Climate change poses a wide range of risks to population health – risks that will increase in future decades, often to critical levels, if global climate change continues on its current trajectory. The three main categories of health risks include: (i) direct-acting effects (e.g. due to heat waves, amplified air pollution, and physical weather disasters), (ii) impacts mediated via climate-related changes in ecological systems and relationships (e.g. crop yields, mosquito ecology, marine productivity), and (iii) the more diffuse (indirect) consequences relating to impoverishment, displacement, resource conflicts (e.g. water), and post-disaster mental health problems. Climate change thus threatens to slow, halt or reverse international progress towards reducing child under-nutrition, deaths from diarrheal diseases and the spread of other infectious diseases. Climate change acts predominantly by exacerbating the existing, often enormous, health problems, especially in the poorer parts of the world. Current variations in weather conditions already have many adverse impacts on the health of poor people in developing nations,and these too are likely to be multiplied by the added stresses of climate change. A changing climate thus affects the prerequisites of population health: clean air and water, sufficient food, natural constraints on infectious disease agents, and the adequacy and security of shelter. A warmer and more variable climate leads to higher levels of some air pollutants. It increases the rates and ranges of transmission of infectious diseases through unclean water and contaminated food, and by affecting vector organisms (such as mosquitoes) and intermediate or reservoir host species that harbour the infectious agent (such as cattle, bats and rodents). Changes in temperature, rainfall and seasonality compromise agricultural production in many regions, including some of the least developed countries, thus jeopardising child health and growth and the overall health and functional capacity of adults. As warming proceeds, the severity (and perhaps frequency) of weather-related disasters will increase – and appears to have done so in a number of regions of the world over the past several decades. Therefore, in summary, global warming, together with resultant changes in food and water supplies, can indirectly cause increases in a range of adverse health outcomes, including malnutrition, diarrhea, injuries, cardiovascular and respiratory diseases, and water-borne and insect-transmitted diseases. Health equity and climate change have a major impact on human health and quality of life, and are interlinked in a number of ways. The report of the WHO Commission on Social Determinants of Health points out that disadvantaged communities are likely to shoulder a disproportionate share of the burden of climate change because of their increased exposure and vulnerability to health threats. Over 90 percent of malaria and diarrhea deaths are borne by children aged 5 years or younger, mostly in developing countries. Other severely affected population groups include women, the elderly and people living in small island developing states and other coastal regions, mega-cities or mountainous areas.

Psychological impacts

• Direct - Acute or traumatic effects of extreme weather events and a changed environment
• Indirect - Threats to emotional well-being based on observation of impacts and concern or uncertainty about future risks
• Psychosocial – Chronic social and community effects of heat, drought, migrations, and climate-related conflicts, and postdisaster adjustment

Extreme weather events

Diseases

Malaria

Increased precipitation like rain could increase the number of mosquitos indirectly by expanding larval habitat and food supply. Malaria kills approximately 300,000 children (under age 5) annually, poses an imminent threat through temperature increase . Models suggest, conservatively, that risk of malaria will increase 5-15% by 2100 due to climate change.In Africa alone, according to the MARA Project (Mapping Malaria Risk in Africa), there is a projected increase of 16–28% in person-month exposures to malaria by 2100.

Non-climatic determinants

Sociodemographic factors include, but are not limited to: patterns of human migration and travel, effectiveness of public health and medical infrastructure in controlling and treating disease, the extent of anti-malarial drug resistance and the underlying health status of the population at hand. Environmental factors include: changes in land-use (e.g. deforestation), expansion of agricultural and water development projects (which tend to increase mosquito breeding habitat), and the overall trend towards urbanization (i.e. increased concentration of human hosts). Patz and Olson argue that these changes in landscape can alter local weather more than long term climate change. For example, the deforestation and cultivation of natural swamps in the African highlands has created conditions favourable for the survival of mosquito larvae, and has, in part, led to the increasing incidence of malaria. The effects of these non-climatic factors complicate things and make a direct causal relationship between climate change and malaria difficult to confirm. It is highly unlikely that climate exerts an isolated effect.

Environment

Climate change may dramatically impact habitat loss, for example, arid conditions may cause the deforestation of rainforests, as has occurred in the past.

Temperature

A sustained wet-bulb temperature exceeding 35°, is a threshold at which the resilience of human systems is no longer able to adequately cool the skin. A study by NOAA from 2013 concluded that heat stress will reduce labor capacity considerably under current emissions scenarios. The freshwater resources that humans rely on are highly sensitive to variations in weather and climate. In 2007, the IPCC reported with high confidence that climate change has a net negative impact on water resources and freshwater ecosystems in all regions.The IPCC also found with very high confidence that arid and semi-arid areas are particularly exposed to freshwater impacts. As the climate warms, it changes the nature of global rainfall, evaporation, snow, stream flow and other factors that affect water supply and quality. Specific impacts include:

• Warmer water temperatures affect water quality and accelerate water pollution.
• Sea level rise is projected to increase salt-water intrusion into groundwater in some regions. This reduces the amount of freshwater available for drinking and farming.
• In some areas, shrinking glaciers and snow deposits threaten the water supply. Areas that depend on melted water runoff will likely see that runoff depleted, with less flow in the late summer and spring peaks occurring earlier. This can affect the ability to irrigate crops. (This situation is particularly acute for irrigation in South America,for irrigation and drinking supplies in Central Asia, and for hydro power in Norway, the Alps, and the Pacific Northwest of North America.)
• Increased extreme weather means more water falls on hardened ground unable to absorb it, leading to flash floods instead of a replenishment of soil moisture or groundwater levels.
• Increased evaporation will reduce the effectiveness of reservoirs.
• At the same time, human demand for water will grow for the purposes of cooling and hydration.

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