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Bleeding Edge

The Variable Sun

by Cyrus Ance
Apr 17,2003

 

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.

Roleplaying Ideas

  • An Attack on SORCE II: The SORCE experiment was a great success revealing that there is a long term rise in solar output that explains recent global warming. The second version is soon to be launched. The character's are contacted by the CIA who have uncovered a plot to shoot down SORCE II during its launch. Who would do this? Why?
  • A Message from the Sun: A researcher working on SORCE comes to the characters with a puzzle. Buried in the data from the satellite is a message encoded in the signal from one of the UV monitors. It says "Time is running out. This star will explode in ..." The rest is not decipherable. Who is the source of the message? Is it coming from inside the sun? How can that be possible? Where else could it be coming from? Is someone faking this message? Why? What if it is true?
  • The Coming Ice Age: SORCE reveals that the energy output from the sun is falling fast. An ice age is due within the next century. What can be done? Rival plans are floated to revive the sun, flood the atmosphere with green house gases to keep the Earth warm (what happens if these plans goes through, but the sun revives and the Earth starts heating uncontrollably), build generation ships for part of the population (mostly in the rich west) to escape and search for a new planet, relocate the people of mid-latitudes (North America, Europe, etc.) to the tropics, or for more studies. Which plan will the characters support? All of them have huge consequences. The possibility or reality of a fast onset ice age could be the start of a long term campaign.

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What do you think?

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