Climates are not constant - they change over time. The changes are barely noticeable during a human's lifetime, but when you look back further in time, for instance to the Middle Ages (when it was much warmer in the Netherlands than it is now) or even back to the Ice Age (10,000 years ago, when the Netherlands were covered with a thick layer of ice and snow), these differences can be huge.
Many factors influence the climate. These factors can be divided into three groups. Firstly, there are ‘internal’ factors: the changes that originate from the way the climate system works. Secondly, there are natural factors: changes that originate from natural processes. Thirdly, there are human factors: factors that originate from the behaviour of human beings.
Research shows that warm and cold periods alternated throughout the last 400,000 years. It is likely that these alternating periods will continue in the future. These internal factors cannot be changed by humans, since they are the direct consequence of the the nature of the global climate system.
Volcanic eruptions can force so much ash into the atmosphere that the temperature can rise over the next couple of years. Even larger nature disasters, such as the impact of a large meteorite, can severely change a climate instantly. In the long term, the position of the earth in relation to the sun can also influence the climate. Looking even further into the future (hundreds of millions of years) the sun will become hotter and hotter.
There has been a number of ice ages during the last 100,000 years. It was proved that these ice ages were caused by both natural and internal factors. In the last 1000 years, there have been two periods in which the temperature was generally higher (during the Middle Ages) or generally colder (during the ‘small ice age’ around 1700). The causes of the small ice age are ascribed to natural factors, such as the increase in the activity of volcanoes and a small reduction in the output of the sun. Interestingly however, scientists do not know why the temperature was a few degrees higher during the Middle Ages.