Human activities have accelerated biosphere change on this planet. Key drivers of the climate crisis include the burning of fossil fuels and deforestation. As a result of these drivers, organisms have been moved around the planet, habitats have been fragmented, polluted, or entirely lost, and biodiversity has been watered down and, in some areas, vanished. The health impacts of these activities include amongst other things, increased respiratory and cardiac disease as a result of particulate matter (PM) in the air that we breathe, heat stress through rising global temperatures, and increasing infectious disease outbreaks arising from biodiversity loss.
These impacts are set out below.
Zoonoses, or diseases of an animal origin, comprise a large percentage of all newly identified infectious diseases as well as many existing ones. Around 60 per cent of human infections are estimated to have an animal origin.
Evidence suggests that the frequent emergence of novel zoonotic diseases is linked to the changing global environment including fragmentation of natural ecosystems, pollution, intensive livestock cultivation, and unsustainable and aggressive chemical-based agriculture.
Land use has changed significantly in the last 100 years with humans having used up over half of the terrestrial land on the planet. 75% of the land surface is significantly altered, 66% of the ocean area is experiencing increasing cumulative impacts, and over 85% of wetlands (area) has been lost. And as humans transform wild animal habitats, the likelihood of human-animal interactions increases which increases the chance of zoonotic disease emergence. 
Drivers of spillover include greater human and animal contact which likely arises as a consequence of livestock rearing, deforestation, and wildlife hunting and trade. 
Domestic animals play a significant role in the transmission of various diseases to humans and in many cases, they work as amplifiers of pathogens emerging from wild animals. 
It is important to note that infectious diseases arise from a combination of the above drivers that combine to amplify disease risk, rather than any individual driver acting in isolation. 
Significant associations have been observed between epidemics and deforestation mostly concerned the countries of the intertropical zone with high forest cover, such as Brazil, Peru, and Bolivia in South America, Democratic Republic of Congo and Cameroon in Africa, Indonesia, Myanmar, and Malaysia in Southeast Asia, among others. 
In particular, the increase in land areas converted to palm-oil plantations show a positive association with the number of vector-borne disease outbreaks. However, it is not only forest clearance that is responsible of outbreaks of infectious diseases, but also reforestation or afforestation, particularly in countries outside the tropical zone.
In addition, it has been shown that there is a positive relationship between the increase of cattle head and the number of outbreaks of human diseases from the period 1960–2019. 
The baseline expected annual mortality from viral disease epidemics with the current world population is 3.3 million lives. 
Vector-borne diseases, (largely those that are caused by the bite of infected insects like mosquitoes, ticks, and sandflies, which act as carriers, or ‘vectors’), account for more than 17% of all infectious diseases, causing more than 700,000 deaths annually.
For instance, dengue is the most prevalent viral infection transmitted by the Aedes mosquitoes. More than 3.9 billion people in over 129 countries are at risk of contracting dengue, with an estimated 96 million symptomatic cases and an estimated 40,000 deaths every year.
Hot weather temperature extremes
We are witnessing increased frequency and severity of heat waves globally, exposing life to elevated temperatures at extremes unlike anything recorded over the last 150 years.  Particularly in tropical regions, increased warming could mean that physiological limits related to heat tolerance (survival) will be reached regularly and more often in coming decades. 
The hottest years on record since the mid-19th century have nearly all occurred within the past decade.  Thermal stress associated with the heat waves during those warm years has directly, widely, and negatively affected all planetary life, including increased mortality of humans globally. It is proposed that the unusually extreme heat waves of the early 21st century will be the norm during summer into the late 21st century. 
The human body responds to heat stress in two primary ways: redistributing blood flow towards the skin (vasodilation) to improve heat transfer from muscles to skin and subsequently to the environment, and secreting sweat onto the skin, which subsequently evaporates and removes body heat. This physiological response can lead to increased cardiac demand and in those with pre-existing heart conditions, a compromised cardiac oxygen delivery. 
Dehydration is also a problem which can lead to decreased blood volume which exacerbates cardiovascular strain that also results in acute kidney injury and failure. High internal temperatures (39–40°C), combined with ischaemia (poor oxygen supply) and increased oxidative stress after blood redistribution, can cause cell, tissue, or organ damage, with the brain, heart, kidneys, intestines, liver, and lungs at the greatest risk. 
One of the measures of our ability to cope with extreme heat is the heat index. This is a measure of what the air temperature feels like to our bodies when relative humidity is factored in. In addition, there is the wet-bulb temperature, which is the lowest temperature to which an object can cool down when moisture evaporates from it. Once the wet-bulb temperature exceeds 35 degrees Celsius (95 degrees Fahrenheit), no amount of sweating or other adaptive behavior is enough to lower the body to a safe operating temperature. 
Climate models tell us certain regions are likely to exceed those temperatures in the next 30-to-50 years. The most vulnerable areas include South Asia, the Persian Gulf, and the Red Sea by around 2050; and Eastern China, parts of Southeast Asia, and Brazil by 2070. 
Analysis also tells us that those most vulnerable include residents in primarily urban areas with high housing density, less open space, and high proportions of elderly, minority populations, and lower income households. 
Heatwave exposure in the 2010s was largest for the low-income region and smallest for the high-income region (2.4 vs. 1.7 billion person-days per year). Thus, despite similar overall populations during this period, the poorest region observed over 40% higher exposure to heatwaves as compared to the richest region. 
This is mainly due to increasing heatwave trends in highly populated low-income regions, such as eastern India and Bangladesh. In fact, the lowest-quartile income region is expected to experience 1.8- to 5-fold higher heatwave exposure than each higher income region from 2060 to 2069. 
Populations living in a warmer climate are more adapted to cope with high temperatures, and more susceptible to cold weather. In the UK, without adaptation, heat-related deaths would be expected to rise by around 257% by the 2050s from a current annual baseline of around 2000 deaths, and cold-related mortality would decline by 2% from a baseline of around 41,000 deaths. 
However, given the speed of climatic changes and numerous physiological constraints, it is unlikely that human physiology will evolve the necessary higher heat tolerance, highlighting that outdoor conditions will remain deadly even if social adaptation is broadly implemented.
Unless global warming is curbed as a matter of urgency and appropriate adaptation measures are taken, about 350 million Europeans could be exposed to harmful climate extremes on an annual basis by the end of this century, with a 50-times increase in fatalities compared with now. 
One of the primary components of air pollution is particulate matter, which is fine soot that is produced through the combustion of fossil fuels. It is classified according to its size in microns. The smallest, PM 2.5, is about thirty times smaller than the width of a human hair. The diameter is so small that it is able to cross into the bloodstream from the lungs.
Exposure to PM 2.5 has been associated with increased risk of death due to cardiovascular disease, cerebrovascular disease, chronic kidney disease, COPD, dementia, type 2 diabetes, hypertension, lung cancer, and pneumonia. This burden is disproportionately borne by those from disadvantaged communities.
Risk remains elevated even at levels below the current European and North American standards and WHO guideline values. In fact, there is no safe level of exposure to PM 2.5. This also includes for exposure to nitrogen dioxide, and black carbon. 
Analysis of the impact of air pollution in England on future morbidity burden estimates that there could be an additional 1.3 million cases of disease attributable to PM 2.5 exposure between 2017 and 2035. This includes an additional 350,000 cases of CHD, and 274,000 cases of diabetes. 
Future premature mortality under projected future emission scenarios could be a possibility under stringent air pollution controls. 
The climate crisis has both direct and indirect impacts on human health, and we stand at a dire crossroads in terms of human health. Mitigation must come in the form of a global effort to reduce carbon emissions and halt deforestation; this will also require a rethink of how we feed our planet.
Nicola Oliver is Director of Life and Health at Medical Intelligence