Extreme Heat Stress trends in ERA Interim 1979-2011

Heat stress is a function of temperature and humidity, and is therefore subject to the covariance of the two quantities. One of the robust predictions from climate change is an increase in temperatures across the planet, and therefore heat stress is projected to increase, however the covariance with humidity is less sure. It has been proposed that in future climate, significant portions of the land surface become subject to life threatening heat stress levels to humans and mammals. There are numerous methods and metrics for calculating heat stress, however, the majority use atmospheric state variables (pressure, temperature, and specific humidity), to measure the thermodynamic state of the atmosphere or estimate thermal load on humans and mammals. Here we present calculations of the evolution of heat stress for the past 3 decades using the European Centre for Medium-Range Weather Forecasts (ECMWF) ERA Interim reanalysis data product from near surface boundary layer state variables. We characterize both spatial and temporal trends with a variety of the most commonly used heat stress metrics (wet bulb temperatures, heat index, etc.). The metrics are calculated from 4x daily values to capture both the diurnal cycle and the daily peak values for these indices.