Fact sheet 1: What is climate, and how has it changed?
When we talk about the climate of a certain place, we mean its average weather over an extended period.
Variations in solar radiation, depending on the sun’s intensity and the Earth’s orbit, have influenced the climate since the beginning, including leading to alternations between ice ages and warmer periods. Changes in ocean currents and concentrations of greenhouse gases (see fact sheet 2) also have a large impact on the climate.
Direct measurements of temperature exist from ca 1860. Variations in the global mean temperature for the period 1880 to 2007 are shown in figure 1.1. The temperature rose from the beginning of the previous century until circa 1940. After that, it was stable or even declined until ca 1970. There has been a steep rise in temperature during the last 30 years. The Intergovernmental Panel on Climate Change (see fact sheet 4
) estimates an increase of 0.74 ºC in global mean temperature from 1906 to 2005. (There is a 90 percent probability that the increase has been between 0.56 and 0.92 ºC.) The mean temperature in Norway from 1900 to 2007 is shown in figure 1.2.
Cooling volcanic eruptions
Volcanic eruptions will have a cooling effect due to the particles that are hurled out into the atmosphere. In 1991, the Pinatubo volcano in the Philippines erupted violently. This caused the global temperature to remain relatively low for the next couple of years.
Ocean and air currents in the Pacific also affect the global temperature. Normally, cold water flows to the surface off the west coast of South America. Sometimes this changes, and then the water here becomes considerably warmer. This usually occurs at Christmas time, and the phenomenon has been given the name El Niño (“boy” in Spanish) with reference to the birth of Jesus. In 1998, there was a particularly strong episode of El Niño, and the global temperature was especially high (see figure 1.1). When the opposite occurs, and the water in this area becomes especially cold, it is called La Niña (“girl”). Autumn 2007 saw the beginning of such a period.
To study the past climate from periods before we could measure it directly, we have to depend on indirect observation methods. By analyzing rock types, fossils, ice layers, sediments, and plant and animal remains, we can learn about the climate earlier in Earth’s history. By studying ice cores bored out in the Antarctic and Greenland, researchers have discovered much about the temperature and the concentration of greenhouse gases in the atmosphere going back almost one million years (figure 1.3). The climate has alternated between colder and warmer periods. We are now in a warm period that started a good 10 000 years ago. It is estimated that a new ice age will not occur for at least 30 000 years, even without anthropogenic emissions of greenhouse gases.
More detailed information is available for the last 1300 years, based on such indicators as tree ring records. There is of course some uncertainty connected with such calculations, but there is little doubt that the last 20–30 years have been unusually warm compared to the rest of this period.