Such global warming is generally attributed to an increase in greenhouse gases, particularly carbon dioxide and methane. Most of these gases have been added to the atmosphere by human activity, notably the clearing of forests by slash and burn farming, changes in agricultural land use, increased use of fossil fuels and rampant urbanization/industrialization. Since about 1860, levels of atmospheric carbon dioxide have increased by more than 30% and methane levels have more than doubled.According to the passage, what is not a cause of global warming?