We tend to think about climate change as an environmental problem.

But it's the impending impacts on our health that have medical experts sounding the alarm.

Last November, planetary health professor Tony Capon co-authored the first national report to track Australia's progress on climate change and human health.

It coincided with the release of a global report from leading medical journal The Lancet, which warned climate change is "the biggest global health threat of the 21st century".

"When we understand the connections between climate change and human health, it makes it clear that this is urgent," Professor Capon said.

Since then, calls for climate action from health bodies and medical professionals have grown louder.

In November, the World Health Organisation director general Tedros Adhanom Ghebreyesus warned the world could no longer "sleepwalk through this health emergency".

In April, some of Australia's leading health bodies published an open letter calling on political parties to recognise "the significant and profound health impacts of climate change to Australian people".

And last week, more than 1,000 doctors in the UK and 70 public health bodies in the US called for "radical action".

But how exactly does a warming climate pose risks to our physical and mental health?

Rising temperatures and heatwaves

In Australia, heatwaves cost more lives than all other natural hazards combined.

They lead to an increase in heat-related illnesses, such as heat exhaustion and heat stroke, and drive up hospital admissions and death rates, particularly among older people and people with chronic illnesses.

At a global level, 157 million more people were exposed to heatwave events in 2017 compared with 2000, according to The Lancet report.

Executive director of the Climate and Health Alliance Fiona Armstrong said heatwaves were getting "longer, hotter, and more frequent".

"They don't just affect people's health. They impact our power supply, transport systems, and water supply," she said.

Infectious disease

Even small changes in temperature, rainfall, and humidity can create the right conditions for infectious disease to spread, according to Professor Capon.

"One of the pathways is the changing distribution and abundance of mosquitoes," he said.

"In certain parts of the world, mosquitoes that transmit malaria are now able to breed at a higher altitude."

In Australia, changes to the distribution and abundance of mosquitoes has meant people are contracting dengue fever and Ross River virus in areas where they weren't previously at risk.

"There is also potential in a warming climate for higher rates of food-borne diseases, which are very sensitive to air temperatures," Dr Capon added.

The same goes for water-borne diseases, such as cholera, which can arise as a result of water scarcity (during droughts) and water pollution (during floods).

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Extreme weather events

If we already get droughts and floods, what's the big deal, you ask?

The latest IPCC report on climate change predicts that under warming of just 1.5 degrees Celsius, which is already a near certainty, both droughts and floods are likely to become more frequent and intense.

Extreme weather events can cause physical injuries, respiratory problems, psychological distress, and in some cases, death, Professor Capon said.

"The changing frequency, distribution, and intensity of extreme weather events has a range of health implications, both direct and indirect," he said.

Indirect consequences include food and water insecurity and the onset of mental illness, which can be exacerbated by the destruction of people's homes and livelihoods.

"With prolonged droughts and desertification, there's also concern about the availability of food, and prospects for famine," he said.

By some projections, climate change — if left unmitigated — is expected to result in a further 1.4 billion instances of people being exposed to drought per year, and 2 billion instances of people being exposed to floods by the end of the century.

Weather extremes can also lead to the displacement of millions of people. ( Unsplash )

Air pollution

People in more than 90 per cent of cities around the world are currently breathing air that is "toxic" to their cardiovascular and respiratory health, according to The Lancet's global report.

"Between 2010 and 2016, air pollution concentrations worsened in almost 70 per cent of cities around the globe, particularly in low-income and middle-income countries," the authors wrote.

"In 2015 alone, fine particulate matter was responsible for 2.9 million premature deaths, with coal being responsible for more than 460,000 of these deaths."

Closer to home, it's not just people living near coal-fired power stations that are affected, Fiona Armstrong said.

"People in our cities are being exposed to dangerous air pollution every day."

Mental health

While the causes of food insecurity are complex, climate change has already been shown to affect Australia's (and the world's) agricultural production.

"The changes in our prevailing weather patterns, whether it's increasing heat waves or longer, unprecedented and unrelenting drought, is really impacting our ability to grow food," Ms Armstrong said.

In addition to the direct damage to crops, research suggests rising temperatures can affect their nutritional quality.

Subsequent reductions in farm yields can lead to increased food prices — especially of fresh, healthy food — which can compound issues of food affordability and accessibility, most notably in low-income communities.

"This has impacts right across the population, and really affects people who are already struggling to access healthy food," Ms Armstrong said.

Perhaps more urgently, the declines seen in farming productivity pose challenges to rural community morale and the mental health of farmers and their families, Professor Capon said.

"In relation to the very deep drought we're having in Australia at the moment, we're concerned about the mental health impacts on farmers and farming communities," he said.

Last year's Australian report identified for the first time an association between mean annual maximum temperatures (driven up by climate change) and suicide rates across states and territories.

"In Australia, hot days have a damaging effect on whole-population mental health equivalent to that of unemployment and predict hospitalisation for self-harm," the authors wrote.

Health benefits of climate action

The silver lining to all this, according to Professor Capon, is the many health benefits that come from combating climate change.

"If we transition to more sustainable ways of living … then as well as reducing greenhouse gas emissions, there are health benefits from the reduction in toxic pollution from the burning of coal," he said.

Both the Lancet papers stressed the need for governments to focus on decarbonising economies, in order to reduce rates of cardiovascular and respiratory disease, and reduce risk factors linked to infectious disease and mental illness.

Professor Capon said policies focused on mitigating climate change were most urgent, as was a focus on vulnerable communities.

"The people who are least responsible for climate change — people living in low income countries and in poor communities — are the most vulnerable to these health impacts," he said.

"[Climate change] is affecting the health of Australians and people around the world.

"This is not a future issue. It's already happening."