20th century global warming - "There is nothing new under the Sun" - Part II

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Solar Activity and Climate

It is already more than 200 years since Sir William Herschel claimed that variations in solar activity affect climate on Earth. Since he did not have any reliable temperature measurements, Herschel looked for indirect proxies. He compared the price of wheat in the London wheat exchange to the solar activity as mirrored in the sunspot number, and found a correlation between them.

In the 1970’s, it was Jack Eddy who pushed the idea that solar activity may be affecting the terrestrial climate. He found a correlation between long term variations in solar activity and different climate indicators. For example, he found that the nadir of the period called “the little ice age” in Europe during the latter half of the 17th century took place in a period during which solar activity was very low. This low activity culminated in a several decade period during which there were almost no apparent sunspots, which was called the “Maunder minimum”. On the other hand, there were other periods, such as the end of the middle ages, during which solar activity was as high as the latter half of the 20th century, and the temperatures were roughly as warm as today. During the “medieval optimum”, Vikings could settle in Greenland (and call it a “green land”) and catholic monks adopted sandals suitable for warm climates.

Presently, there is a large number of different empirical indicators showing that changes in solar activity has a non negligible effect on the climate. Changes in solar activity manifest themselves as changes in the strength of the solar magnetic field, changes in the sunspot number, in the strength of the solar wind (which is responsible for the impressive cometary tails) and other phenomena. These changes can be separated into three time scales.

The basic variation is an activity cycle of about 11 years, which arises from quasi-periodic reversals of the solar magnetic dipole filed. On longer time scales (of decades to millennia) there are irregular variations which modulate the 11-year cycle. For example, during the middle ages and during the latter half of the 20th century, the peaks in the 11-year cycles were notably strong, while these peaks were almost absent during the Maunder minimum. On the other hand, eruptions may appear on the time scale of days. Today there is evidence linking solar activity to the terrestrial climate on all these scales.

Since the work of Jack Eddy, many empirical results show a correlation between different climatic reconstructions and different solar activity proxies. One of the most beautiful results is of a correlation between the temperature of the Indian Ocean and solar activity (see fig. 5).

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Figure 5: The correlation between solar activity—as mirrored in the 14C flux, and a climate proxy, the 18O/16O isotope ratio derived from stalagmites in a cave in Oman, over a centennial to millennial time scale. The 14C is reconstructed from tree rings. It is a proxy of solar activity since a more active sun has a stronger solar wind which reduces the flux of cosmic rays reaching Earth from outside the solar system. A reduced cosmic ray flux, will in turn reduce the spallation of nitrogen and oxygen and with it the formation of 14C. On the other hand, 18O/16O reflects the temperature of the Indian ocean—the source of the water that formed the stalagmites. (From Neff et al., nature 411, 290, 2001). [/collapse]
It is much harder to see climate variations over the 11-year solar cycle. There are two reasons for that. First, if we study the climate on short time scales, we find that there are large annual variations (for example, due to the el-Ñino oscillation) which introduce cluttering “noise”, hindering the observation of solar related signals. Second, because of the large oceanic heat capacity, it takes decades until it is possible to see the full effects of given changes in the radiative budget, including those associated with solar variability. It is for this reason, that climates of continental regions are typically much more extreme than their marine counterparts.

If, for example, a given change in solar forcing is expected to give rise to a temperature change of 1°C after several centuries, then the same radiative forcing varying over the 11-year solar cycle is expected to give rise to temperature variations of only 0.1°C or so. This is because on short time scales, most of the energy goes into heating the oceans, but because of their very large heat capacity, large changes in the ocean heat content do not translate into large temperature variations.

Nevertheless, if the global temperature is carefully analyzed (for example, by folding the global temperature of the past 120 years over the 11-year solar cycle), it is possible to see variations of about 0.1°C in the land temperature, and slightly less in the ocean surface temperature.

Moreover, when studying directly the total ocean heat content, it is possible to see that the amount of heat going into the oceans is at least 5 times larger than can be expected from just the changes in the total solar irradiance (e.g., see this blog entry and references therein). Thus, one can conclude that there must be at least one mechanism amplifying the link between solar activity and climate.

Theoretically, there are two types of mechanisms which can amplify solar activity. The first type is hypersensitivity to one of the non-thermal solar components. One such mechanism was proposed by Joanna Haigh from the UK, and it is hypersensitivity to variations in the UV. This kind of sensitivity can arise because UV is almost entirely absorbed in the stratosphere and although it only includes about 1% of the solar output, the stratospheric structure (and = thus the tropospheric-stratospheric interface) is determined by this 1%. Numerical simulations have shown that by including the variations in the UV and their effects on the stratosphere, one can amplify the surface climate variations by as much as a factor of two, namely, it can be a large effect. However, it still cannot explain the large amounts of heat seen entering the oceans every solar cycle.

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