Global Warming in the Past
Since its formation approximately 4.65 billion years ago, the Earth has heated and cooled many times. Global climate changes depended on many factors, such as massive volcanic eruptions that increased carbon dioxide in the atmosphere, changes in the intensity of the energy emitted by the Sun and variations in the position of the Earth relative to the Sun, both in its orbit and in the inclination of its axis of rotation.
Variations in Earth's position, known as cycles, are believed to be responsible for the repeated advance and retreat of glaciers and ice sheets during the Pleistocene, when the Earth entered fairly regular cycles of colder glacial periods (also known as ice ages) and warmer interglacial periods. Glacial periods appeared at intervals of approximately 100,000 years.
Greenhouse gases and temperatures are increasing. Before the end of the 19th century, the average temperature of the Earth was almost 15 degrees Celsius. Over the past 100 years, the average temperature of the earth's surface has risen by around 0.7° C, with a steeper rise since the 1970s. Scientists have linked this increase to numerous changes around the world, such as the melting of mountain glaciers and polar ice, rising sea levels, the most severe and prolonged droughts, the most intense storms, waves of more frequent heat and changes in the life cycles of many plants and animals. Warming has been most noticeable in the Arctic, where temperatures have even risen to twice the global average.