Molnens påverkan på jordens strålningsbalans och klimatsystem

Sammanfattning: The role of clouds in the climate system has been studied and their impact on the Sun's impact on the Earth's energy balance. The Earth's albedo affects the energy balance by influencing how much of the Sun’s radiation is reflected or absorbed. Clouds affect the Earth's albedo and therefore play an important role in this balance. Cloud formation occurs almost 100% in the troposphere and everything we call weather takes place there. Clouds form when warm moist air rises in the atmosphere and condenses. There are a variety of types of clouds, low so-called "Warm clouds" e.g. cumulus clouds up to 2-4 km, which consists only of water droplets and there are high "cold clouds" above about 8-12 km such as cirrocumulus, which consists only of ice crystals. There are also clouds like altocumulus in between that contain a mixture of water droplets and ice crystals. What they all have in common is that they reflect solar radiation and at the same time they absorb heat from the Earth’s own radiation. Cloud formation and the processes behind it are therefore of great interest. In order for clouds to form, the relative humidity, RH, is required to be above 100%: Under real conditions, neither water droplets nor ice crystals can form if no aerosol particles are present. Aerosols act as nuclei on which water or ice can grow. The composition of aerosols is very complex and not fully understood. They are formed for natural reasons as e.g. from the oceans that create aerosols of various salts from the winds, as well as e.g. of pollen from forests and forest fires. Anthropogenic aerosols from industries and transports that emit large amounts of soot particles that act as condensation nuclei also affect cloud formation. During the last 25 years or so, cosmic radiation has also been studied as a cause of cloud formation and much attention arose when a research group claimed that all global warming during the 20th century could be attributed to this effect. Through various studies, attempts have been made to investigate the magnitude of the effect of cosmic radiation, but without conclusive results. However, the effect seems to be far too small to explain the global warming that has taken place since the middle of the 19th century. The IPCC's latest and best future projections for the temperature increase up to the year 2100 are between about 1.5 oC to 4.5 oC. This is partly depending on which “confidence level” is chosen (in the climate model used) or which so-called RCP - "Representative Concentration Pathway", i.e. which emission scenario (of CO2) is considered. However, this large difference is also due to the fact that cloud formation and aerosols and their feedback effects on the energy balance are poorly understood. A better understanding of these processes could reduce the large difference in the IPCC's future projections of the temperature increase to 2100.