Fig. 2

Response of the Nile to explosive volcanism. a CMIP5 ensemble mean precipitation minus evaporation (P–E) response to five 20th century volcanic eruptions (color shading, mm per day). The response is the average P–E over the first summer season (May–October) that contained or followed the eruption, relative to the five summers preceding the eruption. Only anomalies statistically significant at the 5% level are shown (Methods section). Blue contour is the 2 mm per day isoline of P–E. b As in a but showing the Nile watershed, which is outlined in green, with major Nile watercourses shown by thin blue lines. c Annual Nile summer flood heights from the Islamic Nilometer composited relative to the ice-core-estimated dates of eruption years for 60 large eruptions (Methods section) between 622 and 1902 CE (eruption years are represented at point 0 on the horizontal axis; years 1 to 8 then represent the first to eighth years after these eruptions, and years −1 to −8 the first to eighth years before). Data associated with secondary eruptions within this compositing window are excluded (Methods section). Shading indicates the 2-tailed 90% confidence interval, estimated using the t-distribution. Nile summer flood heights average 22 cm lower in eruption years (P < 0.01). d Annual Nile summer flood heights from the Islamic Nilometer ranked by percentile for eruption years (red; N = 61 (Methods section)) and non-eruption years (black; N = 1030). Gray shading indicates the 2-tailed Monte Carlo 90% confidence interval based on 1,000,000 draws from the non-eruption-year distribution. The blue line indicates the 1-tailed P-value for each of 61 ranks, when tested independently without accounting for multiple tests. e Annual Nile flood quality index for the Ptolemaic period, 305–30 BCE (Methods section)²⁹, ordered by percentile for eruption years (red; N = 8) and non-eruption years (black; N = 88). High flood quality index values equate to documentary indications of high summer floods