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 (17–19), 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.