The Variable Sunby Cyrus Ance
The Variable Sunby Cyrus Ance
The Variable Sun
The sun is not constant. That has been known since the sun was first imaged by Galileo in 1613 and he saw differing patterns of sun spots, dark regions on the sun's otherwise bright surface. In 1859 Heinrich Schwabe discovered that the number of sun spots rise to a peak and fall to minimum in an 11 year cycle. There are other variations. From 1650 to 1715 there was a much reduced number of sun spots, the Maunder Minimum, which may be related to the Little Ice Age when European winters were longer and colder. The starting dates for both of these are uncertain as data earlier than 1700 is sparse and suspect thus the exact start of the Maunder Minimum is not clear, some time after 1620, and there is poor European temperature data making the start of the Little Ice Age unclear, it could have been as early as 1250 or 1450. European winters clearly got warmer during the second half of the 19th Century (1850-1900) which is clearly after the end of the Maunder Minimum. Since the end of the Maunder Minimum sun spot activity has been generally rising.
This all matters a great deal because the energy from the sun is the basic driver of the Earth's climate. It is thought that the rise in solar output since the end of the Maunder Minimum may be responsible for about half of the global temperature rise that has been observed in the 20th century. As models of climate have grown more complex it is clear that what matters is not only the total solar output, but also the output of various types of radiation. For example ultraviolet (UV) radiation interacts most strongly with the ozone layer in the upper atmosphere. If more of the sun's radiation appears in the UV it would lead to more of the sun's energy being deposited in the upper atmosphere causing a climate variation without any change in the total solar energy output.
The best place to study variations in the sun is from space. On Earth the sun's radiation is absorbed by the atmosphere in a complicated way that depends on temperature, the amount of water vapor, exact gas composition, and other factors. The atmosphere absorbs 20-25% of the solar radiation striking the Earth, 45-50% is absorbed, and the remainder is reflected. Thus ground and atmosphere based observations are hopeless since the natural variation is 50 or more times larger than the basic variation.
Satellite observations of the energy output of the sun have been done. They show that over the 11 year sun spot cycle that the variation is only 0.1%. These satellite observations, five different measurements, cover the last 20 years, do a good job of measuring the sun spot cycle variation, but are not accurate or long enough to measure any long term trends and only cover a few types of solar radiation.
The answer to the long term variation in total solar output and a precision measurement of variations in the solar radiation spectrum is the SORCE (SOlar Radiation and Climate Experiment) (much of this column is based on the SORCE web site and its links) satellite which was launched on 23 January 2003. An interesting note is that SORCE was not launched by the Space Shuttle or a vertical lift off rocket, but rather took off under the belly of a converted jet liner and was then boosted into orbit by a Pegasus Rocket.
The goals of SORCE are to measure the total solar energy output with an accuracy of 0.001%, measure two different sorts of solar UV output with an accuracy of 0.5%, and measure the solar visible light output with an accuracy of 0.01%. The satellite is designed to last for five years. The data from this experiment can be combined with the older measurements to give a measure of long term variation in the total solar output of 0.01%.
SORCE will provide key data to answer the question of whether the Earth's climate is being driven by variations in solar output or if something else is at work. The current evidence is that solar variations drive long term climate trends, but shorter term variations are as yet unexplained. It is a good bet that Earthly ice ages are caused by solar variations. The question of whether the observed global warming since the end of the Little Ice Age is a long term trend driven by the sun, a result of green house gas emissions by human industrial activity, or a natural climate variation remains open.
One very open question that remains unanswered is the cause or causes of variations in the sun. As yet there are not even any theories that offer plausible explanations as why the sun, an object that is billions of years old, should show variations on the 11 year time scale of the sun spot cycle, the 100 year time scale of the Maunder Minimum, the 1000 year time scale long term climate trends, and the longer time scale of Earthly ice ages.
The sun is an endless source of fascination. Describing its workings and what we learn about the universe from studying it is worth several more columns.