Ian Castles commented earlier this evening that:
According to the ‘passionate claim’ introducing this thread [C02 Drives Climate: Svante Arrhenius], “we have no alternative to the enhanced greenhouse effect, we have no alternative theory of atmospheric radiation, we have no explanation of the warming based on physically credible models, and we have no basis to believe the greenhouse effect stopped functioning beyond 280ppm of CO2.”
The ‘sceptics’ are accordingly challenged to produce an alternative theory that explains the observed warming over the past century.
The argument is a powerful one if the prevailing consensus explanation does in fact offer a satisfactory explanation of what has happened. Several contributors have claimed that it does. They point to the conclusion in Meehl et al (2004) that “the late-twentieth-century warming can only be reproduced in the model if anthropogenic forcing (dominated by greenhouse gases) is included.”
ABW notes that this has “not been disputed in any peer reviewed journal”, and describes the paper as “very nice work.”
Coby provides a link to a Wikipedia entry with a graph and a table derived from Meehl et al at http://en.wikipedia.org/wiki/Image:Climate_Change_Attribution.png . These show, prima facie, that, “The temperature trend hindcasted over the last century matches observed temperatures very well, and this requires CO2′s radiative forcing.”
There is however a problem. According to the models, anthropogenic forcing is the net outcome of a positive forcing resulting from increasing atmospheric concentrations of greenhouse gaess (GHGs) and a negative forcing from emissions of sulphate aerosols.
The well-mixed GHGs presumably have a similar warming impact in both hemispheres, whereas the cooling effect of sulphate emissions should be concentrated in the northern hemisphere where 90% of such emissions are generated. This is explained, with illustrative maps, on the climateprediction.net website at http://www.climateprediction.net/science/s-cycle.php . The text states:
“The regions of high anthropogenic source emissions of sulphur dioxide leads to high concentrations of sulphate aerosol over the northern hemisphere continents. Unlike greenhouse gases, the distribution and concentration of sulphates varies a lot with location, as can be seen by comparing the sulphate concentration over the North Pole with that over North America.”
So if the prevailing explanation of warming is correct, the greater increase in temperature should be in the southern hemisphere. Yet between 1976 and 2000, according to the IPCC, the average decadal rise in the northern hemisphere was 0.24°C per decade, compared with 0.11°C per decade in the southern hemisphere: see Table II.2 at http://www.grida.no/climate/ipcc_tar/wg1/053.htm .
And according to the latest satellite records, as reported at http://vortex.nsstc.uah.edu/data/msu/t2lt/tltglhmam_5.2 , the difference between the two hemispheres is even greater. The average decadal rise from 1978 to the present was 0.200°C in the northern hemisphere, but in the southern hemisphere (where the increase should have been GREATER because of the much lower average levels of concentration of sulphate aerosols) the trend rise was 0.059°C per decade.
In the past fortnight, the mystery has deepened. In the wake of the discovery of a major error in one of the files being used in the BBC Climate Change Experiment, it was announced that models had been inputting greatly reduced levels of man-made sulphate emissions throughout their runtime. The consequence was “that aerosols responsible for “global dimming” (cooling) are not present in sufficient amounts and models have tended to warm up too quickly.”
The Principal Investigator of climateprediction.net, Myles Allen, said in a message to the participants in the experiment that, “In essence, what your models have done is show how much the world would have warmed up over the 20th century if it weren’t for the masking effect of global dimming.. “.
This was illustrated in a chart produced by one of the Oxford University researchers which showed that, for the average of 66 models that had ‘made it to at least 2005’, the global average temperature anomaly in that year was 1.9°C in the simulations, compared with a global average anomaly of only 0.5°C according to the real-world observations estimated by the Climate Research Unit (CRU) at the University of East Anglia. (The anomalies represent the temperature difference compared with 1941-50).
Thus the simulations suggest that the world would have warmed by no less than 1.9°C in the half-century or so to 2005, had it not been for the masking effect of sulphate aerosols. With these effects taken into account, the observed increase in mean temperature should have been only slightly less than this in the southern hemisphere, but much less in the northern hemisphere where almost all of the sulphate emissions are generated.
Yet in the real world, the opposite has occurred. All of the observations show that the average increase in temperatures was SMALLER in the southern hemisphere.
I don’t conclude that the greenhouse effect stopped functioning at 280 ppm, or that rising atmospheric concentrations of greenhouse gases don’t contribute to global warming, or that there is no need to be concerned about climate change.
I do conclude that the causes of climate change are not yet adequately understood (and may never be). It is not to the point that ‘sceptics’ haven’t produced an adequate explanation either, or that the Meehl et al paper hasn’t been disputed in any peer reviewed journal.
If the close correspondence between modelled results and observations that holds at the global level falls down at the broadest level of disaggregation (the two hemispheres), the explanatory power of the model must be seriously questioned. I would welcome comments on this heresy.