Headline: Atmospheric Chemistry and Climate in the Anthropocene (2012)

The effects of human activities are increasingly overwhelming the geologic, biological and chemical processes that drive changes in the abundance of trace and greenhouse gases (GHGs) in the atmosphere and Earth System behaviour. Thus, mankind has opened a new geological epoch or age – the Anthropocene. This development is driven by the strong population increase which may result in ten billion people by 2100. With this population increase, human demands for food and animal protein in particular, clean water, natural resources and nutrients such as fixed nitrogen (N) and phosphorus (P), land and energy will continue to increase strongly. It is also hypothesised that human enterprise is responsible for the sixth mass species extinction. Further, releases of gases such as sulphur dioxide (SO2), nitric oxide (NO) and chlorofluorocarbons (CFC) into the atmosphere are several times higher than natural emissions. CFCs give rise to highly active radicals in the stratosphere which destroy ozone (O3) by catalytic reactions. However, CFC emissions have been drastically reduced and stratospheric O3 concentrations are increasing. In contrast, concentrations of GHGs such as carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O) continue to increase and are now well above the preindustrial levels as increasing human demand for energy is met by burning fossil fuels. As a consequence, land and ocean temperatures are increasing, glaciers are melting and sea levels are rising. Strong reductions, in particular, of the anthropogenic CO2 emissions are needed to minimise risks of future warming and its consequences as CO2 is the major GHG with part of it having a long atmospheric residence time. However, CO2 emissions continue to increase even during the periods of global economic crises and reduced industrial activities. Thus, geoengineering or climate engineering techniques are discussed to cool Earth indirectly by removing CO2 from the atmosphere or directly by increasing backscattering of solar radiation into space. For example, injecting sulphur (S) into the stratosphere has been proposed as sulphate particles reflect sunlight but many issues remain unresolved. Thus, a strong reduction in anthropogenic CO2 emissions is needed to mitigate climate change.