Solar Influence On Earth's Weather: A 250-Million-Year View

(An expanded version of an Eyepiece article based on a Hayden Planetarium lecture by Dr. Sallie Baliunas)

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A coronal mass ejection, a huge outburst of ionized gas
ejected from the Sun. Sometimes these plasma storms
encounter the Earth, causing aurorae, radio disruptions,
and occasionally power blackouts. [Animation made from SOHO
(Solar and Heliospheric Observatory) images, courtesy
of the SOHO/LASCO consortium. SOHO is a project of
international cooperation between ESA and NASA.]

Those who attended "Solar Influence," a lecture by Dr. Sallie Baliunas, an astrophysicist at the Harvard-Smithsonian Center for Astrophysics, at the Hayden Planetarium on November 26 were treated to a quarter-billion-year history of changes in Earth's climate, and a discussion of the Sun's role in these changes. Baliunas also chronicled the development of human thought as it relates to the Sun, and ended with a weather forecast for the next few million years: cold periods 100,000 years long, in which much of North America will be inundated by mile-thick glaciers, interspersed with 10,000-year periods of relative warmth. We are deep into one of these warm stretches, the Holocene Epoch, and she believes—despite current concerns about global warming—that within a few thousand years it is likely to end and the glaciers return.

These periods of warm and cold are ultimately caused by the Sun's gravitational tug on the orbit and spin of Earth, causing the precession of Earth's axis, its wobble, obliquity and changes in the ellipticity of Earth's orbit. The Sun delivers, and the Earth receives, different amounts of radiation on different land and ocean fractions that change with time, triggering the advance and retreat of glaciers. The current cycle of glaciation began 2 million years ago when the Earth cooled enough for the great ice sheets—which hadn't been seen in 250 million years, since before the dinosaurs—to return. Our species appeared about 200,000 years ago; our intelligence leapt forward in a relatively short period of time, largely through having to adapt to two major glaciations. Early humans wondered about the Sun and incorporated it into their myths and religions, but it wasn't until Galileo imaged it with his telescope in 1610 and discovered sunspots and the solar rotation that much about its nature was revealed. In 1843 an amateur astronomer, Heinrich Schwabe, discovered the 11-year sunspot cycle.

George Ellery Hale was a great promoter of astronomy in the early 20th century. He was particularly interested in the Sun, and in possible effects of changes in the Sun on the Earth; he also encouraged the study of sunlike stars to shed more light on the processes that our own Sun undergoes. In 1889, while a student at MIT, he invented the spectroheliograph, and in1908 used it to discover the magnetic nature of sunspots. Hale built the largest telescopes of his time: the 40-inch Yerkes refractor, the 60- and 100-inch reflectors on Mt. Wilson and the 200-inch Palomar reflector, as well as the 60- and 150-foot solar telescopes on Mt. Wilson. He was responsible for transforming Pasadena from a small town into an international center of learning, and for recruiting many promising young astronomers. His most famous protégé was Edwin Hubble. Working with Milton L. Humason, Hubble determined once and for all the “island universe” nature of galaxies (and devised a system for classifying them), and of course demonstrated that the universe is expanding, making preliminary estimates as to the rate of expansion. [Despite his lack of formal education, Humason became Mt. Wilson’s greatest observational astronomer. He came to Mt. Wilson as a mule driver who helped transport parts of the observatory up the mountain. He got a job there as a janitor/night assistant, and because his inquisitive nature and mechanical aptitude, eventually convinced the staff to train him in using the telescopes. The rest, as they say, is history.]

Mt. Wilson Observatory has been particularly involved in observing sunlike stars. Although starspots are not directly observable, one can detect these stars’ magnetic fields through examining their spectral lines. There is now a 35-year record of several thousand sunlike stars to go along with solar records. By observing the radiocarbon in tree rings of ancient trees, scientists can determine the strength of the Sun's magnetic field over long periods of time. (When the Sun's magnetic field is strong, more cosmic rays arriving from deep space are deflected and less carbon 14 appears in tree rings.) Some records go back 10,000 years, adding dramatically to telescopic records. They reveal that the Sun has undergone major changes in magnetism. Solar magnetism was quite high about 1,000 years ago for several hundred years, then dropped to very low levels, as it does every 300 years or so. In the 17th century, the coldest century of the last millennium, the solar magnetic field was extremely low, and the Sun had very few spots for 70 years (the so-called “Maunder Minimum”). Earth cooled when magnetism was low; in the past 20 years, NASA satellites have found a correlation between the Sun's magnetism and energy output, and changes in temperature on Earth. Our weather may also be affected by the galaxy's flux of cosmic rays; data indicate that cloud cover is greatest when the Sun's magnetic field is least. Perhaps cosmic rays act as seed nuclei to change cloud cover a few kilometers above the earth.

How does the Sun's possible influence compare to that of increasing greenhouse gases through burning fossil fuels? The 20th century was warmer than the 19th century. The past century's weather has had three major trends: a strong warming which peaked around 1940, a cooling trend until the late 1970s and warming since then. Baliunas noted that casting doubt on the notion of runaway global warming is the fact that we put most greenhouse gases in the air during the past few decades, so warming in the early 20th century couldn't be man-made; it may have been due to solar brightening. Theories say that the lower troposphere (between 5000 and 25,000 feet) should warm with the increase of carbon dioxide, but although there has been warming at the surface, satellite measurements have not shown any such atmospheric trend. One theory holds that clouds may act as thermostats, so that if there is a human effect, it must be slow to develop, which gives us time to put into place effective policies on climate change. (Dr. Tyson asked her, if we are truly facing another ice age, if burning more fossil fuels might be a way to stave it off! She responded by saying that the increase in carbon dioxide levels has not shown any significant effect on the lower troposphere, so pumping yet more carbon dioxide into the atmosphere probably would not change much.)

The observatory at Mt. Wilson is undergoing a renaissance. It is the new home of the world's largest interferometer—1,000 feet across, with six 1-meter telescopes whose light is brought to a common focus—and the world's most powerful infrared interferometer. these instruments will enable astronomers to look at other stars, their starspots and flares, and perhaps even their planetary systems, examining their atmospheres and seeing how they change in response to how their suns are changing.

E-mail to tonyhoffman [at] earthlink [dot] net