Human-induced climate change is increasing the frequency of heat waves and heavy rainfall 1,2,3,4,5,6,7. These impacts have begun to manifest themselves in the form of various health hazards8,9,10, wildfires11, floods12, and other types of damaging events13,14,15,16. Since the Paris Agreement, in addition to mitigation efforts that act to slow the progression of climate change, adaptation efforts that symptomatically reduce the damage caused by climate-related events have been promoted1,3,6, but the limitations of these measures are of concern17.
Since climatic conditions change to levels unfamiliar to the flora and fauna of various regions, organisms will either need to adapt to the new climate conditions or change their habitats and behaviours to survive18,19. Humans are no exception here, and people are working to transform their socioeconomic systems to adapt to the changing climate, such as introducing different varieties of crops from other regions20,21. However, human society’s ability to adapt is not always sufficient in relation to the speed of climate change22. In recent years, there has been an increase in the number of physical health problems caused by extreme weather events8,9,10,11,22.
Significant risks can arise when the magnitudes of climate change exceed the adaptive capacity of a region1,3,6,17. Previous studies have noted that an unprecedented climatic risk could have potentially serious impacts on socioeconomic systems1,3,6,17,23,24,25. These studies identified when and from which region climate departures from a range of past changes will occur23,24, and how human habitation niches will shift as a result of climate change26. For each region, the magnitude and the speed of the changing climate within the region was of the utmost interest3. However, for humanity as a whole, departure from known climatic conditions would be highly problematic. Indeed, it is the extreme weather events that have the potential to adversely affect human health8,9,10 and physical safety11,12 that deserve heightened attention.
Here, we used changes in temperature and precipitation, which represent the most fundamental, representative, and influential climatic elements, to identify climatic risk boundaries outside of which the combined risks of extremely high temperatures and intense precipitation will be greater than those of current climatic conditions and virtually no human being has ever lived with such risks before. Then, we estimated the number of people who will live in such regions in the future under different representative concentration pathways27 and socioeconomic scenarios28.
In this study, data from four general circulation models (GCMs)29,30,31,32 employed in the Coupled Model Intercomparison Project Phase 6 (CMIP6)33 were used to predict the changes in extreme temperature and precipitation, and the relationships between these data and human settlements are discussed. The combined risk of extreme temperature and precipitation in each grid within human settlements was estimated for the present climatic conditions (1980–2009) and for future climatic conditions (2070–2099) under two scenarios (RCP8.5-SSP5 and RCP2.6-SSP1, where RCP refers to the representative concentration pathway27 and SSP refers to the shared socioeconomic pathway28, respectively). The rim of the 2D histogram of the present situation was regarded as the climatic risk boundary, and the people in the regions classified as outside of the climate risk boundaries will be exposed to unprecedented climate risks.
Supplementary Figure 1 shows the changes in the magnitude of once-in-20-year (hereafter 20-year) temperature and the coefficients of variation between the models for each scenario. Under the RCP8.5-SSP5 scenario, the 20-year temperature (Supplementary Figure 1a) is expected to rise significantly over the Northern Hemisphere mid-latitudes and the Amazon region with a maximum increase of ~6 °C. In the equatorial regions of Africa and Asia, the changes will be smaller but will still amount to nearly a 2 °C rise. Similarly, the predicted increase of the 20-year temperature under RCP2.6-SSP1 scenario (Supplementary Figure 1b) will be ~3 °C at maximum in the mid latitudes of the Northern Hemisphere and ~1 °C at minimum in the low latitudes of Africa and Asia.
The rate of increase in 20-year precipitation and the coefficients of variation among the models are shown in Supplementary Figure 2. Under the RCP8.5-SSP5 scenario, the increase in the 20-year precipitation in the Sahel and southern Arabian Peninsula will be more than double than that at present. Similarly, in the RCP2.6-SSP1 scenario, the increase rate will be ~1.5-times higher in the Sahel, Arabian Peninsula, and western India than that at present.
Figure 1 and Supplementary Figure 3 show the 2D histograms of the 20-year precipitation and temperature estimated for present and future conditions under the RCP8.5-SSP5 and RCP2.6-SSP1 scenarios, respectively. The colour density corresponds to the population under the climatic risks of 20-year temperature and precipitation. The blue histogram represents the present (1980–2009), and the red histogram represents the future (2070–2099). A greater shift can be observed in the histogram from the present to the future under RCP8.5-SSP5 compared to that under RCP2.6-SSP1, and correspondingly, a larger population will transgress the climatic risk boundaries.
Figure 2 and Supplementary Figure 4 show the locations of the populations outside of the climatic risk boundaries around the world at the end of the 21st century under the RCP8.5-SSP5 and RCP2.6-SSP1 scenarios, respectively. Under the RCP8.5-SSP5 scenario, central India and portions of the Sahel region will overstep the climatic risk boundaries and be exposed to unprecedented climatic risks involving extreme temperatures and precipitation. Similarly, the Arabian Peninsula, northern India, and the Sahel region will be exposed to unprecedented extreme temperature risks. People in Southeast Asia and East Asia also will be exposed to unprecedented extreme precipitation risks. In total, there will be approximately 1.72 billion people, 23.3% of world population, living in areas that overstep the climatic risk boundaries under the RCP8.5- SSP5 scenario, whereas it will be approximately 401 million people, 5.81% of world population, under the RCP2.6-SSP1 scenario (Supplementary Table 1).
In this study, we have proposed a new concept of the climatic risk boundary34,35. This name was devised in relation to the so-called planetary boundaries, which are nine earth system limits that serve as prerequisites for sustainable development34,35. The climatic risk boundary concept is meant to supplement the idea of climatic elements within the planetary boundaries. According to the planetary boundary argument, exceeding these limits will lead to non-linear and abrupt changes within the system, with potentially catastrophic effects34,35. On the other hand, this study elucidates the data-driven boundaries of human settlements in terms of known climatic risks. Extreme weather events that occur beyond these boundaries are unknown to us and could cause direct catastrophic damage to humanity through health hazards and climatic disasters8,9,10,11,12,13,14,15. Under the RCP8.5-SSP5 scenario, more than 1.7 billion people, nearly a quarter (23.3 %) of the world population, will transgress the climatic risk boundaries and live under unprecedented climatic risks before the end of the 21st century. Mitigation measures can reduce the number of suffering people down to 401 million under RCP2.6-SSP1 and it corresponds to 5.81 % of the world population (Supplementary Table 1), however, still the significance of adaptation measures will remain.
Köppen’s climate classification36,37,38 and the Intergovernmental Panel on Climate Change’s (IPCC) SREX report3 estimates of changes in extreme weather events have used two basic climatic elements, viz. temperature and precipitation, for their assessments. This study also focused on the extremes of temperature and precipitation to delineate climatic risk boundaries. However, because we did not consider extremely low temperatures and precipitation, the climatic risk boundaries related to extremely low temperatures and/or droughts should be researched in the future39.
Human societies have been built to accommodate a variety of climatic factors, including wind speed, humidity, pressure, and sunshine hours40,41,42. Therefore, the construction of climatic risk boundaries that also consider the extremes of other climatic elements will make the potential risks of climate change even clearer. In addition, extremes of longer time scales, in particular for precipitation, should be considered for continental scale floods and droughts43.
In this study, uncertainty in climate models has been taken into account. Besides that, climatic risk boundaries may depend on the definition of the return period and ‘precedented’ risk. Therefore, climatic risk boundaries should be set with regard to the intent of the assessment. Furthermore, it should be noted that even within the climatic risk boundary proposed in this study, i.e. the domain of known weather risk, the damage may not be necessarily minor if climate change progresses rapidly and the changes are significant44,45.
We must also mention that we presumed that the known climate risks can be adapted to if the risk has been experienced by human beings somewhere in the world, without considering any barriers to the transfer of adaptation measures. However, there is no guarantee that weather risks can be effectively managed by learning best practices from the people in the regions already under higher extreme risks. Cultural, social, or economical gaps may also prevent smooth transfer of knowledge and experiences from the regions where extremely high climatic risks have been managed to the regions that will be exposed to the risks for the first time17.
For example, the existence of people living under the risk of hotter and wetter weather does not necessarily mean that people in other parts of the world will be satisfied with the way they adapt to weather risks. The acceptability of the adaptations currently being employed by people in other regions cannot escape the influence of cultural and social aspects17. Figure 3 shows climatic risk boundaries of populations in Europe, East North America, East Asia, and South Asia. The change in climatic risks for some large cities are shown as arrows. Compared to the histogram for the entire world in Figure 3, it can be seen that the majority of the population in each region will be still within the climatic risk boundaries of 20-year temperature and precipitation. Nonetheless, the histograms for the present and future are almost exclusive for each region, and results indicate that most of the population will be living outside of the current climatic risk boundaries for that particular region. For example, many large cities in Central Europe will overstep the climatic risk boundaries for people living in Central Europe and people in those cities will be exposed to unprecedented risks, even though these are known risks for humanity as a whole (Figure 3a). Similarly, people in each city on the East Coast of the United States will be exposed to unprecedented climatic risks for that region, but some people in the world may have already been exposed to the similar risks (Figure 3b). On the other hand, the results show that some cities in East Asia and South Asia will be exposed to truly unprecedented extreme climatic risks (Figures 3c and 3d).
These figures allow us to judge whether the future extreme climatic risks are unprecedented. If the risks are known, we can ascertain which cities are currently experiencing the climatic risk and learn appropriate adaptation measures. More reasonable assessments of the risks will also be enabled by considering the transferability of knowledge and technologies in relation to social and economic disparities17. These findings highlight the importance of smooth exchanges of knowledge and technologies through global partnerships based on multilateralism for implementing applicable adaptation measures to climate change.