Answer to Question 1
The rise of agricultural societies may have had two impacts that could cause climate change. One is changes in land use by replacing forests with crop land. The crop-land surfaces would probably have higher albedo, which could cause warming. However, there may also be less photosynthesis compared to the previous forest, so that would lead to an increase in atmospheric carbon dioxide. Some agricultural practices, like growing rice, increase the atmospheric concentration of methane, an important greenhouse gas.
Answer to Question 2
Water vapor concentrations are not thought to change dramatically from year to year because the hydrologic cycle efficiently moves water molecules from vapor to liquid and back again. So, although water vapor is abundant in the atmosphere, it probably doesn't change very much, so it is insignificant for explaining changing temperature. In contrast, measurements of air samples and air bubbles trapped in glacial ice show that carbon dioxide and methane concentrations have increased dramatically in the last century.
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