Attention has begun to shift from local, short-term seasonal patterns of temperature, rainfall, other elements of the weather, toward longer-term trends that can affect the entire Earth, se long-term (typically 30-year) weather trends are called "climate." It is therefore important understand the difference, as well as the relation, between "weather" and "climate."

An example of the relationship between weather and climate is El Nino, which is weather with local, short-term consequences as well as with global, long-term importance. In the ort-term. El Nino can bring a dry summer for some regions and a wet winter for others; however, over the course of many years, the number of times El Nino conditions occur may decade changes in the global climate.

Variations in the behaviour of the weather over long time periods, such as from one century another, are referred to as climate change. Climate itself adjusts from the times of 'ice ages,' hen huge ice sheets covered large areas that are currently ice-free, to periods similar to today hen ice sheets are largely confined to Antarctica, Greenland, and the floating Arctic sea ice. Paleo-climate records indicate that much of the climate changes over the last two million years occured in a rather cyclical manner; with glacial periods lasting roughly 100,000 years with warmer interglacial periods of 10,000 years occurring in between.

The sun, of course, is the ultimate source of heat energy reaching the Earth, fueling our weather systems, and establishing our major climate zones. There is, however, good evidence that larger variations in the sun's activity do occur. For example, during the last half of the 17th century, there was a period of greatly reduced solar activity.

This was also a time of harsh winters and extended bitter cold referred to as the Little Ice Age. Scientists do not yet understand the underlying