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Despite the considerable progress achieved during recent years in quantifying and modeling climatic and ecological processes caused by increasing concentrations of greenhouse gases in the atmosphere, there are still major uncertainties regarding the potential effects of increasing concentrations of either CO2 (carbon dioxide) or future climate change in arid ecosystems. General Circulation Models predict varying patterns of moister or drier conditions in deserts for the next century, but the results of climatic and ecosystem modeling in relation to deserts in a future "greenhouse effect" climate are complex and contradictory. Nevertheless, if deserts do respond more dramatically to global temperature change, as they did during the Holocene and, especially the last interglacial era (130,000 years ago), they might act as globally significant sinks of carbon into soils and vegetation. Some growth chamber experiments have indicated that increased CO2 will significantly affect desert shrubs, whereas other chamber and field experiments suggest that rising levels of atmospheric CO2 may not dramatically affect desert ecosystems, although certain individual species may be strongly favored. It is difficult to make a firm statement whether there are any valid analogs between the climate changes of the past and future climate change induced by greenhouse gases.