As previously indicated, Akasofu believes the changes in the sun to have been the primary mechanism for increased temperatures, but this is a theory that runs into immediate problems. Consider the following graphs:
As you can see, there are wide variations in estimates of changes in TSI and temperature over the course of the LIA. For my initial discussion, I will use the values indicated by Lean, 2000; and by Moberg et al., 2005 (the bright red line in the temperature reconstructions). Partly, this is because these reconstructions provide figures closest to those accepted by Akasofu.
Close examination of the charts shows a change of temperature between 1620 (the coldest period of the LIA) and 1850 of around 0.5 degrees C. There is a further increase of temperature of 0.3 degrees between 1850 and 1940; and from 1940 to 2004, another increase of just over 0.4 degrees C. That is reasonably close to the same increase over each interval.
In contrast, the increase in TSI over the same intervals are, respectively, 1.5 w/m^2, 1 w/m^2, and 0.05 w/m^2. Allowing that the increase in TSI is entirely responsible for the increase in temperature between 1620 and 1850, then we expect a 0.33 increase in temperature per 1w/m^2 increase in TSI. In other words, we would expect a 0.33 degree increase between 1850 and 1940, close enough to the correct value; but only a 0.17 degree increase in temperature between 1940 and 2004. So, <b>if we assume changes in TSI are responsible for all increases in temperature up to 1950, then that demonstrates that they cannot be responsible for increases in temperature post 1950</b>.
This argument does not depend on any assumptions about solar modulation of cosmic rays, or any other speculative theory yet to be proposed. The effects of such modulations, if real, would have included in the temperature increases from 1620 to 1850, as also those from 1850 to 1940. Further, the temperature rise cannot be explained as the impact of multi-decadal oscillations. Both 1940 and 2004 were at, or near the peak for both the Pacific Decadal Oscilation and the Atlantic Multidecadal Oscillation; and both were El Nino years. Consequently, any temperature difference between them due to oceanic oscillations should be minimal, and of no consequence.
As noted previously, there is significant uncertainty in reconstructions of LIA temperatures and TSI. It may occur to some people that Akasofu can escape the logic of my analysis by the appropriate choice of a temperature reconstruction and TSI reconstruction. However, the problem is not that easily solved. Taking Wang et al., 2005 for his temperature reconstruction, for example still leaves him with a 0.6 w/m^2 difference between 1620 and 1850, a 0.4 w/m^2 difference between 1850 and 1940, and at best a 0.2 w/m^2 difference between 1940 and 2004. That predicts a temperature difference between 1940 and 2004 or only 0.17 degrees C.
In fact, direct measurements show that there has been no increase in TSI since 1975, showing that no selection of TSI reconstructions can both explain temperature changes in the LIA, and temperatures changes in the late 20th century:
(Return to Part 1)
(Continue to Part 3)
Tom,
ReplyDeleteIf you go to http://www.co2science.org/data/mwp/mwpp.php and look at the best and most complete data set of the MWP and LIA, I think you might find more complete data than you have shown. It is clear that temperature have varied 1 or more degrees up and down many times over the Holocene. Since human activity can't be blamed for most of that time, what do you attribute the cause of variation?
Leonard, I am aware of CO2 Sciences fairly extensive data base of paleographic climate studies. However, I have found there data base to be highly selective. They show IMO a strong tendency to not include studies showing cooler temperatures durring the MWP; in there collected statistics, they will often include multiple studies from the same sites as though they were distinct peices of evidence; and they do not attempt to distinguish which studies overlap in time, so that a site showing warmth for just 50 years of the 500 odd year MWP is given the same weight in their collected statistics as one showing warmth over a much larger period. The upshot of this is that using CO2 Science as a source of paleoclimatological information is likely to distort your view of the past.
ReplyDeleteUnfortunately, while I do not trust CO2 Science, I can provide little better. It is generally accepted that the global temperatures where 1 to 2 degrees C above the 1960's average durring the Holocene Climactic Optimum, and that they have gradually declined since then with some large scale fluctuation at 4,000 years ago (Minoan Warm Period), 2,000 years ago (Roman Warm Period) and 1,000 years ago(Medieval Warm Period). However, the evidence to date is not good enough to determine if the Roman and Minoan warm periods were global, hemispheric or continental in scope, and the error bars on reconstructions are large compared to the temperature fluctuations even for the fairly well characterised Medieval Warm Period.
More troubling, CO2 levels rose by 20ppm from the Holocene Climactic Optimum (8,000 ago) to 1850, whereas with a declining temperature we would have expected it to decline by an equivalent amount. The source of the extra CO2 is from changing land use, primarilly from human deforestation for cropland, fuel and ships, and also from the spread of rice farming and cattle grazing, although earlier in the period the desertification of the Sahara provided a large natural supplement to this.
If temperatures have declined over the Holocene as most indications show, humans may have contributed between 20 and 30 ppm to CO2 levels over that period, which translates out as a 0.3 degree increase in global temperature - not inconsequential in the face of temperature fluctuations of only 1 to 2 degrees C.
But at least one analysis suggests that the CO2 concentrations are best explained by a rise in SST of at least 1 degree C over that period.
The upshot is that we are too uncertain about temperature fluctuations over that period to be sure about causes and effects. Human activity is a significant, but not a dominating part of the mix, in explaining the overall trend, should we ever determin what that is. But natural factors are probably more important in governing the major temperature fluctuations such as the MWP.
I don't think I will say anymore than that at this time; but hope to revisit the issue in a later post.
Tom,
ReplyDeleteI agree with your analysis of the CO2 science source data, but it does include the widest range of sources I have seen. It bothers me some what that such large emphasis is made that the MWP is lower or higher than the present, because it obviously was at least close either way, and that proves nothing except that the present warm period is not that special. It is clear to me that there was a period of unusual cold (LIA), and that claiming recovery to a more typical temperature (for the Holocene) is somehow requiring human domination, is not logical. I think there is some AGW (and so do many skeptics), but no evidence (that convinces me) that it is large or dangerous, and that is where the disagreement lies.