Introduction
Climate change is a threat multiplier that has significant impacts on
all systems and sectors in the world. Future climate projections suggest
that the intensity and frequency of climate extremes will increase
disproportionately around the globe (1, 2). For instance,
global warming at 1.5 ℃ and 2 ℃ above pre-industrial levels is expected
to result in spatially non-uniform increases in climate extremes, such
as hot days, heavy rainfalls, and high streamflow (3). Some of
these impacts tend to expose disparities in vulnerability between
developing and developed countries (2, 4, 5),
increasing inequality between countries (6), which is among the
most serious implications of climate change.
Disparity imposed by climate change is manifested not only at a global
level but also at local and regional levels. For example, the United
States (US), characterized by various climate regimes (7) (Fig.
S1), a wide range of physical and ecological systems (8), and
large diversity in socio-economic conditions (9, 10), is
facing different types of climate risks across the regions
(11–14). The disproportionate risks of climate change
can, for instance, differentially affect the states’ economies. Tourism,
which is one of the largest and fast-flourishing economic industries in
the US, is projected to be adversely affected by climate change,
especially in prominent metropolitan areas and protected areas of the
country (15), triggering shifts in tourist preferences and
recreation destinations (16). It is important to note that this
industry is generally susceptible to the number of days with moderate
and pleasant temperatures (with no precipitation) in a given region.
We define outdoor days as those days with pleasant weather allowing for
outdoor activities (see Materials and Methods section). We assumed that
an outdoor day can be defined as a day with an average temperature
within the range of 10 to 25 ℃. As shown and discussed later, including
in the Supplementary Materials, our conclusions are not sensitive to
this assumption. Although we mainly utilized the range of dry-bulb
temperature from 10 to 25 ℃ to define an outdoor day, an online
interactive tool is provided at
https://eltahir.mit.edu/globaloutdoordays/ to enable the user to invent
their own definition of an outdoor day using dry-bulb temperature,
wet-bulb temperature, and precipitation, which also allows comparison
between various definitions of outdoor days.
The concept of outdoor days is of significant importance to quality of
life for communities since a pleasant day occurs more frequently
compared to days with extreme temperature (17–19).
Somewhat similar concepts to outdoor days have been proposed previously
at multiple spatial and temporal scales. Examples include global-scale
studies (18, 20), national-scale studies (17,19, 21–23), and local-scale studies (16,24–26). However, previous studies for the US are limited
in their number and largely confined to local scales. A few studies
considered how climate change is likely to increase the number of hot
days and reduce the number of days with mild weather (16).
To mitigate climate change, the participation of the US in global
efforts is crucial since the US is a significant emitter of
anthropogenic greenhouse gases - the main drivers of global warming -
and consequently in part responsible for climate change (27). The
success of these mitigation policies depends largely on public opinion,
support, and behavior (28, 29). However, there is a large
portion of the US population who considers climate change as a hoax
(30). Recent surveys found that more than half of Americans do
not perceive climate change as a threat in their lifetime, with a great
variation based on political views (31), indicating a largely
politicized issue. The divide between these two groups may stem from a
lack of awareness regarding the impacts of climate change. For instance,
a study by Maibach et al. (32) reported that around 61% of the
US population lacks adequate knowledge about the impacts of climate
change on human health. Differences in Americans’ perspectives on
climate change are leading to conflicting climate policies (28).
The withdrawal of the US from the Paris Agreement in 2017 was partially
a manifestation of these debates and has its roots in these different
political perspectives.
Achieving global climate change mitigation requires a substantial
increase in public awareness and subsequent public support for climate
change policies (33). A wealth of theoretical, empirical, and
experimental evidence of climate change processes from various
perspectives could improve public understanding of the science of
climate change. To date, studies on the impacts of climate change mostly
focus on mean climate conditions and/or frequency and magnitude of
climate extremes (7, 34–44). However, we propose a
novel perspective that highlights the potential impact of climate change
on the patterns of outdoor days. This approach has the capacity to bring
the science of climate change closer to human societies, resulting in
broader implications for public awareness and engagement.
Temperature rise associated with climate change might trigger
irreversible changes in the spatial and temporal patterns of outdoor
days. Studying the change in patterns of outdoor days – a highly
relevant variable to people’s everyday activities – would provide more
convincing evidence as to why we should address climate change. The
spatial-temporal disparity in the intensity of these changes and their
impacts across the US are still understudied. Therefore, the aim of the
current research is twofold: (i) investigate, based on observations, the
spatiotemporal variation of outdoor days across the US and their
historical trends, and (ii) project future regional climate change in
terms of outdoor days in the US based on multiple climate models.