Introduction
Climate change presents an evolving global healthcare crisis with the adverse impact of air pollution, extreme weather conditions (temperature, wind, humidity) and environmental hazards having both direct and indirect impacts on communicable and non-communicable diseases.(1) Furthermore, changing weather conditions adversely affect food and water security, particularly in low-income countries, where the most vulnerable populations are disproportionately affected.(2)
Rapidly changing climatic conditions and extreme weather events promote the transmission of airborne, waterborne, foodborne, and vector-borne pathogens, possibly undermining decades of progress made towards eradicating certain infectious diseases.(2) Air pollution and seasonal variations in temperature and environmental humidity are associated with increased transmission of respiratory viruses such as influenza.(3,4) It remains unclear whether the same environmental factors impact the spread of severe acute respiratory distress syndrome coronavirus 2 (SARS-Co-V2), the aetiological agent of the current COVID-19 pandemic, which has highlighted the urgency of understanding the established and impending effect of our climate crisis on environmental and global human health. In this review we provide an outline of the impact of climate change on the emergence of new pathogens, including SARS-CoV-2 and detail the lessons learnt through the ongoing COVID-19 pandemic for the prevention of future pandemics.
Climate change and the emergence of new pathogens
A pathogen is defined as any biological agent, regardless of size or taxa, capable of causing a disease.(5) Viruses and bacteria are the typical causes of plagues and pandemics, however the emergence of new pathogens that have never been seen in humans before can pose a greater threat to humanity. A recent review by Mora et al, demonstrated 277 individual pathogens that cause infectious disease in humans that are directly exacerbated by environmental hazards.(5) The emergence of a human pathogenic disease depends not only on the interplay of biological, ecological, environmental and social factors, but also on the degree of contact between the pathogen and the person, as well as the magnitude to which an individual's resistance is reduced and pathogen strengthened by the climatic hazards.(5) Geographical shifts of species brought about by climate change brings people closer to pathogens and vice versa.
SARS-CoV-2, avian influenza, HIV and Ebola are viruses that have overcome pre-existing natural barriers to infect other animal species.(6) This is known as the zoonotic spill over theory and these natural barriers can be suppressed or enhanced by climate change.(6) The effects of climate change on the ecosystem is complex, with multiple factors contributing to the emergence and transmission of pathogens (Figure 1). The threat of novel infections in humans, brought about by climate change, highlights the urgent need to better understand the relationship between humans, the environment and the transmission of pathogens, so that action can be taken to reduce the impact of climate change on civilization.
The effect of climate change on the emergence of pathogens. Various forms of climatic hazards influence the transmission and emergence of new pathogens. Climate change modifies factors of the environment, host and pathogen. This ultimately leads to adverse health and social related outcomes, including the emergence of new pathogens
The impact of climate change on the environment
Pathogens are brought closer to humans by climatic hazards including warming, precipitation changes, drought, heatwaves, wildfires, storms and floods that cause habitat disruptions.(5)These climatic changes may cause an expansion of vectors implicated in the outbreak of various pathogens, with potential spill over from viruses into human populations.(5) These included the West Nile virus, Zika virus and Ebola to name a few.(5) Direct and foodborne transmission of norovirus, hepatitis, and hantavirus have been associated with floods whilst exposure to once-frozen pathogens that have accumulated with time can occur with warming and thawing of permafrost.(5)
The impact of climate change on the pathogen
In the systematic review by Mora et al, 24% (63/286) of infectious diseases were diminished with climate change. The spread of viruses, including SARS-COV-2, influenza, SARS, rotaviral and noroviral enteritis, were reduced with warming. However, most diseases display variability with climatic hazards whereby differences in climate can either aggravate or diminish a disease.(5)Climatic hazards may enhance the ability of a pathogen to survive and increase its virulence by upregulating gene expression of proteins affecting transmission.(5)
Viruses and bacteria are usually the causative pathogens in pandemics. However, the ability of organisms to undergo evolutionary mechanisms of thermotolerance to elevated temperature caused by climate change can ultimately lead to an increase in new pathogens and potential outbreaks of existing organisms that not only have an impact on humans, but also other animals, crops and native plants.(7)
The impact of climate change on pathogen transmission
Disease transmission occurs through direct or indirect mechanisms. Changes in climate patterns may directly influence the viability of certain pathogens whilst an indirect impact may occur if a change in human and vector (or host) behaviours results in a change in transmission route.(8)
Transmission of disease may be adjusted by alterations in temperature or rainfall.(2,9,10) Transmission of infectious diseases may also be altered by strong winds and dust storms as airborne pathogens and viruses can be transported by winds and spread from endemic to non-endemic regions.(8) During dust storm seasons, downwind Japanese and South Korean regions are affected disproportionately by avian influenza outbreaks.(11) Similarly, human influenza virus could be carried by prevailing winds over the Pacific Ocean to America, from Asia.(12)
Climate variability can result in altered human-pathogen, human-vector or human-host contact patterns and thus transmission of diseases. An example of this is that during heavy rainfall or floods, deer mice invade human residences for food and shelter, altering human-pathogen-rodent contact patterns. This results in an increased transmission of Hantavirus to these humans and increased cases of Hantavirus pulmonary syndrome.(13)
Patterns of disease transmission can be influenced by changes in human behaviours due to climate change. These include migration, seasonal work and summer/winter recreational activities.(14,15) European winters result in people spending longer hours indoors and are associated with seasonal patterns of influenza.(16) Holiday season live poultry markets serve as origins of human infected avian influenza. Transmission of these viruses may be related to interaction with migratory birds.(17,18)
The potential impact of climate change on COVID-19
The link between COVID-19 and climate change is unclear. Due to the recent onset and progression of the pandemic, much is still to be learnt on long-term effects of climate change on COVID-19.(19)The COVID-19 pandemic was likely caused by a spill over event from animals, and its possible interactions with air pollution is a reminder that we are a part of a larger ecosystem, with our health closely connected with that of our environment.(3)
Habitat disruption and climate change may play a role in forcing species to shift habitat and change their geographical range, thereby bringing animals into closer contact with livestock and humans which can serve as a vehicle for zoonotic transmission.(20) In Wuhan, China, the initial epicentre of the COVID-19 pandemic, bats and pangolins are hypothesised as vectors for coronaviruses as they are housed in crowded, open markets in close and often sordid conditions.(20) Climatic hazards caused by climate change therefore bring pathogens closer to people and bring people closer to pathogens. It may also strengthen a pathogen's ability to cause disease, as well as impair an individual's ability to resist infection.(5)
The impact of climate change and the transmission of SARS-COV-2
It has been postulated that ambient air pollution may play a role in the transmission of COVID-19, with air particles serving as carriers for virus-laden respiratory particles.(3) However, when analysing their relative sizes and concentrations in outdoor air, it was thought to be unlikely that air pollution particles play a role as virus carriers in the transmission of SARS-CoV-2.(3) However, long-term air pollution exposure can lead to a chronic airway inflammatory response, even in young healthy individuals, and this can contribute to rapid and high COVID-19 transmission.(19) Particulate matter with diameter 2.5 m (PM2.5), ozone (O3), and nitrogen dioxide (NO2) levels in ambient air have a strong correlation with COVID-19 outbreaks and an increased risk of COVID-19 infection is associated with exposure to higher air pollutant levels including PM2.5, NO2, SO2, and O3, particulate matter with diameter <10 µm (PM10) and carbon monoxide (CO).(19,21)
Temperature was initially thought to have an influence on COVID-19 transmission.(19) SARS-COV-2 is primarily transmitted by aerosols and the apparent limited spread of COVID-19 in tropical climates, thus far, has suggested that high temperatures in combination with high humidity may inhibit the aerosol route of spread. It has been hypothesised that higher temperatures and humid climates would lead to a reduced transmission rate with SARS-COV-2, largely because some other viruses in the same family (respiratory syndrome viruses) do not survive in such conditions.(19,22) Investigations, however, discovered no significant correlation between temperature and transmission with COVID-19. Furthermore, it is no longer considered that warm weather, per se, curbs the spread of COVID-19.(19,22) It has been noted that arid and tropical climate conditions were less conducive to viral spread so there is possibly an interactive effect of daily temperature and relative humidity on COVID-19 incidence.(19) Previous studies have reported that a low humidity favoured COVID-19 transmission and high absolute humidity slows down transmission, however this remains to be confirmed.(19)
Access to clean and safe water is inseparable from climate change and integrally related to limiting the spread of the COVID-19.(20) Rainfall, however, has not been shown to have any direct effect on COVID-19 transmission, while there are conflicting reports regarding the effect of wind.(19)
Does Climate Change Worsen the Severity of COVID-19?
Whilst climate change appears to influence the transmission of SARS-CoV-2 there may also be an association between the changing climate environment and COVID-19 severity, including increased rates of ICU admission and mortality. It is well known that climate change has a significant impact on health and an impact on both communicable and non-communicable disease, including cardiovascular and renal disease as well as respiratory diseases.(23,24) Pollution-related diseases accounted for 16% of deaths globally in 2015, triple the number of deaths from malaria, tuberculosis and HIV/AIDS combined, with 92% of these deaths occurring in low- and middle-income countries. Importantly, 71% of these pollution-related diseases are non-communicable, including deaths from cardiovascular disease, chronic obstructive pulmonary disease (COPD) and lung cancer.(24) This is important to note in the context of COVID-19 given the association between these non-communicable diseases and COVID-19 severity.(25)
Air pollution appears to be associated with mortality in COVID-19, with increased mortality rates in areas with higher levels of air pollution measured as fine particulate matter, PM2.5.(1,26) An American study, assessing county-level data of COVID-19 mortality and PM2.5, showed a positive correlation with mortality rates and increasing concentration of PM2.5, an association that remained when accounting for several confounding factors. Similarly, in a Dutch study, cases of COVID-19 were significantly higher in municipalities with higher concentrations of PM2.5 and there were greater numbers of admissions and deaths due to COVID-19.(26)
This increased severity may be due to both a direct effect on the respiratory system, with impaired immune function and increased receptor expression leading to increased uptake of SARS-CoV-2 by respiratory epithelial cells, as well as an indirect effect given the increase in COPD, diabetes mellitus and hypertension associated with air pollution.(1,25)
Climate Change and COVID-19 in the South African context
South Africa (SA) continues to be one of the major suppliers of mineral commodities and ranks in the top 20 CO2 emissions per capita globally and has amongst the highest per capita emissions in the developing world.(27) SA's greenhouse gas (GHG) emissions have increased by 14.2% since 2000, with the energy sector contributing 80% to this increase.(27) The devastating impact of climate change on many African countries including SA is already obvious and continuing this path without transitioning to a low carbon society, will have overwhelming consequences. Unfortunately, the brunt of this environmental catastrophe will be born predominantly by impoverished communities. South Africa was exposed to a prolonged drought since 2013, leading to eight provinces declaring drought as a disaster between 2016-2017, with the consequent wildfires having a ravaging impact, costing government at least R 3 billion.(27,28) The question is what is being done to facilitate this transition to a low carbon society and how are we currently doing?(29)
Coal is a major contributor to GHG emissions, however a major challenge in shifting to a low carbon society is South Africa's economic dependence on this resource.(28) Coal remains SA's primary energy source and still remains the highest earner in SA's mining revenue contributing 21.4% (R 130.57 billion) in 2020.(30) Renewable energy is becoming a more sustainable option with solar, wind, hydropower and geothermal heat as potential natural carbon sinks. An increase in renewable energy uptake was noted in 2017, contributing 20% to the total South African energy supply.(27)
The challenges faced with climate change have been intensified by the COVID-19 pandemic, which exacerbated the existing inequalities and there is no doubt that change is necessary. A just green transition will be challenging as millions of jobs rely on the country's primary energy source, coal. The just transition to a low carbon society requires integration of national interests and a financially sustainable climatic model.(28,29)
What lessons have we learnt from COVID-19
The central theme from the COVID-19 crisis was the similarity shared by COVID-19 and climate change on the effect on global microeconomic fundamentals. This should serve as a reality check for the revision of our global climate policy, with its neglect coming at an extremely high cost to society.(31) COVID-19 has served as an acid test of what we as society may be faced with in the future. The main difference being that the outcomes during the COVID-19 pandemic were accelerated and tangible as opposed to climate change which will happen over decades, albeit with the potential for the same devastating effect on society.(32)
Some of the main lessons learnt from the enforced change in our social behaviour during the hard lockdown was the marked reduction in GHG emissions.(33) Unfortunately, this goal was realised in the most socially expensive way - a significant macroeconomic cost to the global community with the low- and middle-income countries bearing the brunt.(34) Another inadvertent benefit of COVID-19 was that the human species, the ultimate parasite, was forced by government policy and fear into reducing the demands we place on nature, and paradoxically making the world a safer place to live in. COVID-19 served to remind us of the drain we place on nature globally. Climate change and COVID-19 share one of the most striking similarities in that both entities are transboundary phenomenon with pollution originating in one country but able to cause damage in another country's environment.(35)
However, one of the main differences between climate change and the COVID-19 pandemic was the tangible loss we experienced – the immediate mortality and morbidity within such a short period of time being unacceptably high. Despite our advances, with the industrial revolution in its fourth iteration, we were unable to prevent the death from COVID-19, just as we were helpless during the 1918 Spanish flu pandemic.(36) The most important lesson we learnt from COVID-19 was that the cost of inactivity by some governments had devastating and irreparable consequences to their population. Similarly, the unperceived cost of inaction for climate change will be much higher to the global community.
Unfortunately, in the peak of the COVID pandemic we saw human nature rearing its ugly head again. Except for pockets of humanitarians, the self-preservation of wealthy nations at the expense of the low-income counties was plain for all to witness. Likewise, climate change will not affect the global community equally. Low income and resource limited communities who suffered the most from COVID-19 lockdowns, with rising unemployment and unexpected medical costs, will once again bear the brunt of climate change.(35) Rising global temperatures, changes in precipitation, extreme events, and rising sea levels will have different and unequal effects on various communities. COVID-19 has taught us that we must have a contingency plan for the most vulnerable in our community - the elderly, the sick and the poor.(37) They must be at the centre of all agreements and policies to minimize the impact of this social inequality when another crisis strikes.(34)
Conclusion
The COVID-19 pandemic has not only exposed most countries’ inefficient public healthcare system and disaster management plan, but it has exposed human vulnerability to the extensive burden of disease. Furthermore, the COVID-19 pandemic together with climate change has the potential of significant environmental, social and economic consequences and should be considered a global emergency. The main difference between climate change and the COVID-19 pandemic is that the global, as well as individual effects were accelerated and tangible as opposed to climate change which will happen over decades. Climatic hazards can aggravate the transmission and emergence of new pathogens leading to numerous adverse health related outcomes. Multidisciplinary research with a focus on bridging epidemiologists and climatologists is critical in developing a sustainable risk assessment model, offering an opportunity for a “Greener Future.”