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Sun and Earth Climate

IN SHORT - The solar energy received at Earth is varying. A famed diagram of the 1970's is showing the long-scale variations of it, as, since 1000 A.D. various, natural, cycles may be spotted, leading to such periods like more warmth during the early Middle Ages and cooling and a little ice age

between the 12th and the 18th century. Larger scale pattern cycles may be spotted to far in the Earth's past history. Such variations may be included in the reflexion about the global warming as, further, 5 main astronomical cycles may affect the temperatures at Earth

Sun total energy -which is said the "solar constant"- varies. This variation is 0.1 percent on a 11-year solar cycle duration. 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. A famous diagram by John Eddy ("Science", 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

click to Eddy's diagram

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. 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

click to solar cycles between 1750-2000. diagram courtesy Marshall Space Flight Center