What was done
The authors applied advanced spectral methods to fill in data gaps and locate interannual and interdecadal periodicities in historical records of annual low- and high-water levels on the Nile River over the 1300-year period A.D. 622 to 1922.

What was learned
Several statistically significant periodicities were noted throughout the 1300-year record, including cycles at 256, 64, 19, 12, 7, 4.2 and 2.2 years.  With respect to the causes of these cycles, Kondrashov et al. say the 4.2- and 2.2-year oscillations are likely the product of El Niņo-Southern Oscillation variations.  The 7-year cycle, on the other hand, is possibly related to North Atlantic influences, according to them, while the longer-period oscillations may be due to astronomical forcings.

In addition to revealing the stated periodicities, the results of the analysis and annual-scale resolution provide a "sharper and more reliable determination of climatic-regime transitions" for tropical east Africa, including documentation of fairly abrupt shifts in river flow at the beginning and end of the Medieval Warm Period, as well as for other periods throughout the record.

What it means
In the words of the authors, the "fairly sharp shifts in the amplitude and period of the interannual and interdecadal modes over the last millennium-and-a-half support concerns about the possible effect of climate shifts in the not-so-distant future."  Those living near the Nile and who are dependent on it for their sustenance should be particularly concerned, for abrupt changes in flow rates and river levels have punctuated the river system for 1300 years or more; and there is no reason why similar changes will not continue in the future, independent of any change in atmospheric CO2 concentration.