Theory Sun and Earth Climate

Sun total energy -which is said the "solar constant"- varies. This variation is 0.1 percent on a 11-year solar cycle duration, which is a important variation, given the amount of energy released by the Sun. Sun's brightness is also increasing a bit when large numbers of sunspots and accompanying bright spots called faculae are present as going down slightly when sunspots and faculae are sparse, generally. Stratospheric temperatures vary by about 1 °C (1.8 °F) over the course of a solar cycle. A variation of this same magnitude occurs too due to sunspots daily-weekly activity. On a longer term, solar constant may vary up to 0.2 to 0.6 percent over many centuries. When the Sun is reaching a historic lowest at one of its minimum, like in 1913 for example, this translates, in terms of an impact unto the global warming, with 3 tenths of a degree F. (2 tenths of a degree C.) in less at the Earth (as the greenhouse effect is of a value of 13 times more than the variabilities of the Sun, generally). Sientists believe that sustained changes of as little as 0.25 percent in total solar irradiance over periods of decades to centuries caused significant climate change in Earth's distant past. Some regular cycles of variation in irradiance on a other hand, are linked to the alignment of planets and their gravitational tug on the Sun. The Sun, Earth and Jupiter are aligned in their orbits every 1.09 years, and we see a bump in solar irradiance every year at that time, That's just one of many cycles we have found. Recentest studies show that the solar constant further might vary differentially according to the wavelengths considered. A steep decrease in the ultraviolet, for example, coupled with the increase in the visible and infrared, however does even out in terms of solar irradiance (which is not the case in terms of absorption by the Earth's atmosphere). A famed diagram by Boulder, Co. astronomer John Eddy in a seminal paper in "Science" by 1976, is showing a series of interesting correlations between solar constant, sunspots cycles, tree rings carbon-14 rate, and comprehensive weather. As carbon-14 is produced by cosmic rays in upper atmosphere and then fixed into tree rings, it is a good marker for Sun's activity: when Sun is weak, heliosphere -this solar domain bubble- is weak too and more cosmic rays reach Earth. The solar magnetic field might also be involved in the diagram as, when high it shields from cosmic rays as those are supposed to trigger clouds in the Earth's atmosphere, hence less solar energy reaching the surface. Scientists have noted changes in the Sun's energy by observing from Earth's surface for more than a hundred years, but were only able to begin to determine their magnitude and impact on Earth's climate with more accurate measurements from space, starting in 1978 with measurements of the total solar irradiance made by NASA's Nimbus 7 satellite

What is interesting is a strong correlation between temperature, sunspots number, and carbon-14 curves: each time sunspots number is high, carbon-14 rate is low, and temperature is high. And reciprocally. This diagram is similar in its conclusions to several other studies about climate: warm Middle-Ages from 850 to 1150 AD, cooling and Little Ice Age from 1200 AD to 18th century's end. The Little Ice Age, eventually lifted up around 1850. It seems likely that such links are a reliable explanation for Earth's climate variation as far as historical times are concerned: when Sun activity is increasing, so are temperatures at Earth; when it is decreasing, temperatures at Earth are decreasing too. Before 1100 AD, warm and cold periods alternated: first two centuries AD cold, 200 AD- 600 AD warm, 600-800 AD cold. On a longer timescale, global figures are showing large cold eras at -2.2 billion, -1 billion/-800 million, -440 million, and -290 million years. On average warm periods were longer than cold ones -with two neat warm peaks at -3.8 billion years and -120 million years. Latter period is taking place in a clear long, warm period extending from Triassic beginning to a little before Tertiary's end, i.e. on a 220 million-year time span. Such short and long-term Earth's climate variation might be linked to solar activity. Since it was created 4.5 billion years ago, Sun activity varied. Astronomical cycles and local conditions may also have affected Earth. No less than 5 main astronomical cycles are operating about Earth over timespans between 21,000 and 100,000 years, some of them described by Milutin Milankovitch, a Serbian scientist in the 1920s. Earth orbit may vary from a circle to an ellipse, Earth axis' tilt or perihelion-aphelion axis are varying, affecting Earth climate. Geological evolution at Earth also surely had some influence. Further, it may be thought that such processes are at work at any of solar system planets. Mars progressively lost its atmosphere, Venus developped a greenhouse effect. As far question of manmade induced greenhouse effect since the 1750s is concerned, 250 years is a short time for any real astronomical forcing about Earth climate, based on long term astronomical cycles. Sunpots cycles however are showing that since Little Ice Age's end solar activity high and low-levels alternated. Lately solar activity increased since the 1940s. Related to Eddy's diagram and sunspots-temperature correlation one might say that this late increased solar activity is surely for a part involved into Earth global warming. The 'Maunder Minimum,' during Modern Era, corresponded to what's called the Little Ice Age in Europe—a time of colder weather, heavier snowfall and the freezing of unusually large bodies of water such as the Thames, Dutch canals and even the Baltic Sea. Advancing glaciers destroyed northern European towns. Little Ice Age lasted longer than the Maunder Minimum, and there are other potential causes. Nevertheless it is believed by many scientists that that remarkable solar minima and corresponding decrease in solar energy cooled Earth. One explanation is also that the Little Ice Age might have been caused by a series of large, closely spaced volcanic eruptions at the tropics the particles of which reflected solar radiation back into space, cooling the planet below. After the aerosols left the atmosphere, the cooling effect was strengthened further as it triggered sea-ice growth. That melted into the North Atlantic Ocean, interfering with the normal mixing between surface and deeper waters. This meant the water flowing back to the Arctic was colder, helping to sustain large areas of sea ice, which, in turn, reflect sunlight back into the atmosphere. The cool spell was a major perturbation to the northern hemisphere since the late 13th century, which lasted into the late 19th century. There is also evidence it affected other continents

The solar irradiance began to be monitored by NASA two years after Eddy's paper about long term variance of the solar activity. Dedicated, radiometer aboard NASA weather satellites since 1978 have been measuring the amount of sunlight striking the Earth's atmosphere, or total solar irradiance, validating Eddy's views. The solar irradiance variations mostly are determined through a competition between dark sunspots and bright spots called faculae, with sunspots causing a decrease in irradiance and faculae causing a overall increase. That link to the solar sunspots makes that the irradiance is seen varying along the 11-year solar cycle. Overall, radiometers show that the sun’s irradiance changes by about 0.1 percent as the number of sunspots varies from about 20 sunspots or less per year during periods of low activity (solar minimum) to between 100 and 150 during periods of high activity (solar maximum). Inconsistencies in the data collection or intruments performance unluckily still do not allow to tell whether the solar irradiance has increased over the last decades. The Total Irradiance Monitor (TIM) aboard the NASA Glory mission, being more accurate and stable should allow for a definitive answer. A recent study, by 2009, has shown that, by the maximum of a solar cycle, there really is an interaction from the solar activity into the Earth's climate, as the stratosphere and then the ocean, with their winds, temperatures or clouds, in the subtropical and tropical regions are modified and can affect in turn the global weather patterns. This interaction modulates through the existing weather patterns, like El Niño or the monsoon, whether it's weakening or increasing the effects of those. Generally, for example, during a solar maximum -the El NiNiño let aside- the equatorial Eastern Pacific is dryer and the water colder. By 2011 the 11-year solar cycle was found to influence the weather on Earth during a minimum. A reduction in UV radiation affects high-altitude wind patterns in the northern hemisphere, triggering cold winters. Unusually cold air then forms high in the atmosphere over the tropics causing a redistribution of heat in the atmosphere, triggering easterly winds that bring freezing weather and snow storms to northern Europe and the United States and milder weather to Canada and the Mediterranean. When solar UV radiation is stronger during a solar maximum, the opposite occurs. Studies remain to be performed upon several solar cycles to be confirmed

Studies in 2011 have shown that long or short terms variations of the length of the day, which are correlated to interaction with the Earth's outer core flows, are amazingly correlated too with temperature variation at Earth which might be due to that Earth's magnetic field changes magnetic shielding of charged-particle (i.e., cosmic ray) fluxes that have been hypothesized to affect the formation of clouds. Other possibilities are that some other core process could be having a more indirect effect on climate, or that an external (e.g. solar) process affects the core and climate simultaneously. Luminosity of Sun varies a mere 0.1 percent over the course of the 11-year solar cycle. Even such tiny variations can have a significant effect on terrestrial climate as the study in the field requires a multi-disciplinary endeavour. Of particular importance is the Sun's extreme ultraviolet (EUV) radiation, which peaks during the years around solar maximum. Within the relatively narrow band of EUV wavelengths, the Sun’s output varies by up to factors of 10 or more, which can strongly affect the chemistry and thermal structure of the upper atmosphere. Nitrogen oxides (NOx) created by solar energetic particles and cosmic rays in the stratosphere could reduce ozone levels by a few percent, letting more UV rays to Earth's surface, or even alter the dynamics of the atmosphere below it. Cooling of the polar stratosphere associated with loss of ozone increases the horizontal temperature gradient near the tropopause, altering the flux of angular momentum by mid-latitude eddies. Angular momentum budget of the troposphere controls surface westerlies. On a other hand, Pacific Ocean surface temperature during sunspot peak years show that tropical Pacific shows a pronounced La Nina-like pattern, with a cooling of almost 1° C in the equatorial eastern Pacific and signs of enhanced precipitation in the Pacific ITCZ (Inter-Tropical Convergence Zone ) and SPCZ (South Pacific Convergence Zone) or above-normal sea-level pressure in the mid-latitude North and South Pacific. Some scientists even ponder whether something in the Pacific climate system is acting to amplify solar signals with also bottom-up mechanisms. Most recent studies however show the influence of solar variability is more regional than global and likely does not impact climate warming. If there is indeed a solar effect on climate, it is manifested by changes in general circulation rather than in a direct temperature signal. Or regional cooling in Europe during the Maunder Minimum could have been a drop in the Sun’s EUV output; this is, however, speculative. Latest studies also point to that the Sun could be on the threshold of a mini-Maunder event now. Solar Cycle 24 is the weakest in more than 50 years as there might be too a long-term weakening trend in the magnetic field strength of sunspots. The Sun is not a featureless ball of uniform luminosity but the solar disk is dotted by the dark cores of sunspots and splashed with bright faculae (which do not vanish during solar minima). That would account for that paleoclimate records of Sun-sensitive isotopes C-14 and Be-10 show a faint 11-year cycle at work even during the Maunder Minimum. Better long-term record of the Sun’s irradiance might even be better encoded in the rocks and sediments of the Moon or Mars