Driven by these large-scale forces, the intensity and frequency of rain pulses at a local scale may vary sub stantially with time, and future projections indicate an increasing likelihood of even more episodic climate events and inter-annual variability in deserts. Continental deserts could experience more severe, persistent droughts. In the Americas, temperate deserts are projected to expand substantially under doubled CO2 climate scenarios. About one third of the Sahel was projected to aridify with warming of 2.7-3.6 degrees F (1.5- 2 degrees C) by about 2050, with a general equa-torward shift of vegetation zones. Alternative climate scenarios show less pronounced changes, even shifts to wetter climates in the Sahel zone and movement of vegetation zones, accordingly. The potentially positive impact of rising atmospheric CO2 remains a significant uncertainty, which also could offset current projections, especially because it is likely to increase plant productivity, leading to contrasting scenarios.
Deserts also play an important role in the regulation of climate, air quality, and atmosphere composition of remote areas through wind-blown desert dust; also the desert albedo influences rainfall and bio-geochemistry of other terrestrial and marine ecosystems, in this way increasing vulnerability of remote regions to climate change. The 4th Intergovernmental Panel on Climate Change (IPCC) assessment projects increased dust flux rates, which may increase aridity and suppress rainfall outside deserts, with opposite effects under wetting scenarios, associated with a decline of atmospheric burden by soil aerosols of 20-60 percent. Changes of rainfall in deserts are projected to be in the range of 0.2 mm. day-1.
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