Recent geographic convergence in diurnal and annual temperature cycling flattens global thermal profiles

Warming mean temperatures over the past century(1) have probably shifted distributions(2), altered phenologies(3), increased extinction risks(4,5), and impacted agriculture(6) and human health(7). However, knowledge of mean temperatures alone does not provide a complete understanding either of changes in the climate itself or of how changing climate will affect organisms(8-11). Temporal temperature variation, primarily driven by daily and annual temperature cycles, has profound effects on organism physiology(8,9) and ecology(12), yet changes in temperature cycling over the past 40 years are still poorly understood(1,13). Here we estimate global changes in the magnitudes of diurnal and annual temperature cycles from 1975 to 2013 from an analysis of over 1.4 billion hourly temperature measurements from 7,906 weather stations. Increases in daily temperature variation since 1975 in polar (1.4 degrees C), temperate (1.0 degrees C) and tropical (0.3 degrees C) regions parallel increases in mean temperature. Concurrently, magnitudes of annual temperature cycles decreased by 0.6 degrees C in polar regions, increased by 0.4 degrees C in temperate regions, and remained largely unchanged in tropical regions. Stronger increases in daily temperature cycling relative to changes in annual temperature cycling in temperate and polar regions mean that, with respect to diurnal and annual cycling, the world is flattening as temperate and polar regions converge on tropical temperature cycling profiles.