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Solar radiation, Sunspots and Orbital Variations
Life on Earth could not exist without the Sun and the warmth and light provided by its rays. Although this essential element has always effectively been there and always will be for the next few billion years, the intensity and frequency of the rays reaching our planet varies according to the distance we are from the sun during our annual orbit around it and some solar events.
The Earth tilts slightly during its orbit and rotates every 24 hours. The orbital 28 day path of the moon around the Earth and its gravitational pull creates tidal forces on the Earth. These orbital features combine to give us seasonal changes and high and low tides.
Milankovitch cycles
| Milankovitch cycles based on the precession, eccentricity and axis movements of the Earth have been suggested as influencing climate change in 100,000 year periods which seem to relate to the frequency of Quaternary glaciations. The fact that these cycles do not exactly coincide with climatic conditions has been attributed to CO2 and ice sheet dynamic feedbacks. |
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The Sun
The Sun is the star at the centre of our solar system and it is so massive that it accounts for about 99.8% of all matter in it.
For several hundred years the phenomenon of Sun Spots has been observed. The number of sunspots visible on the Sun is not constant, but varies over an 11-year cycle known as the Solar cycle. At a typical solar minimum, few sunspots are visible, and occasionally none at all can be seen. Those that do appear are at high solar latitudes. As the sunspot cycle progresses, the number of sunspots increases and they move closer to the equator of the Sun.
Sunspots usually exist as pairs with opposite magnetic polarity. The magnetic polarity of the leading sunspot alternates every solar cycle, so that it will be a north magnetic pole in one solar cycle and a south magnetic pole in the next.
History of the number of observed sunspots during the last 250 years, which shows the ~11-year solar cycle.
The solar cycle has a great influence on space weather and is a significant influence on the Earth's climate. Solar activity minima tend to be correlated with colder temperatures and longer than average solar cycles tend to be correlated with hotter temperatures. In the 17th century, the solar cycle appears to have stopped entirely for several decades; very few sunspots were observed during this period. During this era, which is known as the Maunder minimum or Little Ice Age, Europe experienced very cold temperatures. Earlier extended minima have been discovered through analysis of tree rings and also appear to have coincided with lower-than-average global temperatures.
A recent theory claims that there are magnetic instabilities in the core of the Sun which cause fluctuations with periods of either 41,000 or 100,000 years. These could provide a better explanation of the ice ages than the Milankovitch cycles. Like many theories in astrophysics, this theory cannot be tested directly.
Our entire solar system also has an orbit within the Milky Way galaxy22 which takes 225 - 250 million years to complete.
As can be seen in the above graphic, the orbit of our solar systen passes through or near some of the spiral arms of the Milky Way. This possible contact has also been linked to past extinctions.
Although all of these factors may in some way directly effect climate, sea level, continental drift, volcanic activity and asteroids, the evidence is often open to dispute because of the time span involved and the potential error in computer models.
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