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 (1114). 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 (1719). 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, 2123), and local-scale studies (16,2426). 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, 3444). 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.