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Broad threat to humanity from cumulative climate hazards intensified by greenhouse gas emissions

The ongoing emission of greenhouse gases (GHGs) is triggering changes in many climate hazards that can impact humanity. We found traceable evidence for 467 pathways by which human health, water, food, economy, infrastructure and security have been recently impacted by climate hazards such as warming, heatwaves, precipitation, drought, floods, fires, storms, sea-level rise and changes in natural land cover and ocean chemistry. By 2100, the world’s population will be exposed concurrently to the equivalent of the largest magnitude in one of these hazards if emmisions are aggressively reduced, or three if they are not, with some tropical coastal areas facing up to six simultaneous hazards. These findings highlight the fact that GHG emissions pose a broad threat to humanity by intensifying multiple hazards to which humanity is vulnerable. This Review examines the pathways through which humans are impacted by climate change and shows that by 2100 the world’s population will be simultaneously exposed to at least three hazards, and in some locations as many as six, under an RCP 8.5 scenario.

C ontinuous emissions of GHGs are simultaneously shifting many elements of Earth's climate beyond thresholds that can impact humanity 1 . By affecting the balance between incoming solar radiation and outgoing infrared radiation, manmade GHGs are increasing the Earth's energy budget, ultimately leading to warming 1 . Given interconnected physics, warming can affect other aspects of the Earth's climate system 2 . For instance, by enhancing water evaporation and increasing the air's capacity to hold moisture, warming can lead to drought in places that are commonly dry, in turn ripening conditions for wildfires and heatwaves when heat transfer from water evaporation ceases. There are opposite responses in places that are usually humid where constant evaporation leads to more precipitation, which is commonly followed by floods due to soil saturation. The oceans have the added effect of warming waters, which enhance evaporation and wind speeds, intensifying downpours and the strength of storms; storm surges can be aggravated by sea-level rise resulting from the larger volume occupied by warmed water molecules and melting land ice. Other interrelated changes in the ocean include acidification as CO 2 mixes with water to form carbonic acid, and reduced oxygen due to (1) reduced oxygen solubility at higher temperatures and (2) changes in ocean circulation that affect the mixing of surface waters rich in oxygen with deeper oxygen-poor water. These climate hazards and their impacts on human societies occur naturally but are being non-trivially intensified by man-made GHG emissions, as demonstrated by active research on detection and attribution (discussed under Caveats in the Methods). With few exceptions 3 , changes in these hazards have been studied in isolation, whereas impact assessments have commonly focused on specific aspects of human life. Unfortunately, the failure to integrate available information most probably underestimates the impacts of climate change because: (1) one hazard may be important in one place but not another, (2) strong CO 2 reductions may curb some, but not all, hazards (see Supplementary Fig. 1), and (3) not all aspects of human systems are equally challenged by climate hazards. A narrow focus on one or a few hazards may therefore mask the changes and impacts of other hazards, giving an incomplete or misleading assessment of the consequences of climate change 3 .
Here we highlight the broad and heightened threat to humanity from ongoing GHG emissions intensifying multiple climate hazards to which humanity is currently vulnerable. To build our case, we carried out a systematic literature search to identify observed impacts on people from climate hazards and developed a global map of a cumulative index of projected changes in these hazards to evaluate the extent that humanity will be exposed to different concurrent hazards. Integration of these two components revealed that humanity has already been impacted by climate hazards that are projected to intensify even under a best-case scenario. Furthermore, we showed that projected exposure to multiple climate hazards will be very similar between rich and poor countries, but variations in adaptive capacity will probably result in different types of impacts (for example, higher economic loss for developed nations and higher loss of life for developing countries). Our conclusions are not without limitations and we include a description of likely problems from biases in the literature, attribution uncertainty and multimodel uncertainty (see further discussion under Caveats in the Methods). We also provide definitions for certain terms as used here (that is, hazard, exposure, impact, sensitivity, vulnerability and adaptation; see Supplementary Note 1).

Observed impacts on human systems
A systematic review of observed impacts was conducted by creating a table in which ten climate hazards (warming, precipitation, floods, drought, heatwaves, fires, sea level, storms, changes in natural land cover and ocean chemistry) were listed in columns and six aspects of human systems (health, food, water, infrastructure, economy and security) were listed in rows (see Methods). This table was used as a guide for all possible combinations of keywords to search for publications reporting the impacts of climate hazards on key aspects of human life. From over 12,000 references assessed, we identified 3,280 relevant papers that were read in full to find case examples of climate hazards impacting human systems. Our criteria for the selection of impacts required that impacts be observed and supported with traceable evidence (that is, there was a reference to a place and time that could be traced to where and when a given impact occurred). Impacts were subcategorized within each of the six primary aspects of human life to reflect the variety of documented impacts (for example, death, disease within human health; see Fig. 1 and Methods). In total, we found case examples for 89 attributes of human health, food, water, infrastructure, economy and security impacted by the ten climate hazards. Of 890 possible combinations (10 climate hazards × 89 attributes of human life), we found case examples for 467 interactions or pathways by which humanity has been impacted by climate hazards. For brevity, pathways are described and supported with at least one case example; however, very commonly we found numerous similar case examples of impacts, which are listed with their associated paper in a publicly available online database (http://impactsofclimatechange.info). This list is intended to document the vulnerability of human systems to changes in climate hazards.
Health impacts. We found 27 attributes of human health impacted by climate hazards (Fig. 1), of which death, disease and mental health were the most commonly observed. Death was associated with multiple damaging physiological pathways from hyperthermia 4 during heatwaves (for example, from 1980 to 2014, over 780 events of excess human mortality were reported during heatwaves worldwide 5 ), drowning during floods (approximately 3,000 deaths in the 1998 floods in China 6 ), starvation during droughts (approximately 800,000 famine deaths attributed to the Ethiopian drought in the 1980s 7 ), blunt injury during storms (roughly 140,000 deaths occurred in the 1991 Cyclone Gorky in Bangladesh 8 ) and asphyxiation during fires (approximately 173 deaths occurred in the 2009 Australian Black Saturday fire 9 ). The loss of natural land cover impaired coastal protection, probably contributing to increased mortality during storms and floods 10,11 . Warming and changes in precipitation and ocean chemistry caused human death through increased transmission of pathogenic diseases.
Climate hazards were related to numerous conditions that disrupt body function. Increased morbidity (such as cardiac and respiratory disorders) due to heat illness occurred during heatwaves 12 , whereas injuries were common during floods, storms and fires. Respiratory problems were associated with increased ozone pollution from heatwaves and fires 13 , dust from droughts 14 , mould following storms 15 , organic pollutants released from melting ice 16 and pollen released during extended flowering periods caused by warming 17 . By increasing the habitat suitability of pathogens and vectors, warming and precipitation changes contributed to epidemics of malaria 18 , diarrhea 19 , dengue fever 20 , salmonellosis 21 , cholera 21 , leptospirosis 1 , bluetongue disease 1 and campylobacteriosis 22 . Similarly, warming facilitated the range expansion of vectors implicated in outbreaks of plague transmitted by rodents 23 , West Nile virus by birds 24 , schistosomiasis by snails 18 and encephalitis by ticks 25 . Outbreaks also resulted from climate hazards increasing the proximity of vectors to people. For instance, forest fragmentation increased the density of ticks near people, triggering outbreaks of Lyme disease 26 and encephalitis 27 , fires drove fruit bats closer to towns, causing outbreaks of the Hendra and Nipah viruses 28 , drought mobilized livestock near cities, causing outbreaks of haemorrhagic fever 27 , and melting ice due to warming caused voles to find shelter in homes, increasing hantavirus infections 29 . Likewise, floods 30 , heatwaves 31 and intense rain 31 have been related to increases in snake bites due to inhospitable conditions forcing animals to move closer to people. Poor sanitation and contamination of the water supply due to storms and floods resulted in outbreaks of cholera, malaria, leptospirosis 32 and diarrhoeal illness 21 . Changes in ocean chemistry have favoured pathogen growth and harmful algal blooms related to seafood poisoning 21 , cholera 33 and ciguatera 34,35 . Drought was associated with outbreaks of West Nile virus 24 , leishmaniasis 36 and chikungunya virus 37 , and hantavirus when interacting with floods 36 . Drought forced the use of unsafe drinking water, resulting in outbreaks of diarrhoea, cholera and dysentery 38 . By increasing the concentration of particulates during dust storms, drought was also linked to valley fever, a disease caused by a fungal pathogen 39 .
Climate hazards affected mental health. For instance, depression and post-traumatic stress disorder were reported after storms in the United States 40 , floods in the United Kingdom 41 and heatwaves in France 39 . People experienced existential distress during drought in Australia 42 , increased substance abuse after storms in the USA 43 and poor mental health due to climate change in Canada (for example, the loss of sea ice has inhibited cultural practices such as hunting and fishing, leading to depression among Inuit people 44 ). Furthermore, suicidal ideation occurred in victims of drought 45 , heatwaves 46 , storms 40 and floods 47 .
Climate hazards were implicated in pre-and post-natal health problems. Children born to pregnant women exposed to floods exhibited increased bedwetting, aggression towards other children 48 and below-average birth weight, juvenile height and academic performance 49 . Similarly, exposure to smoke from fires during critical stages of pregnancy may have affected brain development and resulted in preterm delivery, small head circumference, low birth weight and fetal death or reduced survival 50 . Finally, salinity in drinking water caused by saltwater intrusion and aggravated by sealevel rise was linked to gestational hypertension, which created serious health issues for both the mother and fetus 51 .
Food impacts. We found ten attributes of food systems that were impacted by climate hazards, of which impacts on the quantity and quality of food from agriculture, livestock and fisheries were most commonly noted (Fig. 1). Agricultural yields were impacted by direct physical loss and indirectly by exceeding physiological thresholds of crop plants. Direct physical losses occurred due to storms (for example, roughly 35% of bean production was lost to Hurricane Mitch in Honduras in 1998 52 ), precipitation (a 10 mm increase in rainfall caused a loss of 0.3 t ha −1 of paddy rice in the Mekong Delta 53 ), floods (over 7,600 ha of agricultural land was destroyed by floods in Vietnam in 2009 54 ), sea-level rise (agricultural land has been lost to saltwater intrusion in Bangladesh 1 ), fires and drought (approximately 33% of grain production was lost to a mixture of fires and drought in Russia in 2010 55 ). Indirect losses due to hazards exceeding crop physiological tolerances were caused by warming (for example, a 3-10% loss of wheat yield per 1 °C increase in China 56 ), drought (a yield decrease of roughly 36% during the 2003 drought in Italy 57 ), heatwaves (one single day above 38 °C reduced annual yields by 5% in the United States 58 ), changes in ocean chemistry (e.g., drought in Australia caused by variability in ocean temperature in the Indian Ocean 59 ), and natural land cover change (e.g., crop yields around the world have been reduced by natural land cover change increasing evaporation and reducing soil moisture 60 ). Climate hazards also impacted the quality of crops by altering nutrient content and increasing the risk of contamination. For instance, protein content in some grains declined due to drought 61 and heatwaves 61 , whereas floods 62 and permafrost thawing due to warming 39 resulted in soil contamination and food spoilage rendering plant material unfit for consumption. Finally, changes in precipitation and drought were linked to crop infections by moulds harmful to people 62 .
Climate hazards have impacted animals used for food. Livestock mortality was associated with warming (for example, the livestock disease bluetongue was positively correlated with increasing temperatures in Europe 63 ), drought (in 2000, three quarters of livestock died due to drought in Kenya 64 ), heatwaves (> 5,000 cattle deaths occurred each year there were strong heatwaves in the US Great Plains 65 ), floods (livestock losses totalled > 236,000 during major floods in Bangladesh in 1987 and 1988 66 ) and natural land-cover change (in Sudan, for example, land-cover change reduced suitable grazing land 67 ). Heatwaves were related to a reduction in grazing, reproduction and milk production in cattle and high mortalities in chickens and turkeys 68 . There were also impacts on hunting, such as warming and melting sea ice in the Arctic shifting the distribution of walrus, leading to the loss of subsistence hunting grounds 69 . Meat quality was also impacted through contamination (higher than normal temperatures were associated with 30% of reported cases of salmonellosis in Europe 63 ).
Climate hazards were found to impact fisheries through reductions in the quantity and quality of fish populations. There were reductions in fish stocks due to warming both directly (warmer temperatures exceeded the thermal-tolerance of cod 70 and high water temperatures reduced oxygen content, severely impacting salmonid reproduction 71 ) and indirectly (warmer temperatures altered food webs by reducing primary productivity 70 ). Direct stock mortality and changes to reproduction were caused by drought (by favouring bivalve predators that decreased shellfish populations 72 ), heatwaves (a heatwave in 1953 warmed Lake Erie, triggering nutrient pollution that caused a large fish kill 73 ) and floods (floods decreased the reproductive capacity of anadromous fish 74 ). Climate hazards also impacted the habitats of stocks, including fires (runoff due to fires increased the heavy metal content in lakes and rivers 75 ), precipitation (rains increased sediment and nutrient loading in lagoons 76 ), sea level (sea-level rise changed the dynamics of coastal lagoons 76 ), ocean chemistry (changes in ocean chemistry increased coral bleaching, which decreased fish habitat 77 ) and natural land cover (introduced water hyacinth in Lake Victoria reduced fish quantity 78 ). The quality of fish was also impacted. Warming increased mercury methylation and has favoured the growth of pathogens involved in food poisoning 79 . Floods, storms and fires were also related to increased heavy metal runoff, causing fish to accumulate mercury and increasing the risk of mercury poisoning in humans 75 .
Water impacts. We found that the quantity and quality of fresh water were critically impacted by climate hazards (Fig. 1). Drought, warming and heatwaves caused wells to run dry and reduced water levels in reservoirs, forcing water shortages and mandatory water restrictions 38,39,80 . Drought, for instance, led to temporary drinking water shortages for over 200,000 people in Puerto Rico in 1997-1998 39 and 33 million people in China in 2001 80 . Decreases in water supply were also attributed to landcover change, including the spread of invasive plant species such as Tamarix spp., which increased evapotranspiration, costing US$65-180 million per year in reduced water supplies 81 , and desertification, which led to losses in water storage in areas such as the Sahel 82 . In mountainous regions, warming resulted in lower snow accumulation and the retreat of glaciers, causing lower groundwater levels and drinking water shortages 1,39,83,84 . Temporary water shut-downs were also experienced as a result of intense storms, such as Hurricane Mitch in 1998, which left over four million residents in Honduras without water 85 .
Water quality was critically impacted by climate hazards. The contamination of drinking water was caused by wildfires and drought that contributed to elevated levels of nutrients (nitrogen, phosphorus and sulfates) 86 , heavy metals (lead, mercury, cadmium and chromium) 87 , salts (chloride and fluorides) 87 , hydrocarbons 88 , pesticides 89 and even pharmaceuticals 86 . Heavy rains and flooding also increased nutrients 24 , heavy metals 90 and pesticides 90 as well as turbidity 91 and fecal pathogens 92 in water supplies -especially when sewage treatment plants were overwhelmed by runoff 24 . For instance, the 2010 Indus flood in Pakistan increased waterborne and infectious diseases, such as Cryptosporidium 93 , whereas torrential rains in upstate New York in 1999 washed wastewaters into aquifers, sickening over 1,100 adults and killing several children 24 . Sea-level rise has led to seawater contamination of drinking supplies globally, including areas in Bangladesh 94 , Spain 95 , New England 17 and the Pacific Islands 96 .
Infrastructure impacts. We found 21 attributes of infrastructure impacted by climate hazards (Fig. 1), of which the electricity, transportation and building sectors were most critically affected. Impacts to electricity and the electrical grid were commonly cited. Heatwaves, for instance, caused overheated power lines to sag into trees and short out 97 . Heatwaves also reduced the efficiency of power conductance and hydroelectric production from a loss of generator cooling 98,99 . Droughts reduced hydroelectric generation due to low water supplies 99 , and dry soil conditions acted as an insulator causing overheating and melting of underground cables 100 . These impacts on electricity generation and conduction frequently coincided with peak demands during heatwaves at times resulting in complete shut-downs. Blackouts due to heatwaves have impacted millions of people around the world. For example, largescale blackouts affected ~670 million people in India in 2012 98  Impacts on transportation infrastructure were common. Storms have flooded roads 106 , railway lines 107,108 and wiped out bridges 109 , ports 110 and levees 111 . Floods have crippled national transport networks 112 , halted rail service 113 , shut down freight transport 114 and stranded city residents 108,110,115 . Heatwaves caused railways 102,116 , and roads to buckle 115 , asphalt to melt 102 , and concrete roads and bridge joints to crack due to thermal expansion 117 . Heatwaves have grounded airplanes because hot air is less dense than cold air, thus requiring additional speed that airplanes may not be able to achieve on short runways 73,118 . Fires have repeatedly disrupted land, air and sea transport (for example, across Southeast Asia 119 ) whereas drought has hampered river navigation (across Europe in 2003 120 , for instance). Warming, and associated permafrost thawing, has destroyed roads and other critical infrastructure in northern latitudes 35,39 .
Direct and indirect impacts to buildings were significant. Floods and storms damaged or destroyed millions of homes (approximately 12.8 million homes in Bangladesh, 8.7 million in China, 1.8 million in Pakistan, 450,000 in Jakarta, 425,000 in the United States, 45,000 in France, 30,000 in Australia and 30,000 in Jamaica). Fires from extreme droughts and heat also destroyed homes (more than 5,500 homes in Australia, 3,500 in California, 2,500 in Texas,and 2,000 in Russia). Glacial lake outbursts due to retreating glaciers in Nepal 121 and landslides 122 swept away entire areas, including villages 123 . Critical 'lifeline' infrastructures such as sewerage and water lines have been disrupted by storms, and electrical supply by heatwaves, with cascading impacts on business districts, hospitals, schools, communications and access to clean water and food [124][125][126] . Loss of cultural heritage sites was attributed to rising seas, flooding and thawing of permafrost 110,127 , whereas droughts and increased salinity due to rising sea level damaged irrigation infrastructure 128 . Rising temperatures and CO 2 concentrations led to corrosion and deterioration of concrete infrastructure 129 .
Global loss of beaches and coastal infrastructure has resulted from increases in sea level, storms, ocean swells and associated flooding, erosion and slumping 1,127 . The loss of coastal land was related to storms and sea-level rise, which claimed entire islands 130 . Warming and the subsequent melting of ice forced the relocation of native villages in Alaska 39 . Natural cover lost in coral reefs, mangroves and wetlands reduced coastal protection, intensifying the effects of storms and tsunamis on infrastructure 131 .
Economic impacts. We found 16 attributes of the economy impacted by climate hazards (Fig. 1), including economic losses, diminished labour productivity, jobs and revenue. Economic losses were often most dramatic after extreme events, and encompassed immediate costs such as those associated with property damage as well as indirect costs. Immediate direct losses included those from drought (for example, US$1. 84  . The loss of natural land cover was also related to economic costs (for example, by reducing coastal protection, storm damages have increased by US$30,000 for each hectare of destroyed wetland in the United States 134 ). Extreme events also had indirect costs, which can have long-term impacts -as in the case of Hurricane Iniki, where the local economy in Kauaʻi, Hawaiʻi, was still suffering losses over a decade later 135 . Indirectly, climate hazards increased commodity prices. For instance, heatwaves, droughts and fires during the summer of 2010 in Russia cut local grain production by one-third, ultimately doubling wheat prices globally 1 . Storms affected access to and the price of insurance. For instance, Hurricane Andrew led to the insolvency of 12 insurance companies 136 and many firms now refuse to issue new policies for properties within a mile of the ocean on the east coast of the United States 17 . Furthermore, a lack of insurance has made it difficult to obtain mortgages for coastal properties in the Bahamas 136 . Climate hazards also affected the cost and availability of energy resources: heatwaves in 2003 and 2006 in Europe led to a 40-fold increase in the cost per megawatt hour in the European Energy Exchange 137 , damages to oil rigs during Hurricane Katrina temporarily increased fuel prices 35 and drought in Brazil reduced sugar crop production, leading to record high sugar prices and a decline in ethanol production 138 .
Climate hazards impacted job availability as well as work capacity. Heatwaves lowered labour productivity, as observed in Australia, where absenteeism increased during heatwaves 139 , and in India and Vietnam, where heatwaves led to longer workdays to compensate for periods of rest during the hottest hours of the day 140 . Storms and floods 141 disrupted the functioning of industries, resulting in an immediate loss of jobs. Job losses were also related to drought (in areas where agriculture is a large part of the economy 142 , for example), warming (in North America timber jobs were lost due to warm temperatures resulting in pine beetle infestations 91 ) and ocean chemistry (in Peru direct and indirect job losses are often linked to climatic impacts on marine fisheries 143 ).
Impacts on revenue-generating activities were documented, with tourism-based economies being particularly sensitive. Climate hazards reduced the number of visitors to national parks in the United States due to increased temperatures 144 , and in Taiwan due to storms 145 . Droughts had distinct impacts on the recreation industry (for example, river-rafting outfitters in Colorado lost 40% of their normal business -over US$50 million to the industry statewide 146 ) as well as other sectors (US$2.5 billion of revenue lost by the cattle industry in Mexico 147 ). The impacts of temperature on winter-and ocean-related activities were particularly acute. Although snow can be artificially produced, warmer winters generally meant fewer visitors and lower revenue to ski resort destinations, as observed in the Alps 148 and Australia 149 . Changes in ocean chemistry degraded coral reef conditions, which were associated with in a decline in recreational dives in Thailand 150 and affected annual whale migrations, causing early closure of the whale-watching season in Australia 151 .

Security impacts.
We identified 11 attributes of human security impacted by climate hazards (Fig. 1), critically related to dislocations, increased conflict and violence, and disruption of the social fabric. Climate hazards forced hundreds of millions of people out of their homes for different reasons and durations, including evacuation (temporary planned movement), displacement (unplanned forced change of residence) and migration (permanent change of residence) 85,152,153 . For example, hundreds of thousands of people were displaced after floods in China and Pakistan 93,152 , and storms in Central America, the United States and Bangladesh 85,154,155 , to name a few. The recurrence of climate hazards also caused temporary displacement to become permanent 39,85 ; in Bangladesh recurring floods forced some rural inhabitants to move to urban squatter settlements 156 . We found several cases of planned migration of coastal communities due to permafrost melting 8 and recurring flooding and sea-shore erosion due to sea-level rise and storms (for example, indigenous communities in the United States 39 , the Solomon Islands 130 and India 157 ). Multiple cases of mass migration have occurred due to droughts, natural land-cover change and extreme precipitation 153,158 . Extreme heat was also the lead driver of migration in rural Pakistan due to the loss of crops and farming income 126 .
Climate hazards contributed to increasing conflict over access to resources and may have acted as a catalyst for violence. Drought, for instance, has triggered conflicts over water rights and access 147,159 . Ocean chemistry was linked to shifts in the distribution of commercial fish stocks 1,16 and the uncovering of new resources under melting sea ice 84,160 generated geopolitical tensions over their use, including military build-up in the Arctic region 161 . Climate hazards, although not necessarily the sole or even primary driver, have been suggested to ripen conditions leading to violence. However, such pathways remain uncertain and are likely to be diverse, including impacts on migration and reduced supply of resources, jobs and commodity prices compounded with socio-economic factors, such as inequality and failing governance 162 . For instance, changes in precipitation and drought resulted in a scarcity of suitable pastoral and crop land, triggering sectarian and intercommunity violence in the Horn of Africa 163 , increased food prices associated with violence across Africa 164 and food shortages that facilitated rebel recruitment in Burundi 165 . Drought was also an influencing factor in migration to urban areas, adding to the unemployment and political instability that contributed to bloodshed in Syria 166 and Somalia 167 . Excess rainfall has also correlated with violent conflict in Africa 168 . The probability of civil conflicts was nearly double during El Niño years compared with La Niña years 169 . Post-1950, warming or a change in precipitation by one standard deviation increased risk of interpersonal violence by 4% and intergroup conflicts by 14% globally 170 .
Impacts of climate hazards on the social fabric were found, including instances of violence, exacerbated gender inequality and breakdown of social order. High temperatures can increase anger and arousal, affecting how people respond to provocation 171 , which can aggravate acts of interpersonal violence and violent crimes during heatwaves 172 . In the United States, for instance, warming by 0.5 o C aggravated rates of rapes by 0.20, robberies by 0.84, burglaries by 8.16, and larcenies by 10.65 per 100,000 people 173 . The breakdown of law and order during extreme rainfall 170 and storms 174 has been linked to interpersonal violent behaviours including battering 175 and rape 176 . Likewise, anomalously high or low rainfall was tied to a two-fold increase in the number of 'witches' murdered in Tanzania 177 . Hydrometeorological disasters have also been associated with increased instances of domestic violence 178 ; for example, after the 1993 flood in the midwestern United States, a significant increase in cases of battered women was reported 179 . It is worth noting that there has been considerable discussion over the relative role of the climate hazards on human conflict 180 .

Global map of cumulative climate hazards
Our overview of observed impacts reveals the high vulnerability of humanity to climate hazards (Fig. 1). As different hazards can impact numerous aspects of human systems (Fig. 1) and may require varied types and costs of adaptation, the simultaneous exposure of future societies to multiple climate hazards constitutes a considerable concern. To provide insight into this issue, we collected projections for the same hazards for which impacts were surveyed in our literature review and constructed a cumulative index of their geographical co-occurrence. Specifically, we collected projections for warming, heatwaves, precipitation, floods, droughts, fires, sea level, storms, natural land cover and ocean chemistry; we also included projections of freshwater scarcity (Fig. 2). Hazard projections were based on the recent Coupled Model Intercomparison Project Phase 5 under Representative Concentration Pathways (RCPs) 2.6, 4.5 and 8.5, which represent a range of mitigation scenarios in which GHGs are considerably slowed (RCP 2.6) or continue to rise throughout the twenty-first century (RCP 8.5), with RCP 4.5 being in the middle of such extremes. Changes in the projected hazards were rescaled to their largest projected change by 2095 under RCP 8.5, and summed to generate an overall cumulative index of climate hazards (see Methods). The index provides a relative indication of the extent to which the largest projected changes in the hazards will co-occur. The effect of multimodel uncertainty in the cumulative index of climate hazards is shown in Supplementary Fig. 4.
Among hazards, the geographical distributions of projected changes were poorly correlated, with no single hazard having a predominant role in the overall cumulative index of climate hazards (Supplementary Table 1). For instance, there was little concordance in the spatial patterns of change in drought, floods and water scarcity compared to precipitation, despite the latter being an underlying driver of the former. This reflects the effects of topography, soil type and human uses that act as modifiers for precipitation patterns. Likewise, warming (which is projected to intensify at higher latitudes) was poorly related to the spatial patterns of change observed in most other hazards (Fig. 2, Supplementary Table 1). Overall, the geographical variability of projected changes in the different hazards highlights the need for analysis that integrates different climate hazards and the potential for underestimation of projected climatic changes when examining one or a few hazards. Globally, the largest intensification of drought is projected to occur in Europe, North America and South America (Fig. 2). Fires are projected to intensify in Australia but decline over the south Sahara. Floods are projected to increase in South America, Southeast Asia and northern Russia. Deadly heatwaves are projected to increase in duration over most tropical areas, while storms are projected to increase in intensity over pantropical regions. Precipitation is projected to increase over tropical areas and high latitudes, but decrease at midlatitudes. Water scarcity will intensify over many regions of Africa and America. When cumulative patterns of change in all hazards are combined, the largest co-occurrence of changes is projected in the tropics, generally isolated to coastal regions (Fig. 2). Coastal areas of Southeast Asia, East and West Africa, the Atlantic coast of South and Central America will be exposed concurrently to the largest changes in up to six climate hazards if GHGs continue to rise throughout the twenty-first century (RCP 8.5, Fig. 2), or three under strong mitigation of GHGs (RCP 2.6, Supplementary Fig. 3).
When we examined how the cumulative patterns of future change relate to human populations (see Methods), we found that globally, half of the world's population will be exposed to the equivalent of the largest change in one full hazard under RCP 2.6 and approximately three hazards concurrently under RCP 8.5 (Fig. 3a-c). This suggests that even under strong mitigation scenarios, there will still be significant human exposure to climate change. Patterns of exposure to cumulative climatic hazards showed similar trends among countries with different levels of wealth (Fig. 3d-f). In our bibliographic search of impacts from climate hazards, we found differential responses from exposure to similar climate hazards, highlighting the variation in adaptation capacity (Supplementary Note 2). The largest losses of human life during extreme climatic events occurred in developing nations, whereas developed nations commonly face a high economic burden of damages and requirements for adaptation (Supplementary Note 2). Thus, while it is commonly noted that developing nations will face most of the burden of current and projected climate change [181][182][183] , our integrative analysis of impacts reveals that developed nations will not be spared from adverse impacts.

Concluding remarks
Our assessment of the literature yielded a small number of positive and neutral responses of human systems to climate hazard exposure (reviewed in Supplementary Note 2). We surmise that the reduced number of positive or neutral impacts may be real, but may also reflect a research bias towards the study of detrimental impacts (discussed under Caveats in the Methods). This small set of positive and neutral impacts, however, cannot counterbalance any of the many detrimental impacts that were uncovered in our literature search, particularly when many of these impacts are related to the loss of human lives, basic supplies such as food and water, and undesired states for human welfare such as access to jobs, revenue and security.
Given the vast number of components in coupled human-climate systems, assessing the impacts of climate change on humanity requires analyses that integrate diverse types of information. Contrasting temporal ( Supplementary Fig. 1) and spatial (Fig. 2) patterns of climate hazards, compounded with varying vulnerabilities of human systems (Fig. 1), suggests that narrow analyses may not completely reflect the impacts of climate change on humanity. Our integrative analysis finds that even under strong mitigation scenarios, there will still be significant human exposure to climate change (Fig. 3d), particularly in tropical coastal areas (Fig. 2); such exposure will be much greater if GHG concentrations continue to rise throughout the twenty-first century (RCP 8.5, Fig. 3) and will not differentiate between poor or rich countries (Fig. 3). The multitude of climate hazards that could simultaneously impact any given society highlights the diversity of adaptations that will probably be needed and the considerable economic and welfare burden that will be imposed by projected climate change triggered by ongoing GHG emissions. Overall, our analysis shows that ongoing climate change will pose a heightened threat to humanity that will be greatly aggravated if substantial and timely reductions of GHG emissions are not achieved.

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Synthesis of impacts.
To compile the observed impacts on people from climate hazards, we searched Google Scholar from February to March 2017 using a full text search in English for publications on the impacts of ten hazards of the Earth's climate system (warming, heatwaves, precipitation, floods, drought, fires, sea level, storms, natural cover change, ocean climate change) on six aspects of human life (health, food, water, infrastructure, economy and security). Our assessment of the Earth's climate system was based not only on mean state changes (such as warming, precipitation, sea level, ocean chemistry) and extreme weather events (heatwaves, flood, drought, storms), but also on disturbances (such as fire). We included changes in natural land cover as one of the hazards because ecosystems are an intrinsic component of Earth's climate system as they are both sources and sinks of carbon, affecting other hazards (for example, warming and precipitation via albedo and evapotranspiration) and directly affecting various aspects of human life 182,184 . We also assessed impacts from changes in ocean chemistry given the key role of the oceans in the climate system and human dependency on ocean goods and services 183 , especially for coastal and maritime societies. Our assessment of these impacts on human systems was based on aspects that we considered essential for human well-being. These six aspects, however, represent general categories within which many other facets of human life were assessed. For instance, we found numerous examples of impacts on cultural practices such as the breakdown of traditional hunting and fishing systems among indigenous communities (that is, a food impact), causing depression or even suicide (a health impact). The diversity of the aspects of human life assessed is reflected by the long list of sub-categories reported (Fig. 1).
To ensure a systematic process, we scrutinized the first 200 references that resulted from using each possible combination of the ten climate hazards and six aspects of human life as keywords. References included the academic literature, grey literature and popular press articles. From those references, we selected papers independent of whether reported impacts were positive or negative. Our search also included the references cited in the publications that were read to be as comprehensive as possible. From > 12,000 references that were screened, including 72 chapters from the five IPCC assessment reports and the most recent National Climate Assessment report for the United States, we identified 3,280 publications that were read to find examples of observed impacts.
For the purpose of quality assurance and standardization, we applied the following approach when searching for impacts in reference abstracts and texts: (1) To ensure standardization, an impact was broadly considered as any case example of "an explicit climate hazard causing a response on an explicit aspect of human life in an explicit or implicit place and time". The criteria allowed us to identify the climate hazard and human aspect that was affected while ensuring the impact was empirically observed (that is, any impact could be traced to a place and time as reported in the literature). Mentions to impacts that lacked such traceable evidence were excluded. For instance, a claim such as "increased mortality has been observed during heatwaves" was not considered. This claim lacks the traceable evidence of when and where the heatwave that killed people happened. In turn, an example of a valid entry is: "During the 2003 European heatwave over 70,000 excess human deaths were observed". This latter entry provides traceable evidence that an explicit climate hazard (a heatwave) impacted an explicit aspect of human life (mortality) in a given place (Europe) and time (2003). (2) We created a public online database consisting of ten columns (one for each climate hazard) and six rows (one for each of the six aspects of human life assessed). We created subcategories (that is, added rows to the online table) within each primary aspect of human life to reflect the variety of documented impacts in the literature (for example, under the primary heading 'food' , entries were separated into agriculture, livestock, marine fisheries and so on; see Fig. 1). On identifying an impact in a given paper, the user placed the reported impact in the online table at the intersection of the climate hazard (column) and attribute of human life (row) explicitly mentioned in the paper. Subcategories were created by the user who read the given paper using the terms provided in that paper, thus avoiding classification biases by the user who entered the data. This initial classification specificity was also intended to prevent 'grouping' of impacts into broad sub-categories and potentially losing the visibility of rare impacts. However, by using a central online database, any created subcategory was automatically available to others entering data thus reducing the duplication of subcategories. When the data entry stage ended, the authors met to integrate similar existing sub-categories as much as possible, while care was taken to avoid generating broad terms that could risk rare impacts being overlooked. For example, we found that climate hazards have numerous types of impacts on the state of mind of people ranging from depression, to addiction, affective disorder, PTSD and even suicide. These subcategories were maintained for better identification of the broad array of psychological consequences from climate hazards. We also performed secondary searches combining key words of climate hazard (column name) and specific (that is, subcategory) attributes of human life (row name) for empty cells in our table to ensure that these empty cells represented a lack of evidence. changes was also calculated separating countries by country-level per capita GDP (Fig. 3d-f). Data on per capita GDP were obtained from the World Bank World Development Indicators Database. We grouped low-, medium-and highincome countries depending on whether annual per capita GDP was smaller than US$4,000, between US$4,000 and US$12,000 and larger than US$12,000, respectively.
A key source of uncertainty in the reported projections of climatic change is the 'precision' with which Earth climate system models predict change in the different hazards. Precision is defined here as the variability in projected changes from replicated Earth climate system models. To assess the effect of this source of uncertainty for each hazard, we gathered the average projections among Earth climate system models and their standard deviations. For each pixel, at each time step, we divided the standard deviation by the mean to calculate the coefficient of variation. We then removed any pixel for which the coefficient of variation was larger than one; that is, pixels for which the multimodel variability was larger than the average projection. We then recalculated the overall cumulative index of climate hazards and compared results from the raw projections and the projections excluding uncertain pixels ( Supplementary Fig. 4). As the effects of multimodel uncertainty were small ( Supplementary Fig. 4), we reported results based on the raw data. To assess the spatial similarity in the projected change of different hazards, we calculated the crosscorrelations between projected changes of all hazards (Supplementary Table 1).
Caveats. Our search of observed impacts yielded a much larger number of negative impacts than positive ones. This result could reflect a real disparity in the occurrence of impacts, but may also reflect a systematic bias of reported impacts. We consider that such bias can emerge from two alternative sources: first, there is a bias in our search of the literature. We minimized this bias by carrying out a comprehensive search of citations on impacts regardless of whether impacts were positive or negative (see Methods). Second, there is a bias in the literature itself towards reporting negative impacts. We consider that this bias could be real, as from a 'risk' perspective a critical concern is those impacts with negative consequences on humanity. However, there is no mechanism for us to quantify such bias within our literature review. This is because publications are probably related to issues of novelty and broad public interests as opposed to how common impacts are. However, even if there is a bias towards negative impacts in the literature, this does not invalidate any of the impacts that have already been observed nor their purpose for this Review, which was to highlight the broad threat to humanity from changes in climate hazards.
From this study it is not possible to quantify the temporal or spatial prevalence of impacts that have been reported. Unfortunately, because our study is based on a compilation of the literature, it is not possible for us to quantify the prevalence of specific impacts as publications are probably related to scientific novelty and interest as opposed to how frequent or important impacts may be. For instance, there may be few examples of impacts of hazards on culture or even loss of islands to sea-level rise because as they may not have garnered broad scientific interest or may not be readily quantified, but these impacts are real and important nevertheless -yet a single report of a case example can reveal that such impacts do occur. Given this limitation, the section on observed impacts on human systems should be taken as descriptive of feasible pathways through which hazards can impact humanity, without indicating the prevalence or importance of such impacts.
We caution that our literature search was restricted to impacts on people from climate hazards, and no other aspects related to climate change. Although our survey of the literature yielded some case examples of adaptations, positive and differential impacts (Supplementary Note 2), these are unlikely to reflect the full scope of the adaptations, opportunities and trade-offs associated with climate hazards. The large array of cases that we uncovered with a systematic literature search on only climatic impacts suggests that a better understanding of those issues (adaptations, positive and differential impacts) will require their own comprehensive analyses. Our assessment of impacts was also restricted to those that affect only people; we excluded impacts on ecosystems unless they had ramifications for human life (such as food and water supply, tourism). The broad impacts of climate change on ecosystems have been the topic of similar analysis 196,197 . We surmise that some aspects of human life lend themselves to more detailed breakdown and analysis, which causes a variable number of subcategories that can be impacted; the more diverse the aspect, the more subcategories were apparent. For instance, there were 27 subcategories of human health affected by climate hazards, but only 4 for freshwater (Fig. 1).
Another potential issue in our literature review relates to the use of Google Scholar as our sole search engine for the identification of publications. We consider that there may be at least two issues that could emerge from using only this tool. One limitation relates to the standards of papers assessed. Curated databases may provide a cleaner set of papers than Google Scholar. The effects of this bias are probably minor in our case because we reviewed the first 200 papers under each pairwise combination of keywords (suggesting that this was a deep search into the literature of specific topics) and because after a given paper was selected, it was read in full and records of reported impacts were curated and validated by our team of authors. The other limitation is that Google Scholar may fail to access records of publications to which other databases may have access 198 . One motivation for using Google Scholar is that it searches over a broad spectrum of the literature as opposed to specialized databases 199 . However, by lacking the