**1. Introduction**

The effect of human activity on observed changes to the climate system over recent decades is widely acknowledged and is a global cause for concern. Anthropogenic (man-made) climate change has led to a rise in annual global mean temperatures since pre-industrial times, with more rapid increases since the mid-1900s [1]. As well as changing weather patterns, increasing average temperatures and, potentially of more concern, is the increase in the frequency of extreme weather events which can have enormous human cost [2]. Climate change is seen as an example of a tragedy of the Commons [3], whereby it is in the interests of individuals to benefit from human activity but the overall impact on all people collectively will be negative unless there is an agreed intervention. It is generally the case that the largest impacts on health are realised in developing regions of the world such as the tropics, whereas the greatest contributors to greenhouse gas emissions are often developed countries

which do not suffer the consequences of extreme events to the same extent [4]. Due to the lifetime of greenhouse gases such as carbon dioxide in the atmosphere, and the timescales associated with ocean warming, even if global CO2 emissions were curtailed immediately, the effects on the earth's climate, including increasing temperatures and sea level rise would continue for a number of decades before starting to plateau [2]. However, this should be seen as a general call to action to reduce emissions as soon as possible, given that the impacts are likely to extend beyond current conditions and there are indications that economic investment now will be likely to reduce costs later [5]. There is also a need to develop adaptations to cope with changes in climate. Recent research highlighting the health benefits of limiting future temperature rises to the more ambitious target of 1.5 ◦C rather than 2 ◦C, in line with the Paris agreement of 2016, further emphasises the need to limit emissions [6].

Climate change affects health in a number of ways, and the impacts vary both geographically and between different populations. A growing and ageing population in much of the world means that the proportion of the population who are vulnerable to the effects of climate change will increase in the future [7]. The most direct impacts from climate change are from the effects of high temperatures, and from acute impacts relating to extreme events such as storms, floods and heatwaves. These physical or meteorological stressors can produce direct health effects, such as physical injury, illness, or mental health impacts due to the consequences of the aftermath. In places where infrastructure or adaptation measures are poor, the impacts will be more severe [2]. In addition to these types of impact on health, climate change is likely to modify or mediate existing health effects and exacerbate inequalities through a number of indirect pathways. These more indirect effects on health occur through climate interactions with ecosystems, water, biodiversity and land use changes. Climate change can lead to environmental degradation; can affect food and water availability and quality; and increase risks to health from pathogens, vectors and infectious diseases [8]. Civil conflict or mass movement of people may be partly driven by environmental degradation and can further increase risks to health. There is evidence to suggest that climate change can be a driver for civil war [9,10].

Waterborne and water-related diseases are sensitive to environmental conditions, some or all of which are likely to be affected by climate change. For example, climate change is likely to lead to changes in the frequency of heavy rainfall events, storms and drought periods [2], melting of polar ice and glaciers, warming and thermal expansion of the oceans causing sea level rise [11], and melting of permafrost, which may contribute to further warming [12]. Changes in interactions between the water cycle and the climate system will modify the risk from waterborne diseases from these physical impacts, as well as from the resulting risk of famine, water shortages, decreased water quality, increasing habitat for mosquitoes, alterations to seasonality of diseases and contaminated recreational waters. However, health impacts of waterborne disease over the longer term may be secondary to other health effects associated with other water issues (e.g., shortage, flooding, famine, the economy, sea level rise and war).

Tackling the climate change problem has focussed on mitigating the effects of greenhouse gas emissions through the Intergovernmental Panel on Climate Change (IPCC), through cross government international agreements on reducing carbon emissions and by providing reliable scientific evidence and reports. While the approach has generally been to reduce worldwide greenhouse gas emissions, the reductions are likely to be slow and work on adaptation strategies to deal with the climate change associated with overall increases in temperature is also being undertaken.
