/ Geophysical Fluid Dynamics Laboratory indicated that the late 20th century Sahel drought was probably a climatic response to changing sea surface temperature patterns, and that it could be viewed as a combination of natural variability superimposed upon an anthropogenically forced regional drying trend. Using GFDL CM2.X, these climate model simulations indicated that the general late 20th century Sahel drying trend was attributable to human-induced factors; largely due to an increase in greenhouse gases and partly due to an increase in atmospheric aerosols. A study published in 2013, done at the University of Washington, suggests that atmospheric aerosols caused a downward shift in the Intertropical Convergence Zone. The shift, the study says, left normally rainy areas in Central Africa much drier. In IPCC future scenario A2 (CO2 value of ≈860 ppm) Sahel rainfall could be reduced by up to 25% by year 2100, according to climate models. A 2006 study by NOAA scientists suggests that the Atlantic Multidecadal Oscillation plays a leading role. An AMO warm phase strengthens the summer rainfall over Sahel, while a cold phase reduces it. The AMO entered a warm phase in 1995 and, assuming a 70-year cycle (following peaks in ≈1880 and ≈1950), will peak around 2020. A 2009 study found further evidence for a link between the AMO and West African drought. Later, a 2013 study found that the East Atlantic (EA) mode also modulates Sahel summer rainfall and further indicated that operational climate forecasting was unable to capture this EA impact on the Sahel. The recovery of Sahel drought since the 1990s, coined "Sahel Greening" by media, is accounted for by enhancements in both the tropical easterly jet and the African easterly jet, both of which are known to induce wet anomalies. Moreover, positional shifts in the African easterly jet and African easterly waves (AEWs) accompanied the northward migration of the Sahel rainband. Change in the African easterly jet and AEWs are coupled to a northward shift and amplification of convective activity. United Nations response In 1973, The United Nations Sahelian Office (UNSO) was created to address the problems of drought in the Sahel region following the West African Sahel drought of 1968-73. In the 1990s, the United Nations Convention to Combat Desertification (UNCCD) was adopted and UNSO became the United Nations Development Programme's Office to Combat Desertification and Drought, as its scope broadened to be global rather than only focused on Africa.