More geographical areas will experience extreme heat causing occupational health hazards, previously only known from operations in tropical areas (10). To allow strenuous exercise during military training, recruits need to get adapted to extreme heat to avoid occupational injury including improved decision-making under heat stress (30, 34). Women seem to be less vulnerable to severe exertional heat illness than men (35). Strenuous exercise in hot environments with a core temperature rise by >1°C does not only affect circulating stress hormones, including catecholamines and cytokines, but also circulating leukocyte counts (36). The health effects of such challenges affect the immune system and may cause several diseases, especially respiratory tract infections.

There is a close relationship between exposure to small particulate matter (PM₁₀ and PM_2.5) and increased mortality or morbidity, both daily and over time. Repeated exposure can lead to increased permeability in lung tissue, elevating susceptibility to viral and bacterial pathogens and aggravation of the severity of chronic lung diseases. Since PM may cause inflammation of lung tissue, there is a high risk of changes in blood chemistry that impact heart function in several ways. Several studies have indicated a correlation between both upper and lower airway symptoms and the military occupational health. For example, it has been shown that exposure to PM_2.5 is a major independent contributor to the development of chronic sinusitis in the active-duty military population (37). The degradation of ambient air quality due to climate change will likely add to the risk of airway symptoms and perhaps also cardiovascular symptoms among military employees (17).

Sea level rises pose an existential threat to nations with large coastal areas–not only due to loss of land surface but also through saltwater intrusion in drinking water wells. Some small island nations are projected to become mainly uninhabitable already during the 21^st century.

The risk of flooding of coastal installations and fortifications in areas with high precipitation requires that the design of military installations is adjusted to climate change. Storm surge and coastal erosion related to climate change will add to the challenges of military installations and the health of servicemen. Flooding related risks are, besides the effects on water quality, a greater incidence of drowning accidents, hypothermia, impaired medical evacuation possibilities, and damaged medical facilities.

Surveillance of vectors and related diseases will become more important for military epidemiologists. The use of repellants and mosquito nets will become mandatory in more areas, requiring education and a new set of skills for military personnel in their home countries (9).

The effect of rising temperatures on the spreading of infectious diseases has been studied more extensively during the last decades. Vector-borne infections are expected to be the most climate-sensitive subset of all infectious diseases and have sensitivity to the greatest number of climate drivers. Arboviruses such as dengue, Zika, yellow fever and chikungunya viruses are transmitted by Aedes mosquito species, which have been spread into new areas due to climate changes, globalization and international trade. These diseases poses serious concern for public health services, not at least the large potential of Aedes mosquitos to transmit viruses in urban settings (38). In a recent study, researchers have also systematically mapped how, not only viruses, bacteria and parasites are affected by climate change, but also how insects and allergens are affected by rising levels of greenhouse gases (39).

In the Nordic countries, zoonoses such as Lyme disease, tularemia and nephropathia epimedica have been increasing for a long time due to habitat destruction and reduced biodiversity, and the increase is expected to accelerate further due to prolonged and more frequent mild winters (40). Since military personnel spend a relatively large part of their working time in field environment, the increased risk of infection poses a threat to the health of military personnel.

Water-borne pathogens often act through two major exposure pathways: drinking water and recreational water use. Higher mean temperatures trigger more intense precipitation events. These events can result in run-off and loading of coastal waters with pathogens, that may adversely affect the health of people using recreational water. Since the risk of gastroenteritis and respiratory infections due to recreational water use are much higher during the rainy season than during the dry season, more intense precipitation during rainy seasons will likely increase that risk, both for the general population and for military personnel (12).

Depending on water treatment capability, the quality of drinking water and hygiene water can be affected due to climate change as well. Precipitation extremes have been linked to both outbreaks and sporadic cases of waterborne illness. For example, studies have shown a link between heavy rain and turbidity to population-level risk of sporadic cryptosporidiosis and giardiasis (41). In developing countries, events such as flooding have been associated with epidemics of cholera and other diarrheal diseases. In Europe, flooding has rarely been associated with an increased risk of water-borne disease outbreaks, although a few exceptions exist (14). Thus, the extent to which climate change will increase the risk for infections from drinking water among military personnel depends on several factors, such as geography, treatment capability and socioeconomic status.

Climate change directly affects food safety, as rising temperatures are associated with a high number of pathogens and bacterial toxins (42). Researchers found in several European countries a linear association between temperature and the number of reported cases of salmonellosis above a threshold of 6°C (43). Vibrio cholerae is endemic in only certain regions of the world, even though it is commonly found all over the world. A complex interaction of abiotic factors, including temperature, salinity, iron concentration, and sunlight, influences V. cholera toxin production (44).

Furthermore, climate change will change the amount and type of mycotoxins in our food. In warmer climate the presence of fungi and mycotoxins such as A. flavus and aflatoxin will increase (45).

Climate change effects on food safety will change both the need for cool storage and the preparedness for food-related infections. As food poisoning has a dramatic impact on soldier performance prevention and monitoring of upcoming gastrointestinal infections will become an important task for defense medical support.

There are few, if any papers specifically investigating military mental health due to climate change over extended time periods, and assumptions in review papers are not entirely based on scientific evidence. Thus, the ability to draw scientifically conclusions with respect to climate change related mental health or mental diseases in military settings are very limited. Symptoms related to mental health in an occupational military setting may be related to both regional climate changes, as well as mission and training. In general, the mental health may not be as jeopardized as in civilian population, due to selection bias of personalities and mental capacities, and the systematic training of military personnel expected to operate in challenging conditions. However, a previous qualitative study among civilians during an annual half-marathon race found a perceived risk profile among runners who collapsed and required hospital care (46). Their personality self-report included descriptions such as being stubborn, ambitious, disciplined and performance oriented. Such personality traits are without a doubt not rare within the military staff, and may relate to profession specific physical and mental outcomes of the military staff performance due to temperature change. Also, military personnel already operate in extreme weather conditions, adding a certain strain on their mental health, that civilians normally are not exposed to.

It is shown that with an increasing air temperature above an optimum, physical and cognitive performance decreases. Cognitive performance may decrease along with increased temperatures (47), and especially complex cognitive tasks seem to be most vulnerable to physical heat in soldiers (48). Also, climate related supply chain disruptions may occur putting basic needs of food and clean water, at risk, both during deployment and in regular training, elevating stress and general anxiety, thus interfering with several aspects of cognitive performance even if military resilience training has been performed (49).

Due to climate change, scenarios in which unacclimatized personnel need to either serve at short notice in areas with severe climate or to over-ride intrinsic or extrinsic thermal safeguards in the face of physical or tactical threats will become much more frequent. These events will cause more military personnel with heat stroke or other heat related symptoms and ultimately degraded capability (30).

The NATO STO Research Task Group on Management of Heat and Cold Stress Guidance to NATO Medical Personnel identified multiple heat acclimatization strategies prior to deployment (50).

Partly because of deployment in areas with polluted air, military troops worldwide have a long and extensive history of airway related diseases and symptoms (36, 37, 51). The distribution of PM air pollutant typically encountered by deployed military personnel, such as desert dusts, burn pit smoke, oil well fires and local industrial pollutants, are expected to be affected by climate change, in a way that may increase the risk for lung disease, no matter whether the air pollution is caused by the military itself or not.

It is therefore important that contingency hospital care is aware of and have a redundance for an increased risk for airway related diseases and symptoms among deployed personnel. As location, duration, and frequency of deployment contribute to the risk of respiratory symptoms they should be part of every soldier's medical history (36).

The greater risk for flooding of lowlands through climate related precipitation and storm surge requires an increased awareness of climate related aspects when planning future operations. The scenario of operating bases being flooded, decreasing military capability, is a likely scenario in future operations in deployment. As flooding causes sanitary challenges with a high risk of drinking water contamination, the importance of military medical presence during operational planning is obvious. Therefore, students attending medical-programs or health-programs need to be trained in preventive medicine aspects related to natural disasters and in austere or deployed environment.

Deployed soldiers are exposed to vector-borne diseases endemic in the deployment area. During World War I and II, soldiers in South East Asia and the Mediterranean have been infected with plasmodium species and brought the malaria to their home countries (52, 53). Meticulous preventive medicine measures–including pesticides and chemoprophylaxis–were successfully containing and in some countries eradicating the disease. But, due to climate change, malaria has resurfaced both due to migration of infected populations, as well as due to increased vector populations in warmer climate (31).

Ingestion of pesticides, as permethrin and pyridostigmine bromide, may affect the human microbiome and may cause chronic inflammatory diseases (54). It is likely that the greater use of pesticides will lead to a higher incidence of adverse effects, which need to be followed cautiously by the medical professions.

Effective water treatment plants in deployed settings (i.e., comprising reverse osmosis, ultrafiltration, or microfiltration technique) normally provides drinking water of high quality to deployed personnel. However, unpredictable rainfall, increased flooding and extreme precipitation in deployed areas will inevitably increase the risk of water-borne diseases and creates conducive breeding grounds for insect vectors at or close to the deployment setting. Moreover, in case of drought, limited and deficient water supplies can perpetuate unhygienic conditions, that might increase the risk of several water related diseases, for example diarrheal disease. Thus, for each contingency hospital care, an awareness of and redundance for increased risk for water-related infections is important.

Warmer climate impairs food safety (55). For instance, the microbial quality of raw milk is threatened by warmer climate, as raw milk contamination with E. coli is dependent on temperature (56). Even the presence of micro-organisms that produce food-borne illnesses such as salmonella and campylobacter may increase with rising temperatures (55). Especially in deployment a well-controlled and functional cold chain for food supply is vital for preventing food-borne illness (57).

As the climate change increases the rate of worm parasites in wild and domestic animals, the quality of the available meat is highly dependent on the ability to perform proper diagnostics for parasites (58). Military personnel in deployment who are dependent on locally acquired or hunted meat need high medical intelligence capabilities as well as parasite detection capabilities.

Future challenges in military mental health are related to both environmental changes involving climate related changes in heat, humidity, cold, malnutrition and natural disasters or extreme weather events. These factors ought to be part of military medical intelligence preparations. These conditions can all affect the mental state via mechanisms of sleep disturbances (lack of sleep or disturbance of circadian rhythms, contributing to anxiety related behavior). Interestingly, in some military settings report of pain is stigmatized, thus complaints about sleep problems, is more accepted as a medical issue (59). In war, lack food and water intake may be part of the logistic disruptions, thus elevating levels of stress and anxiety (starvation is one of the oldest weapons of war being deployed or in war). Calorie deprivation and forced evasion is a unique and severe stress. Performance decrements and psychological changes are evident within 24 h of cessation of nutritional intake. Overall, in any form of environmental stressor on the central nervous system of the operating military personnel, a failure to regulate prolonged periods of excessive cortisol levels may not only elevating risk of mental problems related to both anxiety and depression, but also reduce cognitive capacity, possibly by hippocampal sensitivity (60), being the gateway to complex cognitive functions (61).

Poor water quality and sanitation constitute perhaps the most critical threats to health and safe healthcare (62). Poor water quality does not only affect the availability and quality of freshwater, but also affects the future demand for fresh water. Between now and 2,071, up to 14 % of the global population is estimated to experience a severe reduction in water resources given, a global warming up to 2.7°C (63). Water availability for countries in Europe will likely be affected by increasing runoff and flood magnitudes (64), although perhaps not pronounced at higher latitudes, e.g., in most of Finland, northwestern Russia and northern Sweden (65).

In addition, a global warming of 2°C will also impact water quality in lakes, watersheds, and regions, in terms of a possible increase in chloride levels, that can result in chloride concentrations well above drinking water standards in some regions (66). In a similar fashion, expected increase in leakage of nitrogen and phosphorus from agricultural areas to nearby lakes might affect the usability of fresh water (67). In addition, recent studies on local or regional areas already indicate a decrease in dissolved oxygen in some freshwater basins (68).

Access to fresh water has a profound relevance for military operations in terms of hygiene, healthcare and defense medical logistics. Further research is needed to define defense medical supply system requirements for safe fresh water in times of climate change.

The medical treatment chain requires full access to medical supplies: pharmaceuticals, infusions, diagnostics, blood products, as well as medical devices for prehospital care, outpatient clinic, surgical theater, intensive care unit and ward. Many of these medical supplies are temperature sensitive and lose their effect and may cause harm if the temperature chain is interrupted (69, 70). Climate change forces military pharmacists to improve their fridge and freezing capacity in the cold supply chain, as well as having a higher storage turnover.

Blood products as erythrocyte concentrates and thrombocytes are highly temperature sensitive. As mass casualty incident preparedness includes storage of blood group 0 erythrocytes, plasma and thrombocytes, the storage and transport of these products in warmer areas has to rely on highly effective cooling systems (71).

Many medical devices depend on semi-conductor technology that needs to be cooled during operation. Climate change will be a driver of new semiconductor cooling technology development, and medical devices cooling needs to develop accordingly.

In many countries the armed forces will respond to natural disaster relief in support of the civilian disaster response (10). As the number of natural disasters will continue to increase due to climate change, disaster relief operations will consume more medical and logistic support resources. Climate change acts as a risk multiplier for armed conflict (72). Due to climate related conflicts, disaster relief operations will to a greater extent take place in conflict-ridden countries, putting both military and civilian disaster medical relief personnel at risk for both physical and mental problems, raising the need for regional psychosocial and mental health programs (73). Moreover, due to security issues, a greater deployment of armed forces with medical support to humanitarian relief operations may be needed as well (74–76). Longer-term planning of military capabilities needs to account for humanitarian support during climate change related peace-keeping operations.

The melting ice in the arctic region opens for new waterways allowing shorter sea transport routes between Asia and Europe. This will increase the need to protect these waterways, requiring military presence in the Arctic North more than ever before (4). The changes in operational geography will affect military medicine as well: Prehospital and emergency care will be performed in arctic climate, with consequences of a higher risk of hypothermia, increased bleeding risk, and malfunctioning medication and medical devices. Defense medical support thus needs to establish a medical supply chain fit for Arctic operations. Occupational medicine will need to focus on preventive measures to protect operators in cold environment by leadership development, training, acclimatization, and equipment (77).

Medical facilities use large amounts of electric energy, produce large amounts of waste, even potentially hazardous waste, and leaves a significant contribution to greenhouse gas emissions (78). Indeed, the healthcare sector is responsible for about 4.5% of greenhouse gas emissions worldwide (79). Therefore, it is reasonable to investigate the carbon footprint of military healthcare installations. Bozoudis et al. found hospital electricity to be with 63% the largest contributor of greenhouse gas emissions in a Greek military general hospital, followed by fossil fuels with 32%, refrigerators with 5% and air conditioning with 4%. Waste disposal accounted for < 1% of the carbon footprint of the military hospital. By using a more environmental mix for the electric energy consumption, switching to more fuel-efficient heating systems, and the addition of solar panels on hospital roofs and parking lots, the carbon footprint of military hospitals can be reduced.

Additionally, alternative ways to meet and diagnose patients are available, for instance by telemedicine, diagnostic drones or drone based medical delivery, which may reduce the carbon footprint of healthcare (80, 81). Further research is needed to identify further possibilities for reduction of the military healthcare carbon footprint.