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.
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Ian, I have to apologize for my initial tone with you, and confess I mistook most of the post as Jennifer's words rather than it being all yours. You do strike me as more serious minded about the science in this issue than she, I was therefore a bit too abrasive. Who knows, maybe I don't give Jennifer enough credit, time will tell.
I can't really add anything to your excerpts from Hansen's papers, I hope the point is not to prove there are in fact uncertainties because that is a given. As for regional failures of modeled climate, the question of its significance is necessarily a judgement call. After all, to quote someone (?) "all models are wrong, but some are useful". The failings you point out might lead one to say things are turning out worse than we would have expected, getting more warming where people live and less in the antarctic where it doesn't matter.
You quoted climateprediction.net:
"a PREDICTION of the climate of the 21st century
needs to contain the effects of sulphate aerosol
otherwise the warming trend may be OVERestimated"
I agree they should use the word projected. I don't see how this implies aerosols will increase, though have no knowledge of what they do assume. I really can't say for sure if your inference from roughly equal widening of the non-aerosol vs observations gap in the 1990's really means they are not using an aerosol scenario that levels off in the nineties. The effects may not be so linear.
I do appreciate the obvious time and effort that you are investing here. There are no dumb questions, only dumb answers. I will do my best to pass on what I know and provide resources where I can for what I don't.
Coby, I accept that I should not have said that well mixed GHGs "presumably have a similar warming impact in both hemispheres" (Figure 11 of Hansen et al, third map down in right column refers).
But I don't agree that 'there is no significant discrepancy between modeled and observed behaviours.' For example, on my reading of Figure 18 of Hansen et al (2005), to which Gavin directed me, the observed warming exceeded modelled warming by a significant margin over much the greater part of the northern hemisphere in the 1979-2003 period. In the Antarctic the discrepancy was in the opposite direction.
For the world as a whole, both the observed and the modelled warming in this period was about 0.4°C – i.e., equivalent to less than one-seventh of the vertical axis of the Figure. Visual inspection suggests that, over about one-half of the globe, the DIFFERENCE between the observed and modelled warming for this 24-year period was similar to, or greater than, the observed average warming for the world as a whole for this period. I’m surprised that you think that these aren’t significant discrepancies.
I did not misinterpret the mapping of aerosols and their effects on the ‘Sulphur Cycle Experiment’ page of the climateprediction.net website. The description is quite clear: the map shows ‘the model's surface temperature response to increasing sulphur emissions from pre-industrial levels.. to present day levels..’. The description also says that ‘The cooling effect of sulphate aerosol can be seen throughout the whole northern hemisphere’ (which isn’t strictly true: there is a warming effect in Northern Scandinavia).
The accompanying text says that ‘a PREDICTION of the climate of the 21st century needs to contain the effects of sulphate aerosol otherwise the warming trend may be OVERestimated’ (EMPHASES added). There are two errors here. First, the climateprediction.net simulations aren’t predictions; and secondly, the statement assumes, contrary to most expectations, that sulphate aerosol emissions will increase in the 21st century.
In fact, nearly all of scenarios project that emissions of sulphur oxides will DECREASE in this century. For the four SRES markers, the projected decreases between 2000 and 2100 are: A1, 60%; A2, 13%; B1, 84%; and B2, 31%. For the two illustrative A1 scenarios, the projected decreases are: A1FI, 42%; and A1T, 71%. These decreases lead to significant positive (negative of a negative) forcing in the 21st century, and concomitant WARMING in the IPCC scenarios.
There are huge uncertainties in relation to aerosols, both in respect of the trends in emissions in recent times and in the effects on climate. Having already tripped myself up once in a confusion of forcings and temperatures, I’ll avoid drawing my own conclusions and will simply draw a contrast between what Hansen et al (2005) say and what is posted on the climateprediction.net website:
(a) Hansen et al (2005) assess the total 1880-2003 negative aerosol forcing, including the indirect effect, as equivalent to more than one-half of the effective forcing for the total of the well-mixed GHGs (CO2, CH4, N2O and the CFCs) over the same period (pps. 5, 7). But they say that ‘empirical data for checking model-based temporal changes of tropospheric aerosol amount.. are meager’, and ‘Our largely subjective estimate of the uncertainty in the net aerosol forcing is at least 50%’ (p. 7).
(b) Hansen et al (2005) also say that: ‘Observed global warming, as well as the global warming in the model driven by all forcings, has been nearly constant at almost 0.15°C/decade over the past 3-4 decades, except for temporary interruptions by large volcanoes. This high warming rate has been maintained in the recent decade despite a slowdown in the growth rate of climate forcing by well-mixed GHGs.. The warming rate in the model is maintained because, BY ASSUMPTION, TROPOSPHERIC AEROSOLS STOP INCREASING IN 1990.. The ASSUMPTION that global aerosol amount approximately levelled off after 1990 IS UNCERTAIN, because adequate aerosol observations are not available.. An implicit well-known conclusion is that future global warming may depend substantially on how the global aerosol amount continues to evolve, as well as on the GHG growth rate’ (EMPHASES added).
(c) By (apparent) contrast, the simulations on the climateprediction.net website for the average of 66 models that had ‘made it to at least 2005’, to which I referred in an earlier post on this thread, show ‘overheating’ as a result of the non-inclusion of sulphate aerosols on what seems to be a much larger scale than implied in Hansen et al. Moreover, the widening of the gap between the ‘without aerosols’ temperature simulations and observations appears to be at least as great between 1990 and 2005 as in the decades preceding 1990.
To me, this suggests that the climateprediction.net estimates of aerosol emissions do NOT level off after 1990, and that a continuing growth in such emissions is reflected in the ‘Sulphur Cycle Experiment’ map.
In your initial post, Coby, you told me that for my follow up questions I might find Chapter 12 of the TAR informative, and provided a link. I am in fact quite familiar with what is said in that Chapter on regional climate projections, for which Australia’s John Zillman was Review Editor. If this post was not already overlong, I’d draw on the conclusions of that chapter, and on some of John’s subsequent statements on this subject, to reinforce some of the points made above.
I’m sorry if you think that I am again raising ‘the most elementary complication’ and that, because I ‘have never done anything but the most cursory research, [I] assume no one ever before has ever thought of it’ - and am ‘triumphantly pronounc[ing] climate science as an ignorant religion.’
You alleged, referring to me, ‘that it was completely understandable that most climate scientists are not interested in responding to people who come with pre-formed conclusions that imply they are stupid or frauds’, and said that it is ‘a credit to people like Gavin that they understand the importance of this issue and therefore the importance of overlooking, for the most part, such egregious behaviour.’
And you concluded that ‘the primary consequence is that the real experts in general decide not to waste their time with prejudice matched by ignorance and it falls on non-experts like [you] to spend the time.’
Let me assure you that I don’t come to climate change science issues with pre-formed conclusions that imply that climate change scientists are stupid or frauds. I tried for a long time to stick to my knitting, but found that I was being criticised (notably on this blog) for pleading ignorance of climate change science.
I’ve therefore decided that we should all be prepared to ask some dumb questions in areas outside our area of specialisation and that’s what I’m doing. I’ll have some separate questions to address to Gavin, but in the meantime I hope that you are able to take some time to respond to the issues raised above.