The health impacts of climate change
What do medical students need to know?
Climate change endangers human wellbeing on an almost inconceivable scale. It is the biggest global health threat of the 21st century. It affects health directly—for example, through extreme weather events—and indirectly, by affecting systems on which health depends, such as the food and agriculture system. This brief review describes some of the key effects, and it is largely based on the recently published report on impacts, adaptation, and vulnerability from the Intergovernmental Panel on Climate Change (IPCC). It is important to understand the current and potential impacts of climate change for two main reasons—to motivate action to prevent further climate change (mitigation) and to inform action to minimise harms from climate change that does occur (adaptation). Healthcare professionals have an important role in both of these areas. They will need to adapt to working in a different climate while leading a shift to a more sustainable low carbon way of working and living.
The current warming of the climate system is unequivocal and unprecedented. The atmosphere and ocean have warmed by about 0.8°C since the 19th century, large amounts of snow and ice have melted, and sea levels have risen. The largest contribution to the “radiative forcing” that drives this warming system is increasing amounts of carbon dioxide in the atmosphere. The influence of humans on this system is clear.
When considering the future effects of climate change it is important to understand that climate change depends on the total global cumulative emissions of greenhouse gases. To describe the scale of anticipated impacts it is necessary to think about different emission pathways. Impacts depend not only on the changing climate and efforts to avoid further greenhouse gas emissions, but also on many other environmental and social changes that influence how exposed and vulnerable people are to climate change (box 1).
Box 1: Tips for understanding the health effects of climate change
- Be clear in simple terms about which climate projection is being used. Is it a high emissions, business as usual, worst case scenario? Or is it a medium or low emissions scenario that depends on reducing emissions compared with our current path?
- Frame timescales in relation to yourself—for example, in relation to your retirement date or a specific birthday
- Ask who is most at risk? What can be done to help these people?
- Don’t focus solely on well described risks but try to understand what the biggest risks are likely to be
Climate change will affect many of the systems on which health depends
The fundamental systems on which health depends are threatened by climate change. These include systems related to food and water. It is anticipated that climate change will have a substantial negative impact on food supply and nutrition, notably increasing stunting in children and child deaths related to undernutrition in developing countries. One model (based on a high emissions scenario) estimated that we will see 25 million additional undernourished children under 5 years old in the developing world between 2000 and 2050 unless we improve socioeconomic conditions to compensate. A greater proportion of the global population is expected to experience water scarcity as climate change worsens. The quality of water available for drinking is projected to deteriorate, and competition between sectors for water is likely to increase. Under a high emissions scenario, drought will probably become more common in dry subtropical areas.
The systems on which health depends are also affected by a wide range of other social and environmental changes, which need to be taken into account (box 2).
Box 2: Other planetary boundaries
Climate change is just one of nine proposed environmental “planetary boundaries” inside which humans can safely live. The other boundary most closely related to climate change (and mentioned in the Intergovernmental Panel on Climate Change reports) is ocean acidification, which is caused by carbon dioxide dissolving in the oceans. Ocean acidification has substantial effects on marine ecosystems, particularly coral reefs. The effects on fisheries are complex and difficult to predict. Fish provides more than 2.9 billion people with almost 20% of their animal protein intake, and many coastal economies depend on the marine environment (for example, for tourism or fishing). Ocean acidification could therefore have substantial effects on health. This illustrates the importance to health of developing a better understanding of the risks associated with the other planetary boundaries and considering a range of environmental and social changes together when preparing to protect and improve health in the future.
Extreme weather will make it harder to live and work in many parts of the world
More adverse weather is expected, including heat waves, floods, and extreme storms. Temperatures across the world will not increase uniformly. Some areas may see little temperature rise, whereas others will see substantial increases. Deaths, disease, and injuries from heat waves and fires are projected to increase as temperatures increase. Deaths related to heat waves will probably outweigh reduced mortality related to cold weather, especially in developing tropical countries. In these countries, worker productivity is also likely to drop owing to heat stress. Extreme weather not only affects health directly but also jeopardises key infrastructure, including energy and water supply and emergency and health services. Adverse weather events, such as floods, are associated with increased mental health problems.
Effects on the distribution and incidence of diseases with infectious and environmental causes
The incidence and range of infectious diseases are likely to change. It is anticipated that climate change will increase the incidence of malaria globally. The incidence of other infectious diseases, such as some types of tickborne encephalitis, is also predicted to increase, but it is more difficult to attribute these rises to climate change. Rates of diarrhoeal disease are projected to increase, particularly in developing tropical countries. The incidence of diseases such as asthma and allergic rhinitis, which are related to aeroallergens, may increase because warmer weather generally increases the level of many allergens. Air pollution, such as that caused by low level tropospheric ozone, will probably worsen.
Climate change may worsen conflict and increase numbers of displaced people
Climate change is projected to increase the displacement of people across the world. Migration could be in response to an acute event such as an extreme storm or a gradually changing environment; it could also be an elective adaptation strategy. Forced displacement is associated with a range of health risks including poor nutrition, infectious diseases, and mental health problems. Climate change can increase the risk of conflicts such as civil wars by exacerbating known drivers of these such as poverty. Climate change is increasingly recognised in national security reviews.
Not everyone is affected equally
Climate change is a global injustice. Its effects are not spread evenly across the world. Although the wealthiest people and countries are most responsible for causing the problem, the poorest are often affected the most. In the developing world, effects such as malnutrition and malaria affect the youngest disproportionately. Elderly people are at higher risk from adverse weather conditions, and those with chronic diseases such as cardiovascular or respiratory diseases may also be at higher risk. It is important that health professionals understand vulnerability so that they can identify those most at risk and help protect them from harm—for example, in a heat wave. Many of the above risks cluster together. For example, cities in the developing world may be at risk of urban heat island effect, air pollution, and water shortages. These, combined with vulnerability owing to poverty and poorly developed adaptation measures, could result in disproportionate harms to health.
Adaptation and mitigation must go hand in hand
Adaptation is place and context specific. It is becoming increasingly apparent that our ability to adapt will have a limit. There is a need to avoid as much climate change as possible but also to adapt to the change we are already seeing. Countries are starting to develop national adaptation plans, which set out how they will avoid some of the worst harms, with additional specific plans related to certain risks. For example, England has a national adaptation programme and a specific plan for heat waves (box 3).  England also has a sustainable development strategy for the NHS, public health, and social care. It is the first to bring together a vision for both mitigation of, and adaptation to, climate change with a broader sustainable development agenda across the whole health and care sector.
Box 3: Heat wave plan for England
This plan aims “to prepare for, alert people to, and prevent, the major avoidable effects on health during periods of severe heat in England.” It includes five stages for action, from long term planning to a national emergency. It sets out actions for health professionals and health and care organisations at each level. For example, it recommends establishing systems to identify and, when necessary, contact and take action to protect people at high risk.
Risk factors include:
- In the community: over 75 years old, female, living on own and isolated, severe physical or mental illness, multiple medications and overexertion; urban areas, south facing top flat; alcohol or drug dependency, homeless; babies and young children
- In care home or hospital: over 75 years old, female, frail, severe physical or mental illness, multiple medications; babies and young children (hospitals)
Actions to reduce greenhouse gases often have direct local health benefits, and these can be combined with actions to adapt to climate change so that local communities become more sustainable, more resilient, and most of all healthier. The next 15 years will determine which climate pathway the world follows, and which of the effects described above will be seen and which will be avoided within the working lives of today’s health professionals in training (box 4).
Box 4: Key actions for health professionals
- Learn more about the anticipated effects of climate change in your region, such as national and regional risk assessments, and support local planning processes to help mitigate them
- Learn how to manage patients affected by these (for example, heat stroke, dengue fever, mental health effects of flooding, depending on location)
- Understand who is most vulnerable to these effects and what can be done to help these people
- Consider the impact that these effects might have on you and your ability to work and take action to prevent them
- Dare to be optimistic; imagine a different better future and work to create it
1Public Health England, London, UK
Correspondence to: James.Smith@phe.gov.uk
Competing interests: None declared.
Provenance and peer review: Commissioned; not externally peer reviewed.
Although the author works for Public Health England the views expressed in this review are the views of the author and do not necessarily reflect those of Public Health England
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Cite this as: Student BMJ 2014;22:g2874