Dr Mark Harber is a consultant nephrologist (kidney doctor) and the Royal College of Physicians' special adviser on healthcare sustainability and climate change. He's spent years working at the intersection of medicine and the environment as a member of the Sustainable Kidney Care committee, a clinical lead for the NHS London sustainability group, and an advisor to the UCL Partners Climate Collaborative. Now, he helps shape how the RCP advocates and lobbies on healthcare's role in the climate crisis.This conversation goes deep into the surprisingly stark relationship between heat and human health: why the death rate in London starts climbing at just 24 degrees, why 2026's June heatwave in the UK heatwave produced an estimated 2,900 excess deaths, and why chronic conditions from epilepsy to kidney failure get dramatically worse as the mercury rises. Mark explains why big national statistics can feel strangely meaningless next to a single hospital admission, and why he believes there's little point terrifying the public without giving them something practical to do about it.We talk about the NHS's own outsized carbon footprint (around 5% of the UK's total, more than the aviation industry) and why it became the first health system in the world to commit to net zero, back in 2019. Mark explains the hidden environmental cost of kidney dialysis, which can use 400 litres of water per session and carries 16 times the carbon footprint of an average NHS patient, and why the UK is on track to be 6 billion litres of water short every day by 2050. We also cover the less obvious ways climate change threatens healthcare itself: 90% of English hospitals at risk of overheating, a tenth to a third at risk of flooding, and disease-carrying mosquitoes now established in France and Italy that never used to survive this far north.——Learn more about the RCP's Green Physician Toolkit at https://www.rcp.ac.uk/policy-and-campaigns/our-policy-priorities/climate-and-sustainability/green-physician-toolkit/Find the National Emergency Briefing at https://www.nebriefing.org/Find Real Zero's 7 Acts to Save the World at https://www.realzero.earthSubscribe to the Climate Unf*cked podcast at https://climateunfucked.substack.com/And connect with me on LinkedIn at: /rob-coop https://www.linkedin.com/in/rob-coop/——Chapters