evidence: Now, since people can’t directly control water vapor, the only way we have to influence it is via temperature. If the greenhouse effect boosts global temperature somewhat, we should realistically expect that the amount of water vapor in the air should be increasing. Similarly, if global temperatures drop for some reason (for example, a large volcanic eruption dumping massive amounts of aerosols into the air), we should expect to see water vapor concentrations decrease. In the lower atmosphere, the available data points to increasing water vapor content, but because of large variations in local humidity from day to night, from day to day, and from season to season, no-one currently knows exactly how much more water vapor is going into the air (IPCC Working Group 1 Assessment Report 4, Chapter 3, "Observations: Surface and Atmospheric Climate Change", page 273). And unfortunately, the upper troposphere (the region of the atmosphere believed to be most important for water vapor’s effects on global heating) has no conclusive direct data on water vapor concentrations. Instead, the increase in water vapor in this part of the atmosphere has been indirectly checked by the increase in this region’s temperature. Since water vapor is such a powerful greenhouse gas, any increase in temperature in this region of the atmosphere should be largely a result of the effects of water vapor (IPCC Working Group 1 Assessment Report 4, Chapter 3, "Observations: Surface and Atmospheric Climate Change", Figure 3.21, page 275).