By Revveka Trigka
A JRC-led study analysing the effects of humidity in amplifying the impact of extreme heat waves, both in the past and at different global warming levels, was published on 7 August 2017 in Scientific Reports, a Nature Research scientific journal. The study, described briefly below, indicates the regions of the world where new adaptation measures might be needed to cope with heat stress due to humid heat waves, and highlights the urgent need for international climate change mitigation efforts.
Heat waves are prolonged periods of excessively hot weather, relative to the normal climate pattern for an area. The maximum magnitude of heat waves in a year at a given location has previously been measured using the “Heat Wave Magnitude Index - Daily” or HWMId (developed by the lead author in an earlier study), which takes account of the severity of temperature extremes and the duration of a heat wave. In this new study, an improved indicator - the Apparent Heat Wave Index or AHWI - is introduced, which takes account of the effects of relative humidity during consecutive hot days. The AHWI is computed similarly to the HWMId, except that instead of air temperature it considers “apparent temperature”, which in hot weather is due to a combination of air temperature and humidity, and is called the Heat Index. Heat Index values can be computed using a mathematical equation, or read from Heat Index charts (which also provide colour-coded human health-warnings).
In the JRC study, global HWMId and AHWI values for 1979-2015 were calculated and compared, using as input gridded datasets from two global meteorological “reanalysis” products: (a) ERA-Interim (ECMWF Reanalysis), and (b) NCEP-DOE Reanalysis 2 (National Centers for Environmental Prediction - Department of Energy). In addition, multi-model ensemble outputs from the Coupled Model Intercomparison Project Phase 5 (CMIP5) were used to project future HWMId and AHWI values, under IPCC Representative Concentration Pathways (RCPs) corresponding to three global warming levels (1.5°C, 2°C and 4°C).
The JRC study found, for example, that the severity of major past heat waves, such as in Chicago in 1995 and Shangai in 2003, was strongly amplified by humidity. It also showed that the percentage of area where heat waves are amplifed by humidity, can be expected to increase with global warming. When humidity is considered, at 1.5 °C and 2 °C global warming, highly populated regions such as Eastern US and China are projected to experience the most severe heatwaves ever recorded, with Heat Index values over 55 °C, corresponding to the highest human health danger-levels (e.g. indicated in Heat Index charts as “heatstroke imminent”). The study also found that such extreme humid heat waves, which have never occurred so far, can be expected to happen at least once every two years at 4°C global warming.