saying’ hypothesis.
What we can end up with (I’m neither advocating nor dismissing this), is a picture of recent human impact on climate which is less than half the ‘assumed’ amounts.
Add to this, that the estimate of climate sensitivity (how much warmer it will get if we double CO2) at the lower end of 2-2.5C.
Add to this, the estimates of sea-level rise, assuming no ‘dramatic’ change and using their number, is as likely to be 20cm in the next 100 years as the upper-end estimates of ~60cm.
Where do we end up? Somewhere close to many so-called ‘sceptics’ position on the AGW debate: By then end of the century, global average temperatures may well be a little less than 2C warmer than they are now. The sea will only have risen 8 inches. Though there are still droughts, hurricanes, melting ice-caps and the like, none of these exceeds our recent experience substantially. The net human input exists, but one part of it has largely been offset by another.
I would argue that this is a scientifically tenable and rational interpretation of the facts, physics and numbers as we have been given them by the IPCC.
No horror stories, no dramatic slide into chaos, no death by flooding or super-hurricane. Good reasons to consider where ocean-side development is viable or not, and good reason to support adaptive strategies by/for the most vulnerable nations. But would it be sufficient grounds to justify radical changes in energy policy? Probably not, though this wouldn’t weaken the argument for change based on pollution issues. Reason enough to justify careful monitoring of the various systems and variables which come into play. Good reason to fund scientific research, as well as social/infrastructure investment.
Reason to panic? Absolutely not. Reason to fear that our world will change beyond recognition? Not really. Reason to be cautious about the long-term decisions we make? We always should be, anyway.
There’s only one snag. In order to subscribe to this particular ‘version’ of the climate change future/AGW debate, it is necessary to accept that there is some human impact on the climate, that the solar impact is significant but not sufficient in itself, that the scientists aren’t entirely wrong about the laws of physics. The only ‘victim’ of this version, really, are the climate models: they are seen to be at best rough approximators of possibility, rather than reliable indicators of most likely outcomes.
So, are there any people who have been pooh-poohed as ‘sceptics’ who are willing to subscribe to this version of ‘scepticism, which does not deny climate change or AGW, but calls into question the assumptions about the extent and associated risks, calls into question the political motivations for action (they are worried about energy, not warming), call into question the Political/ethical conclusions being drawn without reference to the equal possibility of not much change occurring, but doesn’t deny the fundamental science?
Thatis my back-of the-envelope effort at responding to the excessive panic and doom-mongering that AGW invokes. Any takers?