Fig. 1. Climate risk is defined by three elements: vulnerability,
exposure, and hazard. While the level of certainty is high and the
significance of evidence is well established regarding disparities
between the global north and the global south in vulnerability and
exposure, certainty is low and evidence is not well established when it
comes to climate hazard. Examples of disparities in climate hazard are
few, and the contrast between the global north and south shown by these
examples is largely not sharp.
The geographically different impacts of climate change are not only
limited to a specific sector but emerge in various fields. Some examples
of disparities caused by climate change include differences in its
impact on economic production (Burke et al. 2015; Callahan and Mankin
2022; Diffenbaugh and Burke 2019; King and Harrington 2018), disparities
in how people allocate their time between work and leisure (Zivin and
Neidell 2014), effects on poverty, urbanization, and migration
(Burzyński et al. 2022), disparities in gender-based health (Sorensen et
al. 2018), and contrasting consequences on income in urban and rural
regions within countries (Paglialunga et al. 2022).
Compensating for disproportionate loss and damage imposed by climate
change has been an integral, however, one of the most debatable and
complex subjects in the international forums (e.g., the
27th Conference of Parties (COP27), held in Egypt in
November 2022) that discuss climate change policies (Dorkenoo et al.
2022) (see text in the online supplementary material regarding the
concept of loss and damage imposed by climate change). This topic is
especially relevant considering the debate about the distribution of
responsibility for causing climate change and who should bear the costs
(Farber 2007), which are difficult to quantify (Rising et al. 2022), but
certainly high (Dietz et al. 2018). Industrial and relatively rich
countries (thereafter, the global north) have historically been the main
emitters of greenhouse gases (GHGs) (Althor et al. 2016; Wei et al.
2016). Being legally liable for GHG emissions has been the main concern
of the global north around the issue of responsibilities and
compensation over climate change. However, the relatively poor
developing countries (thereafter, the global south), which have
contributed less to the problem, are disproportionately impacted by
climate change (IPCC 2022).
In the current research, we emphasize that discussions over differential
climate risk and compensation for the associated loss and damage have
widely been tackled mainly from the lens of disparities between the
global north and global south in terms of vulnerability and exposure.
This is mainly because vulnerability and exposure display clear
differences between these two groups of countries and the evidence for
these disparities are highly significant, certain, and well-established
(Fig. 1). But when it comes to climate hazard - the third component that
defines climate risk - there are only a few significant evidence of
sharp disparities between the global north and the global south.
Additionally, evidence for such a differential pattern are either based
on variables with relatively less significance to society or the
contrast between the global north and south is not sharp. This area of
research regarding north-south disparities in climate hazard has
received relatively little attention (IPCC 2014) albeit its centrality
in shaping climate change impact and defining losses, damages, and
responsibilities.
2. Data and Methods
a. Observations and CMIP data
The observed 3-hourly temperature, dew point temperature and surface
pressure data with a spatial resolution of 0.25° covering the 1959-2021
period were from the ERA5 reanalysis (Hersbach et al. 2020) (available
at http://apps.ecmwf.int/datasets/). We used the daily output of
temperature over the 1976–2100 period from 31 CMIP5 GCMs forced by
historical forcing until 2005 and Representative Concentration Pathway
8.5 (RCP8.5) scenario thereafter (Taylor et al. 2012) and 29 CMIP6 GCMs
forced by historical forcing up to 2014 and Shared Socioeconomic Pathway
5-8.5 (SSP5–8.5) scenario thereafter (Eyring et al. 2016). RCP8.5
represents a business-as-usual scenario with radiative forcing, reaching
about 8.5 Wm-2 in 2100. SSP5-8.5 is approximately
equivalent to RCP8.5 (Russo et al. 2019). For each model, one ensemble
member was used. The list of global climate models and their details
were provided in Table S1. All model outputs from CMIP6 (respectively
from CMIP5) were re-gridded to a common 1° × 1° grid (respectively
1.5° × 1.5° grid). The anomalies in the model outputs are obtained for
the period 1976-2005. The gridded population density was from the
Gridded Population of the World (CIESIN 2018). The gridded global GDP
was provided by Kummu et al. (2018).
b. Derivation of wet-bulb temperature
Wet-bulb temperature (TW) was computed adopting a method developed by
Davies-Jones (Davies-Jones 2008) using surface temperature, humidity,
and pressure, derived from the 3-hourly CMIP outputs and the ERA5
reanalysis.
c. Outdoor days
Our earlier study introduced the novel concept of “outdoor days”
defined as the number of days with moderate temperature, neither too
cold nor too hot, that allows most people to enjoy outdoor activities
(Choi et al. 2023). Here, we adopt the same definition, following Choi
et al. (2023), to estimate “outdoor days” based on dry-bulb and
wet-bulb temperature. That is, the range of dry-bulb temperature from 10
℃ to 25 ℃ is assumed to be suitable for outdoor activity (This range of
temperature is comparable to a wet‐bulb temperature ranging from 8 ℃ to
15 ℃). We assume this definition is valid for all locations on Earth.
However, the exact range of temperature defining an outdoor day may, in
general, vary slightly depending on geographical location, tolerance
levels of the local population, and what exact fraction of the
population is meant when we say “most people”. Our definition of an
outdoor day is discussed further below, where it is demonstrated that
our results are not sensitive to the exact range of temperature we
assume in this analysis. To explore this sensitivity, please visit
https://eltahir.mit.edu/globaloutdoordays/.
Most existing studies that investigated concepts similar to outdoor days
presented herein are limited to local and regional scales (Choi et al.
2023; Gao et al. 2018; Hanlon et al. 2021; Heng and Chow 2019; Lin et
al. 2019; Spagnolo and de Dear 2003; Wu et al. 2017; Zhang et al. 2022;
Zhang 2016). Previous studies referred to pleasant weather conditions as
mild weather (Lin et al. 2019; van der Wiel et al. 2017; Zhang et al.
2022), mild days (Day et al. 2021), good weather (Zhang 2016), outdoor
thermal comfort (Heng and Chow 2019), thermal comfort condition (Gao et
al. 2018), comfortable days (Wu et al. 2017), and thermal comfort
(Spagnolo and de Dear 2003).
Climate variables used in previous studies to analyze changes in the
characteristics of mild weather include temperature (Choi et al. 2023;
Hanlon et al. 2021; Zhang et al. 2022), dewpoint temperature (van der
Wiel et al. 2017), wet-bulb temperature (Choi et al. 2023),
precipitation (van der Wiel et al. 2017; Zhang 2016), relative humidity,
wind speed, sunshine duration (Lin et al. 2019; Zhang et al. 2022),
shortwave radiation, diffuse shortwave radiation, longwave radiation,
and velocity (Spagnolo and de Dear 2003). Temperature is the primary and
common variable used in most, if not all, of the previous studies that
investigated the impact of climate change on concepts similar to outdoor
days. However, the optimum ranges used for the daily maximum temperature
to define mild weather vary considerably. The optimum daily maximum
temperature considered in these studies ranges between 18 ℃ and 31.6 ℃.
A summary of these literatures can be found in the online supplementary
material (Table S2).
The studies of van der Wiel et al. (2017) and Zhang et al. (2023) are
the only ones that investigated mild weather conditions on a global
scale. These studies show a clear contrast in the change of mild weather
between the global north and the global south. However, both studies
considered applying only one or a few GCMs, limiting the reliability of
future projections. Additionally, disparities between the global north
and south were not the central focus of van der Wiel et al. (2017). To
the best of our knowledge, our study is the first to provide more
evidence of its scale in terms of data and models on the north-south
disparities in outdoor days, improving our understanding of how
disparities in climate hazard shape the contrasting risk of climate
change on the global scale.
3. Results
In the current climate, outdoor days occur frequently in most regions of
the world (Fig. 2). On average, across the land areas of the world,
approximately 91 outdoor days are experienced every year (i.e., about
25% of the days of a year), although seasonal and/or regional
differences are evident. If we restrict ourselves to residential areas
(i.e., areas with a population density above 1 person per square
kilometer), more outdoor days are found (165 days per year; i.e., about
45% of the days of a year). Particularly, the global south stands out
due to more frequent outdoor days, compared to the global north. On a
regional scale, high-latitude countries, such as Canada and Russia, are
too cold (Fig. S2a), resulting in fewer outdoor days, especially during
cold months, whereas countries such as Angola and regions like the
southern parts of Brazil show frequent outdoor days regardless of season
(Fig. 2).