Little Ice Age (Solar Influence)

How much of an influence the sun has exerted on earth's climate during the 20th Century is a topic of heated discussion in the area of global climate change. The primary reason for differing opinions on the subject derives from the fact that although numerous studies have demonstrated significant correlations between certain measures of solar activity and various climatic phenomena (Reid, 1991, 1997, 1999, 2000), the magnitude of the variable solar radiative forcing reported in these studies is generally so small it is difficult to see how it could possibly produce climatic effects of the magnitude observed (Broecker, 1999). Supporters of solar effects theories counter by contending that various positive feedback mechanisms may amplify the initial solar perturbation to the extent that significant changes in climate do indeed result. In this summary, we highlight some of the recent scientific literature that demonstrates the viability of such solar linkages and the emerging belief that small changes in the sun's energy output produced the coldest period of the past millennium, i.e., the Little Ice Age.

Many solar-climate studies utilize tree-ring records of ¹⁴C as a measure of solar activity, because solar activity (including variations in the number of sunspots and the brightness of the sun) influences the production of atmospheric ¹⁴C, such that periods of higher solar activity yield a lower production and atmospheric burden of ¹⁴C (Perry and Hsu, 2000). This being the case, it can be appreciated that as trees remove carbon from the air and sequester it in their tissues, they are recording a history of solar activity that could be influencing earth's atmosphere-ocean system. Thus, the history of ¹⁴C contained in tree rings has been examined by a number of authors as a proxy indicator of solar activity and compared with various indices of climate.

A good example of this type of work is the study of Hong et al. (2000), who developed a high-resolution ð¹⁸O record from plant cellulose deposited in a peat bog in the Jilin Province of China (42° 20' N, 126° 22' E), from which they inferred that the most dramatic cold events of the Little Ice Age were centered at about AD 1550, 1650 and 1750. In comparing their ð¹⁸O temperature record with changes in atmospheric ¹⁴C derived from tree rings, the authors report a "remarkable, nearly one to one, correspondence," which led them to conclude that the temperature history of this region was "forced mainly by solar variability."

Verschuren et al. (2000) also utilized a ¹⁴C record as a proxy for solar activity, comparing it with a decadal-scale history of precipitation in equatorial east Africa over the past 1000 years. The results of their analysis revealed that this region experienced relatively wet conditions during the Little Ice Age from AD 1270 to 1850. However, this latter period was interrupted by three periods of prolonged dryness: 1390-1420, 1560-1625 and 1760-1840. These "episodes of persistent aridity," in the words of the authors, were "more severe than any recorded drought of the twentieth century." In addition, they discovered that "all three severe drought events of the past 700 years were broadly coeval with phases of high solar radiation, and the intervening periods of increased moisture were coeval with phases of low solar radiation." Dean and Schwalb (2000) report similar solar-related drought conditions with periodicities of 200 and 400 years for the Great Plains during the main cold phase of the Little Ice Age, as do Haug et al. (2001) for tropical Venezuela.

Other authors have also made the case for a solar-forced Little Ice Age. Vaganov et al. (2000) found a significant correlation between solar activity and temperature during the Little Ice Age in the Asian subarctic. Nearby in Europe, a review of the relationship of extreme weather events to climate during the Holocene implicates solar forcing as the factor responsible for above-average rainfall during the Little Ice Age. According to Starkel (2002), continuous rains and high-intensity downpours coincided with periods of reduced solar activity and were major problems that often led to severe flooding there.

In North America, an analysis of more than 700 pollen diagrams by Viau et al. (2002) indicates a vegetation transition that "culminat[ed] in the Little Ice Age, with maximum cooling 300 years ago." In contemplating the reason for the transition the authors state that "although several mechanisms for such natural forcing have been advanced, recent evidence points to a potential solar forcing (Bond et al., 2001) associated with ocean-atmosphere feedbacks acting as global teleconnections agents."

Bond et al. (2001) examined deep-sea sediment cores in the North Atlantic and cosmogenic nuclides sequestered in the Greenland ice cap (¹⁰Be) and Northern Hemispheric tree rings (¹⁴C) and concluded "it seems almost certain that the well-documented connection between the Maunder solar minimum and the coldest decades of the LIA could not have been a coincidence" and that the Little Ice Age "may have been partly or entirely linked to changes in solar irradiance."

Two additional papers, both model-based studies, also point to a significant role for the sun in producing earth's Little Ice Age climate. Using a version of the Goddard Institute for Space Studies GCM, Shindell et al. (2001) estimated climatic differences between the period of the Maunder Minimum in solar irradiance (mid-1600s to early 1700s) and a century later, when solar output was relatively high for several decades. The results of their analysis led them to conclude that "colder winter temperatures over the Northern Hemispheric continents during portions of the 15th through the 17th centuries (sometimes called the Little Ice Age) ... may have been influenced by long-term solar variations." The second of these studies, by Perry and Hsu (2000), developed a simple solar-luminosity model and used it to estimate total solar-output variations in the Holocene. The model output was well correlated with the amount of carbon 14 in well-dated tree rings throughout the Little Ice Age and prior to it, which finding, in the words of the authors, "supports the hypothesis that the sun is varying its energy production in a manner that is consistent with the superposition of harmonic cycles of solar activity."

So what changes in solar activity are responsible for producing earth's Little Ice Age climate? Several authors have targeted the approximate 11-year sunspot cycle as a primary suspect, but recent work by Rozelot (2001), who noted that "warm periods on Earth correlate well with smaller apparent diameter of the Sun and colder ones with a bigger Sun," has added variations in the sun's radius to the mix.

With respect to the 11-year sunspot cycle, Dean et al. (2002) examined a 1500-year varve thickness time series taken from a lake sediment core in Minnesota, USA, and report the signal from this oscillation to be strongest between the 14th and 19th centuries, during the Little Ice Age. Additionally, Parker (1999), Solanki et al. (2000) and Rigozo et al. (2001) all report relative minima in the mean number of annual sunspots during the Little Ice Age. Relative to the present, sunspot numbers during the Little Ice Age were more than 40 times fewer (Rigozo et al., 2001). Similarly, analyses of other solar parameters by Rigozo et al. indicate that the strengths of the solar radio flux, the solar wind velocity and the southward component of the interplanetary magnetic field were 1.97, 1.11 and 2.67 times weaker during the Little Ice Age than they are presently.

How do these small changes in solar activity bring about significant and pervasive shifts in earth's global climate, such as the Little Ice Age? In answer to this question, which has long plagued proponents of a solar-climate link, Bond et al. (2001) describe a scenario whereby solar-induced changes high in the stratosphere are propagated downward through the atmosphere to the earth's surface, where they likely provoke changes in North Atlantic Deep Water formation that alter the global Thermohaline Circulation. In light of the plausibility of this scenario, they suggest that "the solar signals thus may have been transmitted through the deep ocean as well as through the atmosphere, further contributing to their amplification and global imprint."

Concluding their landmark paper, Bond et al. say the results of their study "demonstrate that the earth's climate system is highly sensitive to extremely weak perturbations in the sun's energy output," noting that their work "supports the presumption that solar variability will continue to influence climate in the future."

Clearly, there is ample ammunition for defending the premise that the global warming of the past century or so may well have been nothing more than the solar-mediated recovery of the earth from the chilly conditions of the Little Ice Age, and that any further warming of the planet that might occur would likely be nothing more than a continuation of the same solar-mediated phenomenon.

References
Bond, G., Kromer, B., Beer, J., Muscheler, R., Evans, M.N., Showers, W., Hoffmann, S., Lotti-Bond, R., Hajdas, I. and Bonani, G. 2001. Persistent solar influence on North Atlantic climate during the Holocene. Science 294: 2130-2136.

Dean, W.E. and Schwalb, A. 2000. Holocene environmental and climatic change in the Northern Great Plains as recorded in the geochemistry of sediments in Pickerel Lake, South Dakota. Quaternary International 67: 5-20.

Dean, W., Anderson, R., Bradbury, J.P. and Anderson, D. 2002. A 1500-year record of climatic and environmental change in Elk Lake, Minnesota I: Varve thickness and gray-scale density. Journal of Paleolimnology 27: 287-299.

Haug, G.H., Hughen, K.A., Sigman, D.M., Peterson, L.C. and Rohl, U. 2001. Southward migration of the intertropical convergence zone through the Holocene. Science 293: 1304-1308.

Hong, Y.T., Jiang, H.B., Liu, T.S., Zhou, L.P., Beer, J., Li, H.D., Leng, X.T., Hong, B. and Qin, X.G. 2000. Response of climate to solar forcing recorded in a 6000-year delta¹⁸O time-series of Chinese peat cellulose. The Holocene 10: 1-7.

Perry, C.A. and Hsu, K.J. 2000. Geophysical, archaeological, and historical evidence support a solar-output model for climate change. Proceedings of the National Academy of Sciences USA 97: 12433-12438.

Reid, G.C. 1991. Solar total irradiance variations and the global sea surface temperature record. Journal of Geophysical Research 96: 2835-2844.

Reid, G.C. 1997. Solar forcing of global climate change since the 17th century. Climatic Change 37: 391-405.

Reid, G.C. 1999. Solar variability and its implications for the human environment. Journal of Atmospheric and Solar-Terrestrial Physics 61(1-2): 3-14.

Reid, G.C. 2000. Solar variability and the Earth's climate: introduction and overview. Space Science Reviews 94(1-2): 1-11.

Rigozo, N.R., Echer, E., Vieira, L.E.A. and Nordemann, D.J.R. 2001. Reconstruction of Wolf sunspot numbers on the basis of spectral characteristics and estimates of associated radio flux and solar wind parameters for the last millennium. Solar Physics 203: 179-191.

Rozelot, J.P. 2001. Possible links between the solar radius variations and the Earth's climate evolution over the past four centuries. Journal of Atmospheric and Solar-Terrestrial Physics 63: 375-386.

Shindell, D.T., Schmidt, G.A., Mann, M.E., Rind, D. and Waple, A. 2001. Solar forcing of regional climate change during the Maunder Minimum. Science 294: 2149-2152.

Solanki, S.K., Schussler, M. and Fligge, M. 2000. Evolution of the sun's large-scale magnetic field since the Maunder minimum. Nature 408: 445-447.

Starkel, L. 2002. Change in the frequency of extreme events as the indicator of climatic change in the Holocene (in fluvial systems). Quaternary International 91: 25-32.

Vaganov, E.A., Briffa, K.R., Naurzbaev, M.M., Schweingruber, F.H., Shiyatov, S.G. and Shishov, V.V. 2000. Long-term climatic changes in the arctic region of the Northern Hemisphere. Doklady Earth Sciences 375: 1314-1317.

Viau, A.E., Gajewski, K., Fines, P., Atkinson, D.E. and Sawada, M.C. 2002. Widespread evidence of 1500 yr climate variability in North America during the past 14,000 yr. Geology 30: 455-458.

Verschuren, D., Laird, K.R. and Cumming, B.F. 2000. Rainfall and drought in equatorial east Africa during the past 1,100 years. Nature 403: 410-414.

Several folks have asked for information about the Sun's role in warming/cooling. Here is recent summary with lots of links.

I appreciate the work CO2 Science Mag. is doing on this subject. Send them a donation(tax deductable) if you do also, and are able, so they can keep it going, and keep it free for all.

Enjoy!

There has been considerable mention on late nite talk radio - you know who you are! - about the color of the sun. It seems that the past couple of years the sun has gotten much whiter.

With the full moon on the fresh snow the past couple days and nights, there was an opportunity to observe the color of the moonlight, which is related to the color of sunlight, without the overwhelming intensity of direct sunlight.

The observation is this: Artificial lighting has gotten warmer, that is, streetlights are pink to orange and interior lights have tended toward longer life bulbs. Longer life bulbs burn dimmer and considerably more yellowish than older style bulbs. All this is acclimatizing our eyes to a warmer spectrum, taking yellow and pink/orange as normal. Then, when we see in direct sunlight, the sun, which is bright but yellowish, seems whiter than it used to. Those who stare at computer monitors every waking hour are also getting used to a different visual spectrum than natural light.

So, the sun is not getting appreciably whiter, but we are getting unused to natural light.

Huh? An article on the mind-boggling concept that the sun may have an influence on our planet's temperature, and no mention of Sallie Baliunas?

I found that odd too, but maybe things are a changing. For a long time she was the voice in the wilderness. Now it looks like she is gaining a chorus.

Holy, Moly!! Look what I just found!

This article at SEPP implies that the IPCC generated assumptions to get the results they wanted:

"If what they say is true, then the methodology used is untenable and hence the results are the same. I can't believe anyone would start with an emissions projection, then make an estimate of the relationship between emissions and growth, and finally calculate growth residually. This is the relationship absolutely backward."

But that, apparently, is exactly what the IPCC's economic project did: It worked backward. Formally known as the Special Report on Emissions Scenarios, the official IPCC group set about creating economic forecasts that appear to support average world temperature increases of somewhere between 1.4 and 5.8 degrees over the next 100 years. But the SRES, by imposing emissions forecasts on economic models, adopted scenarios that economists say make no sense.

It belonged right here. The sun is a variable star, but some are seeing a little more variability than is actually occurring. More whiteness equates to a hotter sun, and some think they are seeing a whiter sun. The trend is toward a hotter sun, but the change wouldn't be appreciable or significant in a generation. Millions of years from now the oceans may boil, etc., but we need to look somewhere else for abrupt climate changes, if they are happening at all.

I have been noticing for several years that natural light appears harsher, especially around mid-day, but when I mention it to people, they say "huh"? I don't think it has anything to do with computer screens or the color of artificial lights.

I also noticed a lot more crows than there used to be, but again, people just say "huh"?

So either I am more observant than most or I've lost my mind, or maybe both.

I've seen the sun's color appear whiter as well, and my explanation above is what I think is causing that perception. Harsher light, seems like a good observation. Some days the sun is brutally bright. Some of that could be physiological, internal to the observer, the eye-brain connection. But you do notice changes. We don't have crows, but it seems like there are more ravens these days.

Right, though, people just say huh? and go on about their business.

"I have been noticing for several years that natural light appears harsher, especially around mid-day....I also noticed a lot more crows than there used to be...."

Well, build it some up more; throw in a few bloody action scenes and lots of T&A and when it comes out on video I'll rent it. And don't forget the contrails!

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