Natural Climate Change
The weather is obviously not the same from day to day. It is actually not the same from age to age. Over long periods of of time the Earth's climate chages. In the past – only a few hundred years ago – Europe had a 'little ice age', with the Thames freezing over every winter. At one time, it is believed the Earth was virtually fozen, with only a 10 km strip of water around the equator where life could survive. There are natural processes causing the Earth's climate to change.
The distance of the Earth from the Sun is not constant. The orbit of the Earth is not a perfect circle. It is actually an ellipse, so at times the Earth is closer than at other times. In addition the orbit undergoes a longer cycle of about 26,000 years. The energy received from the Sun increases as the Earth gets closer.
The power produced by the Sun is not constant. It varies by a fraction of a per cent over periods between 11 and 6,000 years.
There are natural processes on Earth which decrease or increase the amount of energy retained by the Earth. The most important of these natural processes is volcanic eruptions. When a volcano erupts, it may eject lots of dust particles high up in the Earth's atmosphere. These ause temporary cooling: the amount of cooling depends on the amount of dust, and the duration of the cooling depends on the size of the dust particles. Tiny dust-size ash particles thrown into the lower atmosphere will float around for hours or days, causing darkness and cooling directly beneath the ash cloud, but these particles are quickly washed out of the air by water in the lower atmosphere. However, dust tossed into the dry upper atmosphere - the stratosphere - can remain for weeks to months before they finally settle. These particles block sunlight and cause some cooling over large areas of the earth. When Mount Pinatubo erupted in 1991, global temperature fell by an average of half a degree for two years.
The amount of carbon dioxide in the Earth's atmosphere has not been constant since the Earth formed. It was once as high as 7000 parts per milliuon. It has undergone natural cycles for the past few hundred thousand years. When it is low – about 200 parts per million – we have an ice age. When it is high – about 300 parts per million – the climate is warmer. This is a natural greenhouse effect. The amount of carbon dioxide in the atmosphere now is increasing because of human activity, and the Earth's temperature is almost certainly rising as a result of this.
The Earth's continents are moving (very slowly) as a result of continental drift. This has changed rainfall patterns and the flows of ocean currents, both important mechanisms for the regulation of climate.
Land cover has changed. Forested land retains water and absorbs more heat than desert, and experiences more rain. Many areas once forested have become desert for various reasons. Desert has much less capacity to retain heat (and consequently are much hotter during the day, since it takes little to heat them) and colder during the night.