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Solar Changes and the Climate

By Joe D'Aleo
Monday, September 07, 2009

In this week, they reported on an important new paper in Science “Meehl, G.A., J.M. Arblaster, K. Matthes, F. Sassi, and H. van Loon (2009), Amplifying the Pacific climate system response to a small 11 year solar cycle forcing, Science, 325, 1114-1118.”

Weather patterns across the globe are partly affected by connections between the 11-year solar cycle of activity, Earth's stratosphere and the tropical Pacific Ocean, a new study finds.


The study could help scientists get an edge on eventually predicting the intensity of certain climate phenomena, such as the Indian monsoon and tropical Pacific rainfall, years in advance.


The sun is the ultimate source of all the energy on Earth; its rays heat the planet and drive the churning motions of its atmosphere.


The amount of energy the sun puts out varies over an 11-year cycle (this cycle also governs the appearance of sunspots on the sun's surface as well as radiation storms that can knock out satellites), but that cycle changes the total amount of energy reaching Earth by only about 0.1 percent. A conundrum for meteorologists was explaining whether and how such a small variation could drive major changes in weather patterns on Earth.


Earth-space connection


An international team of scientists led by the National Center for Atmospheric Research (NCAR) used more than a century of weather observations and three powerful computer models to tackle this question.


The answer, the new study finds, has to do with the Sun's impact on two seemingly unrelated regions: water in the tropical Pacific Ocean and air in the stratosphere, the layer of the atmosphere that runs from around 6 miles (10 km) above Earth's surface to about 31 miles (50 km).


The study found that chemicals in the stratosphere and sea surface temperatures in the Pacific Ocean respond during solar maximum in a way that amplifies the sun's influence on some aspects of air movement. This can intensify winds and rainfall, change sea surface temperatures and cloud cover over certain tropical and subtropical regions, and ultimately influence global weather.


"The sun, the stratosphere, and the oceans are connected in ways that can influence events such as winter rainfall in North America," said lead author of the study, Gerald Meehl of NCAR. "Understanding the role of the solar cycle can provide added insight as scientists work toward predicting regional weather patterns for the next couple of decades."


The following is our assessment of the ways the sun MAY influence weather and climate on short and long time scales.


The Sun Plays A Role In Our Climate In Direct And Indirect Ways.


The sun changes in its activity on time scales that vary from 27 days to 11, 22, 80, 180 years and more. A more active sun is brighter due to the dominance of faculae over cooler sunspots with the result that the irradiance emitted by the sun and received by the earth is higher during active solar periods than during quiet solar periods. The amount of change of the solar irradiance based on satellite measurements since 1978 during the course of the 11 year cycle just 0.1% (Willson and Hudson 1988) has caused many to conclude that the solar effect is negligible especially in recent years. Over the ultra long cycles (since the Maunder minimum), irradiance changes are estimated to be as high as 0.4%  (Hoyt and Schatten (1993), Lean et al. (1995), Lean (2000), Lockwood and Stamper (1999) and Fligge and Solanki (2000)).


However this does not take into account the sun’s eruptional activity (flares, solar wind bursts from coronal mass ejections and solar wind bursts from coronal holes) which may have a much greater effect. This takes on more importance since Lockwood et al. (1999) showed how the total magnetic flux leaving the sun has increased by a factor of 2.3 since 1901. This eruptional activity may enhance warming through ultraviolet induced ozone chemical reactions in the high atmosphere or ionization in higher latitudes during solar induced geomagnetic storms. In addition, the work of Svensmark (1997), Bago and Butler (2000) Tinsley and Yu (2002) have documented the possible effects of the solar cycle on cosmic rays and through them the amount of low cloudiness. It may be that through these other indirect factors, solar variance is a much more important driver for climate change than currently assumed. Because, it is more easily measured and generally we find eruptional activity tracking well with the solar irradiance, we may utilize solar irradiance measurements as a surrogate or proxy for the total solar effect.


Correlations with Total Solar Irradiance


Studies vary on the importance of direct solar irradiance especially in recent decades. Lockwood and Stamper (GRL 1999), estimated that changes in solar luminosity can account for 52% of the change in temperatures from 1910 to 1960 but just 31% of the change from 1970 to 1999.


N. Scafetta and B. J. West of Duke University, in “Phenomenological Solar Signature in 400 years of Reconstructed Northern Hemisphere Temperature Record” (GRL 2006 and b0 showed how total solar irradiance accounted for up to 50% of the warming since 1900 and 25-35% since 1980. The authors noted the recent departures may result “from spurious non-climatic contamination of the surface observations such as heat-island and land-use effects [Pielke et al., 2002; Kalnay and Cai, 2003]”. There analysis was done using the global data bases which may also suffer from station dropout and improper adjustment for missing data which increased in the 1990s. In 2007, in their  follow up paper in the GRL, they noted the sun could account for as much as 69% of the changes since 1900.


This USHCN data base though regional in nature would have been a better station data base to use for analysis of change as it is more stable, has less  missing data and a better scheme for adjusting for missing data, as well as some adjustments for changes to siting and urbanization. 


An independent analysis was conducted using the USHCN data and TSI data obtained from Hoyt and Schatten. The annual TSI composite record was constructed by Hoyt and Schatten [1993] (and updated in 2005) utilizing all five historical proxies of solar irradiance including sunspot cycle amplitude, sunspot cycle length, solar equatorial rotation rate, fraction of penumbral spots, and decay rate of the 11-year sunspot cycle.


The following includes a plot of this latest 11-year running mean solar irradiance versus a similar 11-year running mean of NCDC annual mean US temperatures. It confirms this strong correlation (r-squared of 0.59). The correlation increases to an r-squared value of 0.654 if you introduce a lag of 3 years for the mean USHCN data to the mean TSI. This is close to the 5 year lag suggested by Wigley and used by Scafetta and West. The highest correlation occurred with a 3 year lag.



In recent years, satellite missions designed to measure changes in solar irradiance though promising have produced there own set of problems. As Judith Lean noted the problems is that no one sensor collected data over the entire time period from 1979 “forcing a splicing of from different instruments , each with their own accuracy and reliability issues, only some of which we are able to account for”. Lean and Froelich in their 1998 GRL paper gave their assessment which suggested no increase in solar irradiance in the 1980s and 1990s.


Richard Willson, principal investigator of NASA’s ACRIM experiments though in the GRL in 2003 was able to find specific errors in the dataset used by Lean and Froelich used to bridge the gap between the ACRIM satellites and when the more accurate data set was used a trend of 0.05% per decade was seen which could account for warming since 1979 (figure 2).



Two other recent studies that have drawn clear connections between solar changes and the Earth’s climate are Soon (2005) and Kärner (2004). Soon (2005 GRL) showed how the arctic temperatures correlated with solar irradiance far better than with the greenhouse gases over the last century (see Figure 3). For the 10 year running mean of total solar irradiance (TSI) vs Arctic-wide air temperature anomalies (Polyokov), he found a strong correlation of (r-squared of 0.79) compared to a correlation vs greenhouse gases of just 0.22.



Warming Due To Ultraviolet Effects Through Ozone Chemistry


Though solar irradiance varies slightly over the 11 year cycle, radiation at longer UV wavelengths are known to increase by several percent with still larger changes (factor of two or more) at extremely short UV and X-ray wavelengths (Baldwin and Dunkerton, JAS 2004).


Energetic flares increase the UV radiation by 16%. Ozone in the stratosphere absorbs this excess energy and this heat has been shown to propagate downward and affect the general circulation in the troposphere. Shindell (1999) used a climate model that included ozone chemistry to reproduce this warming during high flux (high UV) years. Labitzke and Van Loon (1988) and later Labitzke in numerous papers has shown that high flux (which correlates very well with UV) produces a warming in low and middle latitudes in winter in the stratosphere with subsequent dynamical and radiative coupling to the troposphere. The winter of 2001/02, when cycle 23 had a very strong high flux second maxima provided a perfect verification of Shindell and Labitzke and Van Loon’s work.




The warming that took place with the high flux from September 2001 to April 2002 caused the northern winter polar vortex to shrink and the southern summer vortex to break into two centers for the first time ever observed. This disrupted the flow patterns and may have contributed to the brief summer breakup of the Larsen ice sheet.



Recently NASA reported on the use of the Shindell Ozone Chemistry Climate Model to explain the Maunder Minimum (Little Ice Age).



Their model showed when the sun was quiet in 1680, it was much colder than when it became active again one hundred years later. “During this period, very few sunspots appeared on the surface of the Sun, and the overall brightness of the Sun decreased slightly. Already in the midst of a colder-than-average period called the Little Ice Age, Europe and North America went into a deep freeze: alpine glaciers extended over valley farmland; sea ice crept south from the Arctic; and the famous canals in the Netherlands froze regularly—an event that is rare today.”


Geomagnetic Storms and High Latitude Warming


When major eruptive activity (i.e. coronal mass ejections, major flares) takes place and the charged particles encounter the earth, ionization in the high atmosphere leads to the familiar and beautiful aurora phenomenon. This ionization lads to warming of the high atmosphere which like ultraviolet warming of the stratosphere works its way down into the middle troposphere with time.


Here is an example of an upper level chart two weeks after a major geomagnetic storm. Note the ring of warmth (higher than normal mid-tropospheric heights) surrounding the magnetic pole.



A key aspect of the sun’s effect on climate is the indirect effect on the flux of Galactic Cosmic Rays (GCR) into the atmosphere. GCR is an ionizing radiation that supports low cloud formation. As the sun’s output increases the solar wind shields the atmosphere from GCR flux. Consequently the increased solar irradiance is accompanied by reduced low cloud cover, amplifying the climatic effect. Likewise when solar output declines, increased GCR flux enters the atmosphere, increasing low cloudiness and adding to the cooling effect associated with the diminished solar energy.


The conjectured mechanism connecting GCR flux to cloud formation received experimental confirmation in the recent laboratory experiments of Svensmark (Proceedings of the Royal Society, Series A, October 2006), in which he demonstrated exactly how cosmic rays could make water droplet clouds.


Palle Bago and Butler showed in 2002 (Intl J Climat.) how the low clouds in all global regions changed with the 11 year cycle in inverse relation to the solar activity. Changes of 1 to 2% in low cloudiness could have a significant effect on temperatures through changes in albedo.



Recently, Henrik Svensmark and Eigil Friis-Christensen published a reply to Lockwood and Fröhlich - The persistent role of the Sun in climate forcing, rebutting Mike Lockwood's Recent oppositely directed trends in solar climate forcings and the global mean surface air temperature. In it, they correlated tropospheric temperature with cosmic rays. The figure below features two graphs. The first graph compares tropospheric temperature (blue) to cosmic rays (red). The second graph removes El Nino, volcanoes and a linear warming trend of 0.14°C per decade.



Though some might argue the linear warming trend relates to “greenhouse warming” it coincides with the ocean and solar TSI cyclical trends as can be seen in this diagram that overlays PDO + AMO and Hoyt Schatten/Willson TSI and USHCN version 2 temperatures. The 60 year cycle clearly emerges including that observed warming trend. The similarity with the ocean multidecadal cycle phases also suggest the sun play a role in their oscillatory behavior.



Cosmic ray influence appears on the extremely long geologic time scales. Shaviv (JGR 2005 estimated from the combination of increased radiative forcing through cosmic ray reduction and the estimated changes in total solar luminosity (irradiance) over the last century that the sun could be responsible for up to 77% of the temperature changes over the 20th century with 23% for the anthropogenic. He also found the correlation extended back in the ice core data 500 million years.





Though the sun’s brightness or irradiance changes only slightly with the solar cycles, the indirect effects of enhanced solar activity including warming of the atmosphere in low and mid latitudes by ozone reactions due to increased ultraviolet radiation, in higher latitudes by geomagnetic activity and generally by increased radiative forcing due to less clouds caused by cosmic ray reduction may greatly magnify the total solar effect on temperatures.  The sun appears to be the primary driver right up to the current time




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