May 26, 2010

Solar Story Update

Filed under: Climate Forcings, Solar

We have written about the solar control on climate many times in the past, and to say the least, the debate continues to rage regarding the solar influence of Earth’s climate. IPCC has been luke warm on the subject, stating in the Technical Summary that “Solar irradiance contributions to global average radiative forcing are considerably smaller than the contribution of increases in greenhouse gases over the industrial period.” Two articles have appeared recently that provide even more evidence that variations in solar output have a profound impact on regional, hemispheric, and global climatic variations.

The first article comes from a pair of scientists from the Netherlands and Argentina that appeared recently in the Journal of Atmospheric and Solar-Terrestrial Physics. de Jager and Duhau begin stating “The question of the relation between the average tropospheric temperature and solar activity is a matter of debate. There are, however, indications that some relationship does exist. During the Maunder Minimum (1650–1710) the level of solar activity was low while the same was the case for the mean temperature.” We agree that some “debate” continues on the subject of solar control on Earth’s temperature, and we always welcome some new information on the subject.

The pair reports that most studies to date use sunspot numbers as a proxy for solar activity. However, the sunspot numbers reflect activity near the Sun’s equator, not its pole. They define a term or two noting “The toroidal component of the solar magnetic fields shows itself in the solar activity regions, with its many manifestations: sunspots, facular fields, solar flares, coronal mass emissions, prominences are the most important components. A generally used proxy for these activities is the number of sunspots.” Mechanisms that could link sunspots to climate on Earth include “the total solar irradiance directly influencing the troposphere, the UV flux from facular areas around sunspots by their influence on the terrestrial stratosphere, or else it might be based on the flux of magnetized plasma, affecting the rate of cloud formation by the modulation of the incoming flux of cosmic radiation on the Earth.” They again note that “These are related to the toroidal component of the Sun’s magnetic field.”

The authors state that there are indices of activity nearer the Sun’s polar regions and things like “polar faculae” and “bright points” are related to the poloidal component of the Sun’s magnetic field. They used a multiple regression approach to link the temperature of the Earth to both the toroidal and poloidal components of solar activity. They find that “next to the equatorial component, the polar component also influences tropospheric temperature. Its contribution is about 30% of that of the equatorial component.”

They next calculated the expected temperature of the Earth based on toroidal and poloidal components and subtracted it from the observed temperature (To – Te) to produce a residual series. As seen in Figure 1 below, the residuals show a strong upward trend since 1950 – the authors note that you can even see the slight cooling effect of Mount St. Helens in 1980. The global warming alarmists may want to point to the recent non-solar induced warming as a signal related to the buildup of greenhouse gases, but de Jager and Duhau warn:

“Interestingly, the amplitude of the present period of global warming does not significantly differ from the other episodes of relative warming that occurred in earlier centuries.”

They continue to explain the reason that recent global temperatures are unusually high:

“That [the recent period of global warming] actually shows high-temperature excursions in absolute measure is due to the fact that this episode of relative warming is superimposed on a relatively higher level of solar activity than the others, and from that point of view this observation may be a reason for claiming that the present period of global warming is exceptional.”

In other words, the reason that it is particularly warm in recent decades is because anthropogenic (or whatever the cause besides solar) warming has occurred on top of warming from high solar activity. Long-term readers of World Climate Report probably already know this, as we made first mention of it a dozen years ago.


Figure 1. Observed minus predicted temperatures based solar activity (from de Jager and Duhau, 2009)

Our second solar story article came out recently in Climatic Change and was produced by scientists Wu, Yu, Zeng, and Yang; the work was supported by the International Partnership Project, the Centurial Program of Chinese Academy of Sciences, National Natural Science Foundation of China, and National Basic Research Program of China. The team headed to far northwestern China in August of 2001 to extract a sediment core from Ebinur Lake. This is a dry part of China getting less than 4 inches of rain a year, while evaporation rates from the lake are nearly 14 times higher. This place is cold, windy, and dry, but due to the desert climate, the lake is nicely positioned to record changes in regional climate.

Wu et al. carefully examined their 50 inch core and explained that “The fine-grained clay sediments contain 3–17% organic matter (OM) and 9–31% carbonate, and are interrupted by multiple sand and silt layers. These sand/silt layers, having consistently low OM, were found at 700–800, 1000–1100, 1300–1400, and 1700–1750 a.d., with a time spacing of 300–400 years. We interpret that the low OM sand/silt layers were deposited during higher lake levels caused by increased river inflow from the surrounding mountains during wet climate intervals.” Furthermore, they examined oxygen isotope variations from an ice core taken from a glacier in the nearby Tibetan Plateau and found that the wet periods at the lake corresponded to periods of high snow accumulation at the glacier. In the last two sentences of their abstract, they note that “This approximate 400-year periodicity of wet–dry climate oscillations appear to correlate with solar activity” and “Our results suggest that solar activities might have played a significant role in driving wet–dry climate oscillations at centennial scales in the interior of Eurasian continent.”

There are still those who claim that the solar influence on Earth’s climate at various spatial and temporal scales is negligible. But as more an more reports come out to the contrary, this view is becoming harder and harder to hold.

References:

de Jager, C. and S. Duhau. 2009. Episodes of relative global warming. Journal of Atmospheric and Solar-Terrestrial Physics. 71, 194-198.

Wu, J., Z. Yu, H.-A. Zeng, and N. Wang. 2009. Possible solar forcing of 400-year wet–dry climate cycles in northwestern China. Climatic Change, 96, 473–482.




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