Global climate change

Climate, the statistical description of weather including the probabilities for anomalies, changes continuously, since both the interaction among the climate system components (atmosphere, ocean, biosphere, cryosphere, and lithosphere) and the irradiance of the sun vary on all timescales up to billions of years. On timescales which are of most interest for mankind, i.e. up to centuries, major factors causing climate variability are: changing composition of the atmosphere; spectral solar irradiance variations; systematic changes of ocean-atmosphere-land interaction, if resulting in major changes of ocean circulation; volcanic eruptions injecting mass into the low latitude stratosphere (>20 km height); and land cover change modifying the surface energy budget. Two of these influencing factors are now largely anthropogenic, namely changing atmospheric conditions and land cover change. Thus, the climate discussion has become to a large extent one about anthropogenic influence. At different periods, different influencing factors have been more important or dominant. Since the complex interactions are only partly understood, it is difficult to attribute observed climate parameter changes or trends to particular causes. In this brief status report on global climate change, present knowledge is reviewed, mainly based on the second full assessment report of the Intergovernmental Panel on Climate Change.