Lindzen has expressed his concern over the validity of computer models used to predict future climate change. Lindzen said that predicted warming may be overestimated because of their handling of the climate system's water vapor feedback. The feedback due to water vapor is a major factor in determining how much warming would be expected to occur with increased atmospheric concentrations of carbon dioxide, and all existing computer models assume positive feedback — that is, that as the climate warms, the amount of water vapour held in the atmosphere will increase, leading to further warming. By contrast, Lindzen believes that temperature increases will actually cause more extensive drying due to increased areas of atmospheric subsidence as a result of the Iris effect, nullifying future warming. This claim was criticized by climatologist Gavin Schmidt, Director of NASA's Goddard Institute for Space Studies, who notes the more generally-accepted understanding of the effects of the Iris effect and cites empirical cases where large and relatively rapid changes in the climate such as El Niño events, the Ultra-Plinian eruption of Mount Pinatubo in 1991, and recent trends in global temperature and water vapor levels to show that, as predicted in the generally-accepted view, water vapor increases as the temperature increases, and decreases as temperatures decrease.