Global Warming - the two sides of the argument
This global warming business is very confusing - so I thought I would try and line up the two sides to argue it out.
There is no "plateau" where CO2 stops being important. Every time you double CO2, you get another 4 Watts per square meter of radiative forcing, so that the anticipated climate change between present CO2 and doubled CO2 is comparable to that between doubled CO2 and quadrupled CO2.
Water vapor and carbon dioxide are major greenhouse gases. Water vapor accounts for about 70% of the greenhouse effect, carbon dioxide somewhere between 4.2% and 8.4%. Much of the wavelength bands where carbon dioxide is active are either at or near saturation. Water vapor absorbs infrared over much the same range as carbon dioxide and more besides. Clouds are not composed of greenhouse gas -- they are mostly water droplets -- but absorb about one-fifth of the longwave radiation emitted by Earth. Clouds can briefly saturate the atmospheric radiation window (8-13µm) through which some Earth radiation passes directly to space (those hot and sticky overcast nights produce this effect - that is greenhouse but has nothing to do with carbon dioxide). Greenhouse gases can not obstruct this window although ozone absorbs in a narrow slice at 9.6µm. Adding more greenhouse gases which absorb in already saturated bandwidths has no net effect. Adding them in near-saturated bands has little additional effect. ..
Rather obviously, once a window is saturated adding more gases with the same properties will do nothing. This point seems to cause confusion for some people so perhaps consider multiple shades on a window with each shade blocking half the light coming through - pull one shade and you reduce the light source by half, pull another so you block half the light coming through the first shade, etc.. The effect of each shade diminishes as you keep adding more and eventually you get no additional effect - you have saturated or blocked the radiation window and it makes no difference if you double or quadruple the number of shades again.
In global warming and climate studies, figures are often given in Watts per meter squared (Wm-2) -- what does that mean? Surface warming ratios are variously cited as 0.1 °C per Wm-2 forcing through 1.0 °C per Wm-2 -- and most everything in between -- which ones make sense? ..
According to the National Academies' Climate Change Science: An Analysis of Some Key Questions (2001), doubling CO2 (to 600 ppmv) would lead to a forcing of about 4 W/m2..
The IPCC's estimate of additional forcing from all added CO2 since the Industrial Revolution is ≈ 1.5 Wm-2, the equivalent of change in radiative forcing (ΔF) for a value of 370 ppmv. Over the same period they estimate net warming of 0.6 ± 0.2 °C. Professor Roger Pielke, Sr., suggests a figure of 26.5-28% of contemporary warming is attributable to atmospheric carbon dioxide by estimating from IPCC-supplied forcing tables. Of the IPCC's estimated 0.6 ± 0.2 °C that would be 6/10 x 28/100 ≈ 0.17 ± ≈ 0.06 °C. That would give us a value of 1 Wm-2 ≈ 0.11 °C
...What does this 3.7 Wm-2 mean? How much warming does that equate to for the planet's surface? ...
On balance of available evidence then the current model-estimated range of warming from a doubling of atmospheric carbon dioxide should probably be reduced from 1.4 - 5.8 °C to about 0.4 °C to suit observations or ≈ 0.8 °C to accommodate theoretical warming -- and that's including ΔF of 3.7 Wm-2 from a doubling of pre-Industrial Revolution atmospheric carbon dioxide levels, a figure we suspect is also inflated.
The bottom line is that climate models are programmed to overstate potential warming response to enhanced greenhouse forcing by a huge margin. The median estimate 3.0 °C warming cited by the IPCC for a doubling of atmospheric carbon dioxide is physically implausible.
We have often made the case here that equilibrium climate sensitivity is most likely to be around 0.75 +/- 0.25 C/(W/m2) (corresponding to about a 3°C rise for a doubling of CO2). ..
...we know that for a particular model, once you know its climate sensitivity you can easily predict how much it will warm or cool if you change one of the forcings (like CO2 or solar). We also know that the best definition of the forcing is the change in flux at the tropopause, and that the most predictable diagnostic is the global mean surface temperature anomaly. Thus it is natural to look at the real world and see whether there is evidence that it behaves in the same way (and it appears to, since model hindcasts of past changes match observations very well).
So for our next trick, try dividing energy fluxes at the surface by temperature changes at the surface. As is obvious, this isn't the same as the definition of climate sensitivity - it is in fact the same as the black body (no feedback case) discussed above - and so, again it's no surprise when the numbers come up as similar to the black body case.
(an idealised 'black body' (which gives of radiation in a very uniform and predictable way as a function of temperature - encapsulated in the Stefan-Boltzmann equation) has a basic sensitivity (at Earth's radiating temperature) of about 0.27 °C/(W/m2). That is, a change in radiative forcing of about 4 W/m2 would give around 1°C warming)
In a transient situation (such as we have at present), there is a lag related to the slow warm up of the oceans, which implies that the temperature takes a number of decades to catch up with the forcings. This lag is associated with the planetary energy imbalance and the rise in ocean heat content. If you don't take that into account it will always make the observed 'sensitivity' smaller than it should be. Therefore if you take the observed warming (0.6°C) and divide by the estimated total forcings (~1.6 +/- 1W/m2) you get a number that is roughly half the one expected.
Hansen's model is dumping heat into the oceans at roughly 0.8 Wm-2 and the bulk ocean heat rise mid-1993 - mid-2003 sort of matched that -- if only the world would remain constant and conform to the models we'd have this "global warming" thing sorted. Like all happy accidents, however, this good thing came to an end, too.
Lyman et al (2006), using updated data from the same source, show that the period 2003-2005 involves a sudden ocean cooling at a rate of -1.0 ± 0.3 Wm-2 over the period, which means Hansen's model is calculating wrongly in both magnitude and sign. No one expected this loss of one-fifth of the heat stored in the ocean since 1955 and no model predicted it. Its cause is unclear but we appear to be witnessing Earth dumping heat to space via the atmosphere.
So it looks like "Real Climate" is arguing that the models are correct, even thought the earth doesn't realise yet how warm it is meant to be yet and is obstinately refusing to play along, whereas "Junk Science" seems to be trying to see how the models fit into the limited data we have of how the world is actually reacting, and then forecasting from that. I think I know who my money is on.