February 1st 2016
In the grand scheme of things, the Sun is unbelievably constant as a star. While some other Stars have very dramatic pulsations, wildly varying is size and brightness, and sometimes even exploding, the luminosity of our sun varies a very small 0.1% over the course of the 11-year solar cycle. Dawning on the realization among researchers that even these apparently small variations can have a significant effect on Earth’s climate. A report issued by the National Research Council (NRC) “The effects of Solar Variability on Earth’s Climate”, lays out some surprisingly complex ways that solar activity can make itself felt on our planet.
Understanding the Sun’s Climate connection requires a breadth of expertise in field of Plasma Physics, Atmospheric Chemistry, and Fluid Dynamics, Particle Physics, and even Earth’s History. When the Solar Cycle was studied by the (NRC) dozens of experts from various fields combined their efforts to frame the problem in a truly Multi-Disciplinary Context. The University of Colorado’s Atmospheric and Space Physics Department pointed out such a small fraction is still important. “Even typical short term variations of 0.1% in incident irradiance exceed all other energy sources (such as natural radioactivity in Earth’s Core) combined”. The most important is the sun’s extreme ultraviolet (EUV) radiation, which peaks during the years around solar maximum. There is a relatively narrow band of EUV wavelengths, the sun’s output varies not by a minuscule 0.1% but a whopping factor of 10 or more. This can strongly affect the chemistry and thermal structure of the upper atmosphere.
Using space-borne measurements of the total solar irradiance (TSI) show 0.1% variations in solar activity on the 11-year and shorter timescales. The Data has been corrected for calibration and offsets between the various instruments used to measure TST. The University of Colorado had several researchers discussed how changes in the upper atmosphere can trickle down to Earth’s surface. There are many top-down pathways for the sun’s influence. The Goddard Space Flight Center described how nitrogen oxides (NOX) created by solar energy particles and cosmic rays in the stratosphere could reduce ozone levels by a few percent. Because ozone absorbs UV radiation, less ozone means that more UV rays from the sun would reach Earth’s surface. NOAA took this one step further, they described how the loss of ozone in the stratosphere could alter the dynamics of the atmosphere below it. “The cooling of the polar stratosphere associated with the loss of ozone increases the horizontal temperature gradient near the tropopause,”. This alters the flux of the angular momentum budget of the troposphere and controls the surface westerlies. (Western wind flow). In other words, solar activity felt in the upper atmosphere can, through a complicated series of influence’s, can and do push surface storms off their tracks and off their course. How incoming galactic rays and solar protons penetrate the atmosphere. NASA Goddard Space Flight Center, “The impact of Energetic Particle Precipitation on the Atmosphere, “presentation to the workshop on the Effects of Solar Variability on Earth’s Climate. Many of the mechanism proposed at the workshop had a Rube Goldberg-like-quality. (Domino effect) They relied on multi-step interactions between multiple layers of atmosphere and ocean flows. Some relying on Chemistry to get their work done, others were leaning on Thermodynamics or fluid physics. The National Center for Atmospheric Research (NCAR) presented persuasive evidence that solar variability is leaving an imprint on the climate, especially in the Pacific. According to the report, when researcher’s look at sea surface temperature data during sunspot peak years, the tropical pacific shows a pronounced La Nina-like pattern, with cooling of almost 10 deg C in the equatorial eastern Pacific. In addition, there are signs of enhanced precipitation in the pacific (ITCZ) Inter-Tropical Convergence Zone. And in the SPCZ (South Pacific Convergence Zone) as well as above normal sea level pressure in the mid-latitude North and South Pacific, correlated with peaks in the sunspot cycle. The sun spot cycle is strong in the Pacific. Even if you take into consideration the low number of 0.1% during the sunspot cycle you have amplified the surface of the Pacific itself.
My research has shown the yearly averages of sunspot numbers for a period of 400 years (1610-2013), have determined the frequency of similar grand minima. Early estimates of grand minimum frequency in solar-type stars ranged from 10% to 30%, implying the sun’s influence could be overpowering. The sun itself could be on the threshold of a mini-maunder event right now. Ongoing solar cycle 24 is the weakest in more than 50 years. Moreover, there is (Controversial) evidence of a long-term weakening trend in the magnetic field strength of sunspots. By the time cycle 25 rolls around the magnetic field of the sun will be at its weakest sense the maunder minimum.
In the images of the Sun’s upper photosphere shows bright and dark magnetic structures responsible for variations in (TSI). In a concluding panel discussion, the researchers identified a number of possible devices currently used to measure total solar irradiance (TSI) reduce the entire sun’s wavelengths. This integrated value becomes a solitary point in a time series tracking the sun’s output. In fact, Heliophysics points out the situation is more complex. The sun is not a featureless ball of uniform luminosity. Instead, the solar disc is dotted by the dark cores of sunspots and splashed with bright magnetic froth known as faculae. Radiometric imaging would essentially map the surface of the sun and reveal the contributions of each of the sun’s luminosity. This may be why paleoclimate records of the sun-sensitive isotopes C-14 and Be-10 show a faint 11-year cycle at work even during the Maunder Minimum. A radiometric imager will be deployed on some future space observatory, and would allow researchers to develop the understanding they need to project the sun-climate link into a future of prolonged spotlessness. Some researchers stressed the need to put sun-climate data in standard formats and make them widely available for multidisciplinary study. Because the mechanisms for the sun’s influence on the climate are complicated. Researchers from many fields will have to work together to successfully model them and compare competing results.
In summary, the key challenge for the researchers will be the difficulty in deciphering the sun-climate and how it links to the magnetic field and atmospheric circulation. Variations in Earth’s magnetic field and atmospheric circulation can affect the deposition of radioisotopes far more than actual solar activity. A better long-term record of the sun’s irradiance might be encoded in the rocks and sediments of the Moon and Mars. Studying other worlds might hold the key to our own.
The new emerging field of Heliophysics will help answer more of these questions before cycle 25 starts. We should have better data than we have now, because it will come from one source dedicated to just the study of the Sun. But the overall outcome is the Star we call the Sun has a greater effect on our little planet than most thought.
Robert Moore PhD
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