Materials and Methods
Observations and CMIP data
Temperature, dew point temperature, and surface pressure data at a spatial resolution of 0.25 and a three-hourly temporal resolution were taken from the ERA5 reanalysis (48). Daily temperature projections over the 1976–2100 were obtained from 31 CMIP5 models and 32 bias-corrected NASA Earth Exchange (NEX) Global Daily Downscaled Projections (NEX-GDDP-CMIP6 models at a spatial resolution of 0.25). The CMIP5 models are driven by historical forcing and Representative Concentration Pathway 8.5 (RCP8.5) scenario (49) and the CMIP6 and NEX-GDDP-CMIP6 models are driven by historical forcing, Shared Socioeconomic Pathway 1-2.6 (SSP1-2.6), and SSP5-8.5 scenarios (50). The radiative forcing in RCP8.5 (referred to as a ‘business as usual scenario’) rises continuously to reach 8.5 Wm-2 by the year 2100. RCP8.5 is approximately equivalent to SSP5-8.5 (51). As an optimistic climate change scenario, SSP1-2.6 was applied (51). Only one ensemble member was used for each model run. The list of GCMs used in this study was provided in Table S1 with more details. Wet-bulb temperature (TW) was computed using 3-hourly surface temperature, humidity, and pressure derived from the CMIP outputs and the ERA5 reanalysis based on the Davies-Jones method (52). Outputs from CMIP5 and CMIP6 models were re-gridded to 1.5° × 1.5° grid and 1° × 1° grid, respectively.
Definition of ‘Outdoor days’
The concept of an “outdoor day” in this study was defined as a day with thermal comfort conditions enabling most people to do outdoor activities. Specifically, it is determined based on the daily dry-bulb temperature falling in the range of 10 to 25 ℃, or alternatively, the daily wet-bulb temperature lying within the range of 8 to 15 ℃ (Fig. S16). While our study primarily reported results on the range of dry-bulb temperature from 10 ℃ to 25 ℃ as the one possible defining criterion for outdoor days, we introduced a more flexible definition compared to previous works (Fig. 5; refer to https://eltahir.mit.edu/globaloutdoordays/). This flexibility in defining outdoor days is a novel contribution, making the concept more advanced and applicable in the field for investigating pleasant weather conditions. By considering dry-bulb and wet-bulb temperature ranges in addition to precipitation and providing a more adaptable definition, this study enhances the understanding and assessment of outdoor days in relation to thermal comfort, allowing for a more comprehensive evaluation of weather conditions suitable for outdoor activities.
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