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Solar Climatic Effects (Recent Influence) – Summary
In a study of the climatic history of Scandinavia over the past 10,000 years, Karlén (1998) compared a proxy temperature record -- derived from analyses of changes in the sizes of glaciers, changes in the altitudes of alpine tree-limits, and variations in the widths of tree-rings -- with contemporaneous solar irradiance data that were derived from 14C anomalies measured in tree-ring records.  The results of this analysis revealed both long- and short-term temperature fluctuations; and it was noted that during warm periods the temperature was "about 2°C warmer than at present."  Furthermore, the temperature fluctuations were found to be "closely related" to changes in solar radiation, so much so that Karlén concluded that "the frequency and magnitude of changes in climate during the Holocene do not support the opinion that the climatic change of the last 100 years is unique."  In fact, he bluntly stated that "there is no evidence of a human influence so far."  Likewise, Perry and Hsu (2000), who also investigated the possible role of the sun on past and current Holocene climate, concluded that the idea of "the modern temperature increase being caused solely by an increase in CO2 concentration appears questionable."

In focusing on these ideas, this summary reviews several recent studies that provide strong correlative evidence for a solar-climate link that obviates the need to invoke CO2-enhanced radiative forcing to explain the modest global warming of the 20th century.  Specifically, we examine studies dealing with a number of phenomena that influence solar heating of the earth-ocean-atmosphere system that may have been altered over the course of the past century and discuss the potential effects of these phenomena on earth's climate.

We begin with the study of Frohlich and Lean (2002), who extrapolated a record of total solar irradiance back to the seventeenth century.  According to their analysis, "warming since 1650 due to the solar change is close to 0.4°C, with pre-industrial fluctuations of 0.2°C that are seen also to be present in the temperature reconstructions."

In another study, Rigozo et al. (2002) analyzed two data sets (tree-ring widths from Santa Catarina, Brazil and sunspot numbers) in an attempt to determine the influence of the solar parameter (sun spot number) on climate (tree-ring width) over the period 1837-1996.  Their analysis revealed the existence of an 11-year cycle in the tree-ring width data that matched the 11-year sun spot cycle.  When comparing both data sets via cross-wavelet spectral analysis, the authors report that "a good correspondence is observed," which correspondence was strongest during the time of most intense solar activity, i.e., 1940-1970.

A strong sun-climate correlation was also found by Vaganov et al. (2000) for the Asian subarctic.  Using tree-ring width as a proxy for temperature, the authors discovered a significant correlation between temperature variations and solar radiation (R = 0.32) over the past 600 years.  When examining this relationship over the much shorter interval of the industrial period (1800 to 1990) the correlation improved considerably (R = 0.68).

Another pertinent study is that of Pang and Yau (2002), who assembled and analyzed a vast amount of data pertaining to phenomena that have been reliably linked to variations in solar activity, including frequencies of sunspot and aurora sightings, abundance of carbon-14 in rings of long-lived trees, and amount of beryllium-10 in annual layers of polar ice cores.  Over the past 1800 years, these authors identified "some nine cycles of solar brightness change," including the well-known Oort, Wolf, Sporer, Maunder and Dalton Minima.  With respect to the Maunder Minimum -- which occurred between 1645 and 1715 and is widely acknowledged to have been responsible for some of the coldest weather of the Little Ice Age -- they report that the temperatures of that period "were about one-half of a degree Celsius lower than the mean for the 1970s, consistent with the decrease in the decadal average solar irradiance."  Then, from 1795 to 1825, came the Dalton Minimum, along with another dip in Northern Hemispheric temperatures.  Since that time, however, the authors say "the sun has gradually brightened" and "we are now in the Modern Maximum," which may well be responsible for the warmth of the Modern Warm Period.

Pang and Yau also say that although the long-term variations in solar brightness they identified "account for less than 1% of the total irradiance, there is clear evidence that they affect the earth's climate."  And so they do.  A dual plot of total solar irradiance and Northern Hemispheric temperature from 1620 to the present indicates that the former parameter (when appropriately scaled, but without reference to any specific climate-change mechanism) can account for essentially all temperature changes up to about 1980.  After that time, the IPCC surface air temperature record rises uncharacteristically rapidly, although the radiosonde and satellite temperature histories largely match what would be predicted from the solar irradiance record.

In agreement with Pang and Yau's study, Parker (1999) and Rigozo et al. (2001) report that the number of sunspots has more than doubled over the past century.  Furthermore, Rigozo et al. say their "1000-year reconstructed sunspot number reproduces well the great maximums and minimums in solar activity, identified in cosmonuclide variation records, and, specifically, the epochs of the Oort, Wolf, Sporer, Maunder, and Dalton Minimums, as well [as] the Medieval and Modern Maximums," the latter of which they describe as "starting near 1900."  When quantified, for example, the mean sunspot number for the Wolf, Sporer and Maunder Minimums is found to be 1.36.  For the Oort and Dalton Minimums it is 25.05; while for the Medieval Maximum it is 53.00, and for the Modern Maximum it is 57.54.  Compared to the average of the Wolf, Sporer and Maunder Minimums, therefore, the mean sunspot number of the Oort and Dalton Minimums was 18.42 times greater; while that of the Medieval Maximum was 38.97 times greater, and that of the Modern Maximum to the time of Rigozo et al.'s analysis was 42.31 times greater.  Corresponding strength ratios for the solar radio flux were 1.41, 1.89 and 1.97, respectively; for the solar wind velocity, 1.05, 1.10 and 1.11; and for the southward component of the interplanetary magnetic field, 1.70, 2.54 and 2.67.

Lockwood et al. (1999) also examined measurements of the near-earth interplanetary magnetic field, finding that the total magnetic flux leaving the sun has risen by a factor of 1.41 over the period 1964-1996.  What is more, surrogate measurements of this parameter previous to this time indicate that the total magnet flux has risen by a factor of 2.3 since 1901.  Given these increases, Lockwood et al. state that "the variation [in total solar magnetic flux] found here stresses the importance of understanding the connections between the sun's output and its magnetic field and between terrestrial global cloud cover, cosmic ray fluxes and the heliospheric field."

One of the main criticisms of the solar-climate link on decadal and centennial time scales is the belief that solar energy output fluctuations are too small to cause the corresponding temperature changes (Broecker, 1999).  In response to this criticism, we again point to the vast amount of literature in support of such an influence in the Solar Effects section of our Subject Index, in particular, the section on Cosmic Rays.  We also refer to the study of Tobias and Weiss (2000), who, noting that "solar magnetic activity exhibits chaotically modulated cycles ... which are responsible for slight variations in solar luminosity and modulation of the solar wind," attacked the solar forcing of climate problem by means of a model in which the solar dynamo and earth's climate are represented by low-order systems, each of which in isolation supports chaotic oscillations but when run together sometimes resonate.  The results of their analysis showed that "solutions oscillate about either of two fixed points, representing warm and cold states, flipping sporadically between them."  They also discovered that a weak nonlinear input from the solar dynamo "has a significant effect when the 'typical frequencies' of each system are in resonance."  Based upon these findings, the authors conclude that "the resonance provides a powerful mechanism for amplifying climate forcing by solar activity."  Hence, there need no longer be any reluctance to accept as fact the conclusion that the many correlations that have been documented between solar variability and the time histories of various climatic phenomena do indeed have a cause that is of extraterrestrial origin.

With respect to cosmic rays, their intensity has been observed to vary by about 15% over a solar cycle due to changes in the strength of the solar wind, which carries a weak magnetic field into the heliosphere that partially shields the earth from low-energy galactic charged particles (Carslaw et al., 2002).  When this shielding is at a minimum, allowing more cosmic rays to impinge upon the planet, more low clouds have been observed to cover the earth (Kniveton and Todd, 2001), producing a tendency for lower temperatures to occur.  When the shielding is maximal, on the other hand, less cosmic rays impinge upon the planet and fewer low clouds form, which produces a tendency for the earth to warm (Solanki et al., 2000).

So, do solar-mediated changes in cosmic ray intensities influence climate on decadal and centennial time scales?  In a provocative plot that suggests a positive answer to this question, Carslaw et al. depict a composite history of cosmic ray intensities derived from four independent proxies, two of which extend all the way back to 1700.  Comparing this plot with what we believe to be the most accurate temperature history of the Northern Hemisphere, i.e., that derived by Esper et al. (2002), we note that for almost all of the 18th century, cosmic ray intensity declined modestly, while air temperature slowly rose.  Then came a sharp rise in cosmic ray intensity that was immediately followed by a sharp drop in temperature.  This change, in turn, was followed by a sharp decline in cosmic ray intensity that was immediately followed by a sharp upturn in temperature.  Thereafter, the cosmic ray intensity leveled off, rose slightly and then declined in undulating fashion to the end of the record, while temperature leveled off, dropped slightly and then rose in undulating fashion to the end of the record, as would be expected to occur in light of what is currently known about the cosmic ray-cloud connection.

With respect to the past century, Carslaw et al. note that the flux of cosmic rays declined by about 15% over this period, which is not surprising in light of the increase in solar magnetic flux since 1901 that was reported by Lockwood et al.  In addition, Feynman and Ruzmaikin (1999) report that the flux of 300 MeV-protons at the top of the magnetosphere declined by a factor of 5 between solar minima at the beginning of the century and recent solar minima, and that the flux of 1 GeV-protons dropped by a factor of 2.5.  Given these findings, we wonder just how much -- if not all -- of the reported 0.6°C global temperature rise of the last century bears the ultimate fingerprint of the sun.

Another, and totally independent, source of variability in the intensity of solar radiation received at the surface of the earth is provided by aerosols (see the several sub-headings under Aerosols and Clouds in our Subject Index).  In a recent paper on this subject, Stanhill and Cohen (2001) reviewed reports of numerous solar radiation measurement programs from around the world to determine if there had been any trend in this parameter over the past half-century.  The results of their investigation revealed a 50-year reduction that "has globally averaged 0.51 ± 0.05 Wm-2 per year, equivalent to a reduction of 2.7% per decade, and now totals 20 Wm-2."  After reviewing several possible causes of this huge decline, Stanhill and Cohen concluded that "the most probable is that increases in man made aerosols and other air pollutants have changed the optical properties of the atmosphere, in particular those of clouds."

Because of various feedbacks (both positive and negative) and other processes (such as those related to cosmic rays) that are active in the earth-ocean-atmosphere system, it is unclear what effect the past half-century's 20 Wm-2 reduction in solar radiation reception at the earth's surface has had on global climate.  One possible consequence is that it has reduced the amount of evaporation occurring at the surface of the earth.  This is the hypothesis of Roderick and Farquhar (2002), who demonstrated that the observed decrease in pan evaporation in Russia over the past 50 years is both qualitatively and quantitatively consistent with "what one would expect from the observed large and widespread decreases in sunlight resulting from increasing cloud coverage and aerosol concentration."  It would also help to explain the reduction in pan evaporation that has been observed in the United States (Petersen et al., 1995) and elsewhere over the past half-century.

All things being equal, the 20 Wm-2 decrease in surface solar forcing observed by Stanhill and Cohen should have resulted in a significant decrease in global near-surface air temperature.  The fact that such has not occurred and temperatures have actually increased slightly over this 50-year period might therefore suggest that the reported doubling of sunspot numbers and total solar magnetic flux over the course of the past century have offset whatever cooling impetus was provided by the observed decline in solar radiation.  Alternatively, much, if not all, of the 20 Wm-2 of solar energy lost to the planet's surface may have been absorbed higher in the atmosphere by water vapor, clouds, black carbon and other aerosols.  Several authors have analyzed the ability of these atmospheric constituents to absorb solar radiation, and they generally conclude that their impacts have been significantly underestimated by the atmospheric science community (Wild, 1999; Wild and Ohmura, 1999; Hansen, 2000; Satheesh and Ramanathan, 2000; Hansen 2002).

Clearly, there is much that remains to be learned about the variability of the flux of solar radiation that reaches the outer limits of the earth's magnetosphere and how it is subsequently operated upon by the host of independent and interacting phenomena that determine its ultimate climatic consequences.  Until we have a better understanding of these things, it is premature to conclude -- as the IPCC has -- that the historical increase in the air's CO2 content has been a major determinant of climate change over the 20th century.

References
Broecker, W.  1999.  Climate change prediction.  Science 283: 179.

Carslaw, K.S., Harrizon, R.G. and Kirkby, J.  2002.  Cosmic rays, clouds, and climate.  Science 298: 1732-1737.

Esper, J., Cook, E.R. and Schweingruber, F.H.  2002.  Low-frequency signals in long tree-ring chronologies for reconstructing past temperature variability.  Science 295: 2250-2253.

Feynman, J. and Ruzmaikin, A.  1999.  Modulation of cosmic ray precipitation related to climate.  Geophysical Research Letters 26: 2057-2060.

Frohlich, C. and Lean, J.  2002.  Solar irradiance variability and climate.  Astronomische Nachrichten 323: 203-212.

Hansen, J.E.  2002.  A brighter future.  Climatic Change 52: 435-440.

Hansen, J., Sato, M., Ruedy, R., Lacis, A. and Oinas, V.  2000.  Global warming in the twenty-first century: An alternative scenario.  Proceedings of the National Academy of Sciences USA 97: 9875-9880.

Karlén, W.  1998.  Climate variations and the enhanced greenhouse effect.  Ambio 27: 270-274.

Kniveton, D.R. and Todd, M.C.  2001.  On the relationship of cosmic ray flux and precipitation.  Geophysical Research Letters 28: 1527-1530.

Lockwood, M., Stamper, R. and Wild, M.N.  1999.  A doubling of the Sun's coronal magnetic field during the past 100 years.  Nature 399: 437-439.

Pang, K.D. and Yau, K.K.  2002.  Ancient observations link changes in sun's brightness and earth's climate.  EOS, Transactions, American Geophysical Union 83: 481, 489-490.

Parker, E.N.  1999.  Sunny side of global warming.  Nature 399: 416-417.

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.

Peterson, T.C., Golubev, V.S. and Groisman, P. Ya.  1995.  Evaporation losing its strength.  Nature 377: 687-688.

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.

Rigozo, N.R., Nordemann, D.J.R., Echer, E., Zanandrea, A. and Gonzalez, W.D.  2002.  Solar variability effects studied by tree-ring data wavelet analysis.  Advances in Space Research 29: 1985-1988.

Roderick, M.L. and Farquhar, G.D.  2002.  The cause of decreased pan evaporation over the past 50 years.  Science 298: 1410-1411.

Satheesh, S.K. and Ramanathan, V.  2000.  Large differences in tropical aerosol forcing at the top of the atmosphere and Earth's surface.  Nature 405: 60-63.

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.

Stanhill, G. and Cohen, S.  2001.  Global dimming: a review of the evidence for a widespread and significant reduction in global radiation with discussion of its probable causes and possible agricultural consequences.  Agricultural and Forest Meteorology 107: 255-278.

Tobias, S.M. and Weiss, N.O.  2000.  Resonant interactions between solar activity and climate.  Journal of Climate 13: 3745-3759.

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.

Wild, M.  1999.  Discrepancies between model-calculated and observed shortwave atmospheric absorption in areas with high aerosol loadings.  Journal of Geophysical Research 104: 27,361-27,371.

Wild, M. and Ohmura, A.  1999.  The role of clouds and the cloud-free atmosphere in the problem of underestimated absorption of solar radiation in GCM atmospheres.  Physics and Chemistry of the Earth 24B: 261-268.