The fourth assessment report on climate change (AR4) was released in 2007 and the Intergovernmental Panel of Climate Change (IPCC) derive an increase of 0.74 ± 0.18°C in the 100 year global mean surface temperature linear trend between 1906 – 2005. IPCC state further that “there is very high confidence that the global average net effect of human activities since 1750 has been one of warming” (IPCC, 2007). The observed global warming has occurred during the same period as a considerable increase of greenhouse gases in the atmosphere. The most important anthropogenic greenhouse gas is carbon dioxide (CO2) and the global concentration of CO2 has increased by 35% from 280 parts per million (ppm) since pre industrial times to 379 ppm in 2005. The sources of increasing CO2 in the atmosphere are related to human activity through fossil fuel combustion and deforestation.
Other important greenhouse gases that have increased since pre industrial times are N2O and CH4 and their sources are primarily from agriculture. At the same time as the observed global warming, there has been an increasing amount of anthropogenic emissions of particles into the atmosphere. These particles are known as aerosols and have mainly caused global cooling by scattering and absorbing solar radiation. It is proposed that the aerosols have partly masked the warming caused by the greenhouse gases (Charlson et al., 1992,Ramanathan et al., 2001,Kaufman et al., 2002,Chung et al., 2005)
