Scenarios of climate change

This article provides an overview of current and prospected climate changes, their causes and implied threats, and of a possible route to keep the changes within a tolerable level. The global mean temperature has up to 2005 risen by almost 0.8◦C, and the change expected by 2100 is as large as glacial-interglacial changes in the past, which were commonly spread out over 10 000 years. As is well known, the principle actor is man-made CO2, which, together with other anthropogenic gases, enhances the atmosphere’s greenhouse effect. The only man-made cooling agent appears to be atmospheric aerosols. Atmospheric CO2 has now reached levels unprecedented during the past several million years. Principal threats are a greatly reduced biodiversity (species extinction), changes in the atmospheric precipitation pattern, more frequent weather extremes, and not the least, sea level rise. The expected precipitation pattern will enhance water scarcity in and around regions that suffer from water shortage already, affecting many countries. Sea level rise will act on a longer time scale. It is expected to amount to more than 50 cm by 2100, and over the coming centuries the potential rise is of the order of 10 m. A global-mean temperature increase of 2◦C is often quoted as a safe limit, beyond which irreversible effects must be expected. To achieve that limit, a major, rapid, and coordinated international effort will be needed. Up to the year 2050, the man-made CO2 releases must be reduced by at least 50%. This must be accompanied by a complete overhaul of the global energy supply toward depending increasingly on the Sun’s supply of energy, both directly and in converted form, such as wind energy. Much of the information and insight available today has been generated by the Intergovernmental Panel on Climate Change (IPCC), in particular its Fourth Assessment Report of 2007, which greatly advanced both public attention and political actio