Discussion and Conclusions
We reviewed several studies that focused on mild weather conditions using various indices such as thermal comfort (25), good weather (26), comfortable days (23), thermal comfort condition (20, 21), outdoor thermal comfort (24), mild weather (1719), and mild days (16). However, these studies are geographically limited to a few regions, such as Sydney (25), Washington D.C and New York (26), Indiana (16), China (17, 19, 21, 23), and Singapore urban park (24). In addition, there is a limited understanding of how pleasant weather conditions could respond to anthropogenic climate change. Although a few studies projected a change in pleasant weather conditions, they applied one or few GCMs, and thus could not address the uncertainty of future projections (18,19, 21). To the best of our knowledge, our study is the first to demonstrate the climate change impact on the climate of outdoor days in the US with significant implications for future quality of life in different climate regions, and for the distribution of the economic potential of travel and tourism. In addition, the versatility of data (i.e., ERA5 reanalysis, 31 CMIP5 models, 10 CMIP6 models, and 32 NEX-GDDP-CMIP6 models; Table S1), spatial scale, and various ways to define “outdoor day” (see below for details) are unprecedented in previous studies.
Unlike the nearly uniform warming across seasons and the US, recent accelerated climate change has significantly shifted the seasonality of outdoor days with significant reductions of outdoor days during warmer months and a relatively moderate increase in outdoor days during colder months across all sub-regions. In particular, anthropogenic climate change negatively affected the Southeast, South, and Ohio Valley, featuring a decrease in outdoor days. A comparable climate change might benefit the Northwest by gaining more outdoor days, resulting in a significant northwest-southeast disparity and implying disproportionate risks of climate change. Toward the end of the twenty-first century, high emissions scenarios from CMIP5, CMIP6, and NEX-GDDP-CMIP6 models consistently point to significant, robust, and disproportionate risks of climate change in outdoor days. Where we identified the largest reduction in outdoor days such as in the Southeast, South, and Ohio Valley regions, Howe et al. (28) and Marlon et al. (47) point out that people largely do not hear about global warming in the media or discuss it at least occasionally compared to other regions in the US.
Despite the large consistency in the patterns of change in outdoor days, as revealed by the results from various models (Table S1) used in the current study, our findings based on raw GCMs from the CMIP5/CMIP6 archive must be interpreted with some caution. First, the systematic biases in temperature projections, especially in GCMs from the CMIP5 archive, could possibly affect the future projections of outdoor days (Figs. S12-S13). Although the general characteristics of outdoor days are well captured by CMIP models for the reference period, biases in GCM outputs can bleed into the conclusions. To address this issue, the bias-corrected NASA NEX-GDDP-CMIP6 dataset was applied.
Second, there is no universal definition of ‘outdoor days’ as defining them is necessarily somewhat subjective (18). To define mild weather similar to outdoor days presented herein, previous studies have considered various climate variables, such as temperature (19,22), dew point temperature (18), precipitation (18,26), relative humidity, wind speed, sunshine duration (17,19), shortwave radiation, diffuse shortwave radiation, and longwave radiation (25). Nevertheless, there is a consensus in the literature that temperature is the primary variable (but not always), although the threshold to define mild weather varies considerably.
In our study, we primarily defined an outdoor day as a day with a temperature ranging from 10 to 25 ℃. However, we find that our analysis and conclusions are robust and not sensitive to the choices of threshold or the specific variable considered, such as daytime temperature, daily mean dry-bulb temperature, wet-bulb temperature, and precipitation (Fig. 5). Rather than adhering strictly to a single definition, our study suggests a more flexible definition of an outdoor day (Fig. 5). To facilitate this, we have developed an online interactive tool available at https://eltahir.mit.edu/globaloutdoordays/. This tool allows users to explore different definitions of an outdoor day, which also allows comparison between various definitions of outdoor days. This operational and flexible definition takes individual differences into account when defining an outdoor day, providing a more tailored understanding of outdoor day patterns.
Unlike previous studies, wet-bulb temperature is used in “outdoor days” definition. Considering relative humidity, which is embedded in the wet-bulb temperature, highlights the important role of humidity in shaping the human’s ability, in physiological terms, to resist hot weather. Similar to the results using dry-bulb temperature, a significant northwest-southeast disparity is found in the projected change of annual outdoor days defined using wet-bulb temperature, but its magnitude is smaller (Fig. S14). For instance, annual outdoor days are likely to decrease by about 10% at the Charlotte city in Southeast (Fig. S15).
The findings reported here have important implications for understanding the regional climate response in the US to rising greenhouse-gas concentrations. We project that some of the US regions where the populations are known to be more skeptical about the anthropogenic cause of climate change, and those with attractive tourism and recreation spots for outdoor activities, are likely to face negative impacts of climate change, in the form of reduced outdoor days. Life, travel, and tourism in Florida will be very different if and when the number of outdoor days is cut as projected in this study. Hence, our findings not only identify specific regional hotspots and better inform communities of future climate change, but can also be interpreted in terms of future quality of life for those communities, and in terms of the future economic potential of travel and tourism in different climate regions of the US.
Integrating reliable information on potential climate change impacts from various perspectives, including those related to pleasant weather conditions (expressed here in terms of outdoor days), will help, from one side, decision-makers to develop effective adaptation strategies (e.g., adjustments in tourism and outdoor activity seasons), and on the other side, consolidate more cogent evidence to gather public support to climate mitigation policies.