Many studies that examine the role of climate change on mental health underscore the threat of harm to both individuals and communities. However, there are methodological limitations, as studies examining outcomes after severe weather events are mainly cross-sectional, with few studies that include longitudinal pre-disaster data. On the contrary, it is common for studies examining chronic effects, such as rising temperatures, to draw conclusions that are correlative rather than causative. Many studies have endorsed a broader definition of psychological impacts and will utilize self- or parent-reported questionnaires that screen for symptoms of internalizing and/or externalizing behaviors rather than the formal diagnosis of a clinical disorder [14,15]. Furthermore, studies have also looked at outcomes such as emergency department visits and violent crime as proxies for mental health problems [16,17]. This is not to say that these studies do not have value; however, it is also important to further examine the direct impacts of climate change on mental health. 

In order to create more targeted public health interventions, it is integral to recognize risk factors and identify populations that may be particularly vulnerable to the effects of climate change. While individuals that live through a climate disaster may all experience psychological and sociological distress, it is inaccurate to assume that all of them will go on to develop a diagnosable mental health disorder. In fact, many symptoms of the immediate responses to the traumatic event tend to resolve over time and most can fully recover after a disaster [3]; however, some who develop symptoms after a traumatic event will develop a diagnosable mental health disorder [18]. There are many risk factors for developing a mental illness in the aftermath of a climate-related disaster, including the severity of the disaster, being a witness (firsthand or secondhand) of death or injury, female gender, lower education or socioeconomic status, younger age, minority or ethnic status, psychiatric history, family instability and inadequate social support [3,17]. Contrarily, there are also several determinants of resilience that have the potential to increase adaptability following a weather-related disaster. Those who possess self-efficacy regarding coping is an important predictor—those who believe they can cope with whatever comes their way fare better than those that do not [19]. Yet overcoming psychological adversity depends on much more than just will and personal resources. People that are ingratiated into a larger community are significantly more likely to recover if the community is socially supportive—it is hypothesized that a lack of social support can compound mental anguish [20].

Acute, subacute and chronic consequences of climate change are all associated with negative mental health outcomes. While the mechanisms of action that climate change-related events have on the psychopathology of affected individuals remains unknown, several hypotheses have been proposed, such as a pre-existing lower level of resiliency to natural disasters, or the weather event’s magnification of additional psychosocial stressors [4].

There is a substantial body of evidence that has documented the mental health outcomes after acute, short-term (i.e., lasting for days) extreme weather events such as hurricanes, floods, wildfires and heatwaves. Negative sequelae that are commonly associated with acute consequences of climate change include heightened anxiety, acute stress reactions, sleep disruption and a decreased sense of self and self-identity due to displacement from homes and communities [21]. A recent event that exemplifies the intersection of extreme weather events and their potential to wreak havoc on pre-existing vulnerabilities is Hurricane Maria that made landfall on Puerto Rico in October 2017. At that time, Puerto Rico was in the throes of a decade-long recession that had led to skyrocketing rates of unemployment, poverty and separated families due to emigration. In the nine months following the hurricane, acute mental health concerns were greatly exacerbated—suicides rose by a staggering 18% and calls to Puerto Rico’s main suicide hotline rose by 13% when compared to the previous year [22].

Subacute or long-term climate-related changes last for months or years and include events such as drought and long-duration heat waves. These impacts may provoke an intense emotional response in those who experience the effects of climate change both directly and indirectly and lead to anxiety related to an uncertainty about the future of humans, other species and the planet [23]. In a large-scale longitudinal study conducted on two million randomly sampled US residents between 2002 and 2012, Obradovich et al. found that an increase in monthly temperatures between 25 °C and 30 °C to more than 30° increased the likelihood of negative mental health events by 0.5% and that a 1 °C temperature increase over five years is associated with a 2% increase of negative mental health events [24]. Similarly, another study found that suicide rates in the United States and Mexico rose by 0.7% and 3.1%, respectively, for every 1 °C increase in monthly temperature—which will result in a combined 21,770 (95% CI 8950–39,260) additional suicides by 2050 [25]. Additionally, heat suppresses thyroid hormones which results in functional hypothyroidism, which can present as decreased energy, dysphoria and cognitive impairment [26]. Drought presents its own set of outcomes that cast an interesting light on the indirect impacts of climate-related events. One review discovered an economic effects pathway that has particularly severe impacts on rural farming populations, as well as a migration pathway that links drought to mental health [27]. Additionally, a causal-pathways model suggested two indirect mechanisms by which subacute climate change-related events are thought to impact mental health: (1) by negatively impacting physical health through increased heat stress, disease and disruption to food supply and supply chain; and (2) by deteriorating community wellbeing through damage to the economic and social fabric of communities [28].

Even if planned mitigation efforts to combat global climate change are successful, a significant amount of damage has already occurred to many areas of the world. It is important to note that poor countries will continue to be the most adversely affected due to the greater reliance on farming and exposure to higher overall temperatures, which will place even more stress on already limited access to infrastructure and risk management [29]. Perhaps the most profound mental health outcome related to long-lasting climate-related events, however, is the psychological threat to the human existence that comes from the conceptual realization of anthropogenic climate change [30,31]. Indeed, “(t)his awareness contributes to ‘psycho-terratic’ syndromes, including phenomena such as ‘ecoanxiety’, ‘eco-paralysis’ and ‘solastalgia’, the distress and isolation caused by the gradual removal of solace from the present state of one’s home environment” [21]. Research has demonstrated that younger generations, particularly those living in wealthier countries, are believed to be particularly impacted by these syndromes [32].

As outlined above, acute, subacute and chronic consequences of climate change are associated with a variety of negative sequelae, such as increased psychological distress, mental health-related emergency room visits and admissions, suicide, depression, anxiety and development of disorders such as post-traumatic stress disorder (PTSD). The literature focuses primarily on identifying the course of mental health changes following extreme weather-related events, which may result in conditions such as PTSD, depression, anxiety and suicidal behavior. Internalizing behaviors are negative behaviors that focus inwardly towards the self, such as depression, anxiety, social withdrawal and suicide, whereas externalizing behaviors are negative behaviors that are directed outward toward others such as aggression, opposition, defiance and violence [33].

Acute stress reactions, adjustment disorders and post-traumatic stress disorder (PTSD) are commonly experienced mental health conditions following climate change-related disasters. Such events have the potential to disrupt life and cause catastrophic losses, which may lead to psychiatric responses such as hypervigilance, guilt, fear, or flashbacks [2]. In a sense, these experiences could be considered a normal response to an abnormal situation. However, when prolonged, trauma-based symptomatology following a climate change-related disaster has the potential to lead to diagnosable PTSD. The rates of PTSD post-disaster are staggering—estimates range between 25% to 40% and can result from nearly every type of weather disaster, including wildfires, hurricanes and earthquakes, without regard to continent or culture [2,21,59]. Disaster-related PTSD does not impact survivors equally. There is a body of research that indicates that incidence of post-disaster PTSD is higher among females compared to males, older people compared to younger people, those of low socioeconomic status or the unemployed, or those that have pre-existing mental health disorders [60].

The extent of exposure to and experience of a natural disaster appears to yield a dose-dependent relationship with the development of PTSD symptoms. After the Victorian Black Saturday bushfires, probable PTSD was identified in 15.6% of subjects living in highly-affected areas and significantly more than the 7.2% and 1% of subjects in medium- and low-affected areas, respectively (p < 0.001) [37]. Similarly, in an area of extensive flooding in Australia, adults who were personally affected by floods scored significantly higher PTSD scores than those not personally affected (p = 0.001) [14]. Disaster-related PTSD may persist for several years after the initial disaster [38] and may stem from disaster-related experiences including serious injury or death of a someone close, fear for one’s life, or displacement from home [37,38,42,43]. In a study of 130 adults affected by Hurricane Sandy, Schwartz and colleagues examined mental health outcomes (depression, anxiety, PTSD) in relation to several personal (e.g., directly affecting participant or family) and property exposures (e.g., level of property affected, financial hardship) [43]. At follow-up (1 year after initial evaluation post-disaster), PTSD was significantly associated with experiencing personal damage (OR 1.6, 95% CI [1.2–2.2]), property damage (OR 1.3, 95% CI [1.1–1.5]), or both (OR 1.2, 95% CI [1.1–1.4]); however, there were no statistically significant increased odds of these exposures to depression or anxiety, suggesting that the impacts of disaster may uniquely have persisting effects on PTSD.

The specific mechanism of action in which climate change leads to developing PTSD remains unknown; however, it is hypothesized that multiple brain systems may be implicated, both at the structural and chemical level. Studies have shown that patients with PTSD had decreased volume in several regions of the brain, including the hippocampus, left amygdala and anterior cingulate cortex, all thought to play a role in memory processing and emotion regulation [61,62]. Neurotransmitter changes in the adrenergic receptor system result in increased norepinephrine levels and downregulated adrenergic receptors, as well as decreased glucocorticoid levels and upregulated glucocorticoid receptors [63]. These changes in the anatomical and neurochemical pathways are believed to drive the core symptoms of the disorder.

Compared to acute stress responses, which tend to be relatively short-lasting after surviving extreme weather events, depressive symptoms and diagnosable major depressive disorder emerge as a longer-standing effect. Like PTSD, there is a breadth of literature that correlates mood disorders with climate change-related weather events. Multiple studies have shown that, after wildfires, depressive symptoms may be present in between one quarter and one third of participants [60]. Sullivan and colleagues [44] found that 23% of subjects presented with clinically significant depression post-disaster and new-onset depression was markedly more prevalent amongst those with pre-existing mental health problems versus those without (32.1% versus 12.6%, respectively). Depressive symptomatology rates are also reflected after hurricanes. In a study of adults affected by Hurricane Sandy, depressive symptoms were seen in 35.4% of subjects 12–28 months post-disaster [43]. Concerningly, one year after the initial evaluation, the rate of depressive symptoms was not significantly lower (30.8%, p = 0.39), which may reflect the longevity and pervasiveness of depression. A study on children in Grades 2 through 4 who lived through Hurricane Ike further supports this hypothesis–approximately 11% had depressive symptoms at both 8 and 15-months post-disaster, which underscores the continuity of depressive symptoms across time [41].

Chronic consequences of climate change may also have a significant impact on depression and other mood disorders. One study found significant associations between depression and high temperatures consistent with projected global warming increases [48]. Higher probabilities of self-reported anxiousness, hopelessness and meaninglessness have also been strongly linked to increases in temperature variability, another effect of climate change [58].

The pathogenesis for depression is complicated, partially understood and multifactorial, including genetic, psychosocial and environmental factors [64]. The neurobiology of depression may be attributed to both structural and neurochemical changes in the brain, specifically as diminished prefrontal cortex regulation of the amygdala, hippocampus and hypothalamus coupled with dysregulation in the noradrenergic, cholinergic, dopaminergic and serotonergic pathways [64]. Studies have shown harmful environmental exposures from airborne pollutants such as diesel, carbon monoxide (CO), nitrogen oxides (NOx), sulfur dioxide (SO2), ozone (O3) and particulate matter can alter neurotransmitter function, both in the serotonergic and dopaminergic pathways [64,65,66], with implications for many psychiatric illnesses, including depression. Additionally, it is believed that exposure to stress, whether in acute climate change related events such as hurricanes or indirect effects from chronic climate change such as food insecurity or poverty, also increase the risk of depression [64]. Furthermore, climate change has direct neurobiological effects due to heat exposure and the subsequent increase in inflammation that is associated with greater likelihood of developing depression [64]. While there is a relatively large knowledge base examining climate change and the mechanisms by which cognition is affected, little research has been conducted examining the neurobiological implications climate change may have on depression. Future research should observe specific causal pathways of exposure to climate change-related events and neurobiological changes resulting in depression.

While less immediately catastrophic, minor changes to atmospheric composition can also have effects on mental health. Particulate matter such as ozone, sulfur dioxide and nitrogen oxide which are byproducts of climate change have been studied extensively. All have shown to be correlated with either a direct risk of suicide or with neuropsychiatric disorders that are associated with increased rates of suicidality, such as autism spectrum disorder, bipolar disorder, dementia and major depressive disorder [67,68,69,70]. The increased concentrations of these byproducts have been found under a variety of conditions, such as in those living near areas of high-traffic with several freeways, close to coal plants or oil refineries, or other sources of acute and chronic air pollution [68,69,70]. It is proposed that the pathophysiological mechanism behind air pollution and suicide is neuroinflammatory, as the air particulate matter acts as an irritant that generates local and systemic inflammatory responses, which can lead to increased rates of suicide, as well as vascular and pulmonary disorders [71,72].

Rising temperatures have also been independently linked to suicide, with spring and early summer having been identified as the period for greatest temperature-associated suicide risk [73,74]. A 1 °C increase in daily mean temperature was found to increase daily suicide risk by 1.4% in Republic of Korea [75]; in Taiwan, a 1 °C mean temperature increase led to a 1.8%, 2.34% and 1.45% increase in total, male and female suicide death numbers, respectively [76]. Studies have demonstrated that the relationship between temperature and suicide appears to remain stagnant, suggesting limited adaptation despite a consideration of factors including rising income, utilization and availability of air conditioning, or accessibility of mental health services [25,56]. One study also reported that economic factors such as unemployment and labor force participation were insignificantly related to completed suicides, suggesting a stronger role for climactic variables [76], while another study in India found the effect of temperature on annual suicide rates to be significant only during the growing season [54]. Similarly, droughts have been associated with increased suicide risk in farmers [73], as they may interfere with the livelihood of these people and thus increase psychological stress. Thus, while many physiological and behavioral mechanisms such as maladaptive serotonin receptors and disruption of sleep may be the primary basis for increased suicidality [56,77], it is integral not to undervalue the importance of economical mediators, particularly in vulnerable populations whose livelihoods are closely linked to changes in weather.

Several neurochemical endophenotypes including serotonin have long been known to be associated with emotion and behavior, including suicidal behavior. It is hypothesized that hopelessness, neuroticism, impulsivity and aggression have been associated with alterations in serotonin which are correlated with risk of completed suicide [78]. Both the noradrenergic and dopaminergic systems are maladaptive in depression, but the role of these in suicide is still unknown [66]. There is also a body of evidence that supports that stress and hypothalamic-pituitary-adrenal (HPA) axis dysregulation may play a large role in suicidal behavior [66]. Regarding climate change specifically, air pollution may cause hypoxia, which can dysregulate a variety of neurobiological systems, including the production of serotonin [64]. Studies have linked a correlation between hypoxia and suicide [78] and exposure to air pollutants (particulate matter, specifically) were consistently correlated with an increased prevalence of completed suicide [79]. This supports the hypothesis that exposure to air pollutants not only increases the risk of depression but also of completed suicides. These findings also provide evidence of possible neurobiological underpinnings.

In addition to internalizing mental health outcomes, it is worth discussing externalizing behaviors associated with these conditions. Although less studied, externalizing behaviors, maladaptive actions that are directed toward the external environment, remain an important area of research, as changing climate can cause feelings of isolation and related stressors that can trigger aggressive behavior. Historically, the literature has focused more on how climate change affects collective unrest, aggression and violence amongst communities, rather than an individual’s externalizing behaviors. For instance, severe weather events (e.g., droughts) have been linked to societal conflicts and wars through stressors such as resource scarcities, forced displacements and loss of livelihood [80]. Even the chronic, more incremental changes of climate change are strongly linked to events of human conflict around the world: each standard deviation of change in climate towards higher temperatures or more extreme rainfall have been shown to increase frequency of intergroup conflict by 14% [81].

While the societal impacts of climate change on aggression are important, the focu was on outcomes that reflected individual mental health outcomes. Like internalizing behavioral disorders, externalizing behavioral disorders occur significantly more in those exposed to acute climate-related disasters [82]. Studies directly measuring externalizing behavioral disorders were limited. Using violent crime as a proxy for individual aggression, one study found that over 16 year period, monthly mean temperature and violent crime rates were strongly correlated [57]. Measuring peripheral serotonin receptor densities in a small group of healthy males (N = 18, mean age 31.4 y) and male violent offenders (N = 33, mean age = 34.5 y), there also appeared to be a correlation between ambient temperature and peripheral serotonin receptor densities, which itself also correlated negatively with the incidence of violent crime. The authors thus proposed that effects of ambient temperature and individual violence may be mediated by a physiologic mechanism involving serotonergic pathways, also suggested elsewhere [83,84,85], and thus emphasizes the consideration of physiological effects on individual aggression in association with global climate change. Another study suggested a positive association between daily temperature and homicide rates in South Africa. A one-degree Celsius increase in same-day maximum temperature was associated with a 1.5% increase in homicides. South Africa has one of the highest homicide rates on record at an estimated 36 per 100,000 individuals, roughly six times the global average. South Africa is also experiencing climate warming at a faster rate than the global average, which heightens the importance of understanding potential relationships between temperature and adverse health and social outcomes such as homicide.