to land use is that the terrestrial albedo is often altered by use, which leads to radiative forcing. This effect is more significant locally than globally. Livestock and land use Worldwide, livestock production occupies 70% of all land used for agriculture, or 30% of the ice-free land surface of the Earth. More than 18% of anthropogenic greenhouse gas emissions are attributed to livestock and livestock-related activities such as deforestation and increasingly fuel-intensive farming practices. Specific attributions to the livestock sector include: 9% of global anthropogenic carbon dioxide emissions 35–40% of global anthropogenic methane emissions (chiefly due to enteric fermentation and manure) 64% of global anthropogenic nitrous oxide emissions, chiefly due to fertilizer use. Aerosols With virtual certainty, scientific consensus has attributed various forms of climate change, chiefly cooling effects, to aerosols, which are small particles or droplets suspended in the atmosphere. Key sources to which anthropogenic aerosols are attributed include: biomass burning such as slash-and-burn deforestation. Aerosols produced are primarily black carbon. industrial air pollution, which produces soot and airborne sulfates, nitrates, and ammonium dust produced by land use effects such as desertification Attribution of 20th-century climate change Over the past 150 years human activities have released increasing quantities of greenhouse gases into the atmosphere. This has led to increases in mean global temperature, or global warming. Other human effects are relevant—for example, sulphate aerosols are believed to have a cooling effect. Natural factors also contribute. According to the historical temperature record of the last century, the Earth's near-surface air temperature has risen around 0.74 ± 0.18 °Celsius (1.3 ± 0.32 °Fahrenheit). A historically important question in climate change research has regarded the relative importance of human activity and non-anthropogenic causes during the period of instrumental record. In the 1995 Second Assessment Report (SAR), the IPCC made the widely quoted statement that "The balance of evidence suggests a discernible human influence on global climate". The phrase "balance of evidence" suggested the (English) common-law standard of proof required in civil as opposed to criminal courts: not as high as "beyond reasonable doubt". In 2001 the Third Assessment Report (TAR) refined this, saying "There is new and stronger evidence that most of the warming observed over the last 50 years is attributable to human activities". The 2007 Fourth Assessment Report (AR4) strengthened this finding: "Anthropogenic warming of the climate system is widespread and can be detected in temperature observations taken at the