Ik denk wel dat de genoemde 24% de virtueel hoogste schatting is, maar desondanks intrigerend. Over dezelfde studie (http://www.waysnet.org/aggregator/sources/3):
,,For example, Shindell et al (Science, 2001) showed model results that suggested solar forcing could lead to enhanced winter cooling over certain regions of the Northern Hemisphere such as Europe during the Late Maunder Minimum (the coolest part of the European "Little Ice Age", the late 17th and early 18th centuries). The mechanism involves the large-scale dynamical response of the atmosphere to the estimated decrease in solar irradiance and subsequent stratospheric ozone change. The response was associated with a shift towards the negative phase of the so-called North Atlantic Oscillation (NAO)/Arctic Oscillation (AO) which projects onto a pattern of regional warmth and coldness that tends to cancel out in an average over the entire Northern Hemisphere. There is supporting evidence for this idea based on widespread proxy evidence for a negative phase of the NAO during late 17th and early 18th century intervals.
In that paper, the Northern Hemisphere temperatures cooled only modestly (at most a few tenths of a degree C) in response to the associated radiative forcing (lowered solar irradiance). A similar argument holds for the impacts of explosive volcanism, as shown in subsequent work by Shindell and colleagues (including us). However, the dynamical response associated with the NAO/AO pattern, which sits on top of the radiative response, leads to greater cooling in some regions at the expense of less cooling (or even warming) in other regions. This mechanism could explain the enhanced cooling in Europe for instance, but it cannot easily explain the additional evidence for an overall cooling of North Atlantic SST at this time. Shindell et al (2001) alluded, however, to the possibility that a sustained negative phase of the NAO would lead to a minor weakening of the North Atlantic meridional overturning circulation,
The GISS model results and empirical reconstructions both suggest that solar-forced regional climate changes during the Maunder Minimum appeared predominantly as a shift toward the low AO/NAO index. Although global average temperature changes were small, modeled regional cooling over the continents during winter was up to five times greater. Changes in ocean circulation were not considered in this model. However, given the sensitivity of the North Atlantic to AO/NAO forcing (37--Delworth and Dixon, J. Climate, 2000), oceanic changes may well have been triggered as a response to the atmospheric changes (38--Broecker, PNAS, 2000). Such oceanic changes would themselves further modify the pattern of SST in the North Atlantic (39--Visbeck et al, 1999) and, to a lesser extent, the downstream air temperature anomalies in Europe.
Jones and Mann later expanded on this point in a review paper published in 2004 in the journal Reviews of Geophysics (see final paragraph of section 5.3 therein). The argument basically goes as follows: a sustained forced negative state of the NAO pattern would lead, based on the experiments done by Delworth and Dixon (2000), to a small, but detectable [1-2 Sverdrups ('Sv') - or about 5-10%) decrease in the North Atlantic meridional overturning circulation (MOC), which in turn would lead to an additional pattern of cooling over the North Atlantic during the height of the LIA. This seems consistent with the small change in the strength of the "Gulf Stream" inferred by Lund et al study for the LIA.
An alternative view would suggest that rather than the MOC being a consequence of radiative and subsequent atmospheric change, it was in fact the sole factor and that this observed shift was just part of a multi-centennial oscillation. That implies that the southern Atlantic might be expected to warm slightly instead of cool and that the peak cooling would be in the North Atlantic rather than further afield. Of course, it is also conceivable that some combination of both mechanisms are required to explain the observed changes.''
Ik ga de studie van Shindell eens proberen op te duiken. Hij wordt ook gebruikt in het IPCC rapport van 2001 en de resultaten zijn dus ook verwerkt in de schattingen voor de stralingsbalans.
Gr. Ben
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