  Recent observations of the Lyman-alpha forest show large-scale spatial
variations in the intergalactic Lyman-alpha opacity that grow rapidly with
redshift at z>5, far in excess of expectations from empirically motivated
models. Previous studies have attempted to explain this excess with spatial
fluctuations in the ionizing background, but found that this required either
extremely rare sources or problematically low values for the mean free path of
ionizing photons. Here we report that much -- or potentially all -- of the
observed excess likely arises from residual spatial variations in temperature
that are an inevitable byproduct of a patchy and extended reionization process.
The amplitude of opacity fluctuations generated in this way depends on the
timing and duration of reionization. If the entire excess is due to temperature
variations alone, the observed fluctuation amplitude favors a late-ending but
extended reionization process that was roughly half complete by z~9 and that
ended at z~6. In this scenario, the highest opacities occur in regions that
reionized earliest, since they have had the most time to cool, while the lowest
opacities occur in the warmer regions that reionized most recently. This
correspondence potentially opens a new observational window into patchy
reionization.
