Solar Cycle 24
The Earth's surface is heated primarily by the sun. Since we're sitting on a hot ball of iron and nickel, some heating does come from the interior of the Earth. That notwithstanding, a variation in insolation will change the average temperature at the surface of the Earth. We still don't know whether the sun's energy output is constant, although we do have a number known as the Solar constant. The Solar constant, which is the integrated amount of solar energy incident on the Earth at all wavelengths, is about 1366 watts per square meter. This varies about 6.0% over the course of the year, since the Earth's orbit is not circular. When the Earth is nearest the sun, the solar constant is about 1412 W/m2; when it's farthest, it's about 1321 W/m2.
The sun is not a featureless ball of light. Sun spots, cooler regions on the sun's surface that arise when convection is inhibited by magnetic action, were observed telescopically by several astronomers in 1610 and explained by Galileo shortly thereafter. The interesting thing about sunspots is that their occurrence follows a cyclic pattern with a period of about eleven years. Every eleven years there is a period when no sun spots appear, followed about five or six years later by a solar surface marked with about 100-200 sunspots. We are just leaving a sun spot minimum and we're at the start of what's called Solar Cycle 24, which began with an observation of a sun spot of a particular magnetic polarity on January 4, 2008 [1-3]. Solar Cycle 24 is the twenty-fourth cycle since accurate records have been taken during the first cycle of March, 1755, through June, 1766.
One curious thing about sun spots is their near absence in the period 1645 - 1715. During this period, known as the Maunder Minimum after its discoverer, Edward Maunder, only fifty sunspots appeared in toto, as compared to the tens of thousands typically observed in a similar period. In fact, no sun spots were observed at all during 1670. The Maunder Minimum coincided with the Little Ice Age, a period of extremely cold winters. This coincidence may just be anecdotal evidence for an intimate connection between solar activity and Earth's climate, but if it's a real effect, our climate is affected by more than man-made global warming.
Solar cycle 23, which peaked around 2001, was one of the more violent cycles with many solar storms that affected power transmission lines, Earth satellites, GPS navigation, and other radio communications. Solar storms can interfere also with cell phone reception. Predictions have indicated that this solar cycle, which should peak near 2011, will be intense. David Hathaway and Robert Wilson of the Marshall Space Flight Center presented a paper at a 2006 meeting of the American Geophysical Union in which they predicted a sun spot number of 160 +/- 25 at maximum [4]. The sun, however, is sending mixed messages. Solar cycle 24 was expected to start a year earlier than it actually did, and cycles with such a late start are usually less intense. It's uncertainty like this that keeps scientists employed. To track the progress of Solar Cycle 24, see the references [5-6].
References:
1. Tony Phillips, "Solar Cycle 24 Begins" (NASA Science Page, January, 10, 2008).
2. Maggie McKee, "Maverick sunspot heralds new solar cycle" (New Scientist Online, January 7, 2008).
3. Tony Phillips, "What's Wrong with the Sun?" (NASA Science Page, July 11, 2008).
4. Scientists Predict Big Solar Cycle (PhysicsOrg, December 22, 2006).
5. Solar Cycle 24 Prediction Web Site (NOAA Space Weather Prediction Center)
6. Homepage of the Solar Influences Data Analysis Center (Royal Observatory of Belgium)