The Sun is the primary source of energy responsible for governing both the weather and the climate of Earth. For that reason alone one would expect that changes in the amount and type of energy Earth received from the Sun could alter weather and climate on the Earth.

Our Sun is not a constant star and variations in the energy Earth receives from the Sun, both in the form of total irradiance and other types of energy such as outflows of charged particles, are well documented. The variations in solar irradiance are generally cyclic with times ranging from the 27-day solar rotation period, through the 11-year and 22-year solar activity periods, to very long cycles of hundreds to thousands of years duration. The variations in the other forms of energy received by Earth from the Sun exhibit variations of similar time scales.

Much meteorological and climatic data suggest that there are significant responses in Earth+s atmosphere and oceans to variability on the part of our Sun. Drought cycles, variations in global sea surface temperatures, variations in stratospheric temperatures at specific locations, variations in the tracks followed by storms across the Atlantic, variations in year-to-year tree growth as determined by tree-ring studies, and climate variations exposed by glacial ice-core studies have all shown remarkable correlation with various forms of solar variability over time spans ranging up to 100,000 years.

While our Sun is a variable star, so that the energy it delivers to Earth changes with time, and while there is statistical evidence of climate responses to this variability, the exact nature of Earth+s response to those variations has proven elusive. There are a variety of reasons why understanding the details of the interaction between the Sun+s variability and Earth+s response has not been achieved.

Perhaps most important is that the physical processes that connect solar variability to climatic change have yet to be established. In particular, the mechanisms by which the observed changes of less than 0.1% in total solar irradiance can modify temperatures on earth by more than 1 degree C are still open to much speculation and while a variety of theories have been put forth, non have been generally accepted.

Another complicating factor is that solar variability is not the same at all wavelengths. At visible wavelengths, the variations in solar intensity are quite small with changes of only 0.1% on times scales of years. At wavelengths not visible to humans, such as the Ultraviolet (UV) and Extreme Ultraviolet (EUV), the percentage variability is much larger. In the x-ray portion of the solar spectrum, the radiation from the sun can change by factors of 100 or more over time scales of minutes to hours. The contribution to the total solar irradiance from UV and x-ray radiation is small (less than 1% of the total energy). However, because these photons are absorbed at high altitudes where the atmosphere is very thin, fluctuations in the UV irradiance produce large changes in atmospheric temperature and density.

Even if the connection between solar variability and Earth+s climate response were well understood, true predictions would remain difficult for the source of the periodic nature of the sun is not well understood. Solar physicists are still looking for physical processes that could produce the various time variations observed in the solar irradiance record. The more important periods of solar irradiance variability are described below.

27 Day Solar Rotation Period: This is one of the more prominent periods of solar irradiance variability however the amplitude is usually much less than 0.1% and there is very little evidence of atmospheric responses to changes of these time scales

• 10.5 Year Solar Cycle: The most prominent period observable is that of the "Solar Cycle". It has been observed in Chinese sunspot records dating back two thousand years. Many of the observed changes in climate are correlated with 10.5 year solar cycle period. It has been shown that the tracks of storms across the oceans change in latitude with changing phases of the solar cycle. These changes in storm tracks could be the cause of droughts and floods which show periodicities of 10.5 years in some regions of the world.

• 88 Year and 124 year and >300 year cycles: The sun has many subtle periodicities that show up in ice core records and tree ring records that can be taken back thousands of years. These periods are still the subject of ongoing research. The confluence of these cycles have produced climatic changes. The most recent dramatic example of this occurred in the 17th century during which time, the sun went several decades without sunspots. This period of solar minimum is referred to as the Maunder minimum and the climatic changes associated with this period include severe cold in Europe with snow in the middle of summer. In more recent times, it has been hypothesized that between 10 and 40 percent of the increase in the Earth's temperature over the last 100 years (global warming) could be due to an increase in the solar irradiance associated with the superposition of several long-period solar oscillations.

• 23,000, 42,000 and 100,000 year cycles: Over long periods of time (thousands of years) the orbit of the earth around the sun changes due to many factors including the gravitational pull of other planets. The changes in the orbit will change the amount of solar radiation that the earth receives. These variations cause major changes in the climate which cause long periods of cooling. These periods of glaciation of much of the Northern hemisphere, referred to as Ice Ages, have been well documented to have occurred regularly over millions of years and the agreement between the Earth orbit around the sun and the Ice Ages is quite good.

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  February 11, 1997