The most general definition of climate change is a change in the statistical properties (mainly the mean and the spread) of the climate system considered over long periods of time. Fluctuations over periods shorter than a few decades are not regarded as climate change. Current climate change is caused by natural factors, such as variations in solar radiation and volcanic eruptions, but mainly by human activities. By, among other things, burning fossil fuels, and cutting down and burning down forests, people have changed the heat balance so that the average temperature has fallen outside the range that has characterized the recorded (human) history so far. Due to delays in the climate system, the impact of climate change will continue for many decades and – in some cases – many centuries.
Although there is still much to learn, the core phenomena, scientific questions and hypotheses surrounding climate change have been thoroughly investigated. These have survived debates and the evaluation of alternative explanations. Evidence for climate change comes from a number of sources that can be used to reconstruct the climate in the past. For the distant past, the major part of this concerns changes in indicators that reflect the climate, such as vegetation, ice cores, sea-level change, and glacial geology. With regard to later periods, one can rely on archaeological evidence, oral history, and historical documents.
Worldwide reports of surface temperatures are available from the late 19th century. This data was supplemented with atmospheric controls by the middle of the 20th century. Satellite data collected from the 1970s onwards also show that the amount of solar radiation has not increased since then. Heating up in the past 30 years can therefore not be attributed to an increase in solar energy. Climate models are unable to reproduce climate change when only the output of the sun and volcanic activity are taken into account. These only work if human activity is taken into account. More than 95% of the researchers are certain that the phenomenon is caused by increasing concentrations of greenhouse gases and other human activities.
