The lure of solar forcing

This is not to say that there is no solar influence on climate change, only that establishing such a link is more difficult then many assume. What is generally required is a consistent signal over a number of cycles (either the 11 year sunspot cycle or more long term variations), similar effects if the timeseries are split, and sufficient true degrees of freedom that the connection is significant and that it explains a non-negligible fraction of the variance. These are actually quite stiff hurdles and so the number of links that survive this filter are quite small. In some rough order of certainty we can consider that the 11 year solar cycle impacts on the following are well accepted: stratospheric ozone, cosmogenic isotope production, upper atmospheric geopotential heights, stratospheric temperatures and (slightly less certain and with small magnitudes ~0.1 deg C) tropospheric and ocean temperatures. More marginal are impacts on wintertime tropospheric circulation (like the NAO). It is also clear that if there really was a big signal in the data, it would have been found by now. The very fact that we are still arguing about statisitical significance implies that whatever signal there is, is small.

Over the multi-decadal time scales, there is more reasonable evidence for an NAO and surface temperature response to solar changes though the magnitudes are still small. Over even longer time scales (hundreds of years) there are a number of paleo-records that correlate with records of cosmogenic isotopes (particularly 10Be and 14C), however, these records are somewhat modulated by climate processes themselves (the carbon cycle in the case of 14C, aerosol deposition and transport processes for 10Be) and so don’t offer an absolutely clean attribution. Nonetheless, by comparing with both isotopes and trying to correct for climate (and geomagnetic) effects, some coherent signals have been seen.

Some contrarian commentators have recently fallen into the habit of mass mailing any new solar-related abstracts and implying that the existence of solar forcing in the past negates any possible recent anthropogenic impact on climate. Since these studies do not have any implication for the radiative impact of CO2, and don’t change the fact that there has been no effective change in any solar indices since about 1950, it is hard to see a substantial basis for this (implied) argument. For instance, there has been a lot of recent attention paid to Mangini et al. (2005) where a solar link to a new Alpine speleothem record was claimed. However, a quick analysis (right) indicates that the explained variance in the record (smoothed over 25 years) correlated to the 14C-production function (a slightly cleaner solar proxy than the resdiual atmospheric 14C (Muscheler et al, 2005 – see comment/link below)) is only about 5%. Hardly a definitive refutation of anthropogenic greenhouse gas forcing.

A more interesting question is whether our current understanding of how solar forcing works is sufficient to explain the clearest solar impacts in the record. During the most studied period, the Maunder Minimum (MM) in the late 17th Century, sunspots were very rarely seen and that corresponded to a particularly cool period in the Northern Hemisphere (particularly in Europe as is seen in the speleothem record as well – NB. cooler temperatures are associated with increased isotope ratios). In order to assess that, all other forcings that were operating at the same time need to be considered as well. The MM was also a time of enhanced volcanic activity, and the cooling from this was probably comparable with the cooling due to solar effects (an exact attribution is impossible given the uncertainties in both forcings) .Another important factor is that the records of cooling at the MM are predominantly continental and mainly located in North America and Eurasia. This is consistent with the eveidence for a weak NAO at this time in independent reconstructions.

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