What is lethal humidity?
In 2023, the Intergovernmental Panel on Climate Change (IPCC) confirmed that one of the greatest hazards posed by near-term 1.5°C warming is the impact of “dangerous” humid heatwaves on human mortality.
This means that of all the dangerous impacts of the climate crisis, humid heatwaves are one of the nearest dangers we face according to major scientific consensus.
According to The National Aeronautics and Space Administration (NASA), for every degree the planet warms, humidity rises by about 7 per cent, because warmer air holds more water vapour. On this basis, it is likely that humidity has risen roughly 10 per cent since the 1850s. Increasing temperatures combined with high humidity can be lethal for humans. As our oceans warm from rising temperatures, more water evaporates, further increasing humidity in the air. This drives energy into the atmosphere and stirs up violent and frequent weather events like storms and hurricanes.
We are all at extreme risk from the combined threat of lethal humidity and heat.
For every 1°C the planet warms, humidity rises by about 7 per cent as water evaporates from our oceans and waterways [1].
In a dry environment, the upper limit to human survival is reached at around 54°C [2]; However,
with high humidity, our bodies can go into heat stroke from as low as 31-35°C [3].
That’s because with high humidity and heat our sweat can no longer effectively cool our bodies causing our core temperature to rise.
Without intervention, these conditions will cause your blood to thicken and organs to shut down, resulting in death within hours [2].
Open Letter
Lethal humidity and heat are already upon us. Our actions today will determine how many millions of people die or are forced to migrate.
World Leaders,
We, the undersigned, come from the world’s leading research and academic institutions, and consist of a diverse range of climate scientists, public health experts, policy makers and industrialists.
There is scientific consensus that rising humidity and heat pose a serious and growing threat to humanity. At temperatures as low as 30˚C, under conditions of high humidity, the human body struggles to cool down by sweating. Death can result.
As temperatures increase, so does the potential for episodes of high temperature and high humidity beyond what humans and many mammals – including livestock, an important source of protein globally – can endure for more than a few hours.
These impacts will not be equitable. Society’s most vulnerable, such as the elderly, young children and the poor, will suffer the most. At population scale, this is called Lethal Humidity and its impacts will be felt globally, especially in tropical and sub-tropical regions.
Some of the most populous regions on Earth are at risk, including parts of India, China, the United States, Indonesia, Pakistan, Bangladesh, Mexico, Northern Australia and many other countries across Africa, Asia, South America and Europe.
Rising humidity is also driving some of the most violent weather the world has seen and the threat will only grow as the world gets hotter. We must prepare for abrupt environmental and social change.
Signatories
Professor Michael Mann
Presidential Distinguished Professor, Director of Penn Center for Science, Sustainability and the Media, University of Pennsylvania
Professor Li Zheng
Executive Vice President, Institute of Climate Change, Tsinghua University
Professor Nerilie Abram
Deputy Director ARC Centre of Excellence for 21st Century Weather and Deputy Director Australian Centre for Excellence in Antarctic Science
Professor Katrin Meissner
Director, Climate Change Research Centre, University of New South Wales, Australia
Dr Bill Hare
UN High Level Expert Group, Founder and CEO, Climate Analytics
Professor Tim Lenton
Climate Change/Earth System Science, Global Systems Institute, University of Exeter, UK
Professor Steven Sherwood
Atmospheric Sciences, Climate Change Research Centre, University of New South Wales Sydney, Australia
Dr Sylvia Earle
Ocean Elder
Professor Ollie Jay
Heat and Health, Director of the Heat and Health Research Incubator, Faculty of Medicine and Health, University of Sydney, Australia
Professor Kevin Trenberth
Distinguished Scholar, US National Center for Atmospheric Research and Affiliate faculty University of Auckland, New Zealand
Dr Fahad Saeed
Heat and humidity effects; Regional Lead: South Asia and the Middle East, Climate Analytics
Dr Sneha Malhotra
Chief Technology Officer, Office of Principal Scientific Advisor, Government of India
Professor Peter Huybers
Earth and Planetary Sciences, Harvard University
Dr Guang Zhang
Research Meteorologist, Scripps Institution of Oceanography, University of California San Diego, USA
Professor Paromita Chakraborty
Atmospheric pollution, Centre for Research in Environment, Sustainability Advocacy and Climate Change, Institute of Science and Technology
Professor Deqing Zhu
School of Minerals Processing and Bioengineering, Central South University
Dr Jonathan R. Buzan
Climate Physicist, Climate and Environmental Physics and Oeschger Centre for Climate Change Research, University of Bern, Switzerland
Professor Sagnik Dey
Institute Chair, Centre for Atmospheric Sciences. Indian Institute of Technology Delhi
Dr Chloe Brimicombe
Climate Scientist and Extreme Heat Researcher, Karl-Franzens-Universität Graz & University of Reading
Professor Vidhya Venugopal
Country Director (NIHR GRC NCD-EC), Department of Environmental Health Engineering, Faculty of Public Health, Sri Ramachandra Institute and Higher Education, Chennai, India
Professor Trevor McDougall
AC FRS FAA FAGU FRSN, Emeritus Scientia Professor of Ocean Physics, School of Mathematics and Statistics, University of New South Wales, Australia
Professor Ambarish Dutta
Epidemiology Indian Institute of Public Health, Bhubaneswar Public Health Foundation of India
Professor Arnab Dutta
Interdisciplinary Programme in Climate Studies, IIT Bombay
Dr James Smith
MRCGP FFPH, Sustainability Lead, Cambridge Public Health Interdisciplinary Research Centre, University of Cambridge
Dr Amy Booth
Clinician and Researcher, Climate Change and Global Health, Nuffield Department of Primary Care Health Sciences, University of OxfordDPhil Candidate, IRIHS Research Group
Professor Kurian Joseph
Director, Centre for Climate Change and Disaster Management, Centre for Environmental Studies, Department of Civil Engineering, Anna University
Dr Benjamin Jones
Oxford Centre for Global Health Research, Nuffield Department of Medicine, University of Oxford
Dr Andrew Forrest AO
Minderoo Foundation, Tattarang and Fortescue; Honorary Research Fellow, The University of Western Australia
Professor Ken Smith
Walter and Eliza Hall Institute of Medical Research, Australia
Dr Emma Ramsay
Humid heat in urban settlements, Nanyang Technological University Singapore
Professor Stephen Luby
Department of Medicine, Stanford University
Professor Matthew England
Scientia Professor, Director of Research, Centre for Marine Science and Innovation, University of New South Wales, Sydney
Dr Brian O’Callaghan
Smith School of Enterprise and the Environment, University of Oxford; United Nations Economic Commission for Africa; Acadia Infrastructure Capital
Professor Srinivasan Keshav
FRCS, FACM, FIEEE, Robert Sansom Professor of Computer Science, University of Cambridge
Professor Callum Roberts
Marine Conservation, Centre for Ecology and Conservation, University of Exeter, UK
Professor Sarah Perkins-Kirkpatrick
Climate Scientist and Deputy Director ARC Centre of Excellence for 21st Century Weather, Australian National University
Dr Constantin Arnscheidt
Centre for the Study of Existential Risk, University of Cambridge
Dr Shanta Barley
Chief Climate Scientist, Fortescue; Adjunct Lecturer, The University of Western Australia
Ms Kylie Walker
CEO Australian Academy of Technological Sciences & Engineering
Professor Anjal Prakash
Bharti Institute of Public Policy, Indian School of Business
Professor Michael Ellwood
Research School of Earth Sciences, Australian National University
Dr Laurence Wainwright
Departmental Lecturer & Course Director, Smith School of Enterprise & the Environment, University of Oxford
Emeritus Professor Sarah Dunlop
Minderoo Foundation and The University of Western Australia
Professor Jacob G. Foster
Informatics and Cognitive Science, Indiana University Bloomington, External Professor, Santa Fe Institute
Professor Alastair Iles
Sustainability Transitions Department of Environmental Science, Policy & Management University of California, Berkeley
Dr Shannon A. Bonke
Research Fellow of King’s College Cambridge and Department of Chemistry, University of Cambridge
Professor Douglas McCauley
Director, Benioff Ocean Science Laboratory, University of California Santa Barbara
Dr John Hunter
Climate and sea level scientist, University of Tasmania
Dr Larry Marshall
Director, Fortescue; Chair, American Chamber of Commerce in Australia; Governing Council of Australian National University
Dr Vishal Dixit
Interdisciplinary Programme in Climate Studies, Indian Institute of Technology Bombay, India
Professor Kamiar Mohaddes
Cambridge Judge Business School, University of Cambridge, Co-Director of University of Cambridge climaTRACES Lab
Professor Anne-Louise Ponsonby
Head, Neuroepidemiology Research Group, The Florey Institute of Neuroscience and Mental Health
Professor Stephen F. Ackley
Department of Earth and Planetary Sciences, University of Texas San Antonio, Fellow of the American Geophysical Union
Bodhi Patil
Youth Ambassador, Dept. Environment & Sustainability, University of British Columbia
Professor Jean-Louis Tison
DSTE-IGEOS-Glaciology laboratory – Université Libre de Bruxelles, Belgium
Professor Ian Allison
D.Sc., FAA, University of Tasmania, Australia
Dr Maria Luiza Pedrotti
National Centre for Scientific Research, Sorbonne
Safiya Noble PhD
Professor and David O. Sears Presidential Endowed Chair of Social Sciences, University of California, Los Angeles (UCLA)
Chandni Singh PhD
Senior Research Consultant, Indian Institute for Human Settlements
Professor Peter McCulloch
Professor of Surgical Science and Practice, University of Oxford
Dr Robert Glasser
Head of Climate and Security Policy Centre, Australian Strategic Policy Institute
Professor Michael Francis Good AO Duniv
Head, Laboratory of Vaccines for the Developing World, Principal Research Leader and NHMRC Investigator Fellow (L3), Institute for Glycomics, Griffith University
Associate Professor Danielle Stanisic PhD
Research Leader and Principal Research Fellow, Institute for Glycomics, Griffith University
Dr Caroline Buckee
Professor of Epidemiology, Harvard T.H. Chan School of Public Health, Harvard University
Dr Louise J. Slater
Professor of Hydroclimatology, University of Oxford
Associate Professor Anette Mikes
Saïd Business School, University of Oxford
Professor Ben Newell
Director of the Institute for Climate Risk & Response, University of New South Wales, Australia
Rosalind Chaston
Science Writer, MSc Environmental Change and Management, University of Oxford
Dr Dileep Mavalankar
Director, Indian Institute of Public Health, Gandhinagar
Josh Hopkins
Curtin University Sustainability Policy Institute
Dr Ramit Debnath
Assistant Professor, University of Cambridge. Cambridge Zero Fellow and Director of Collective Intelligence & Design Group
Dr Ronita Bardham
University of Cambridge, UK Associate Professor of Sustainable Built Environment, Director – Cambridge Sustainable Design Group
Associate Professor Jason Kai Wei Lee
Yong Loo Lin School of Medicine, National University of Singapore
Dr Robert Rouse
Department of Computer Science and Technology, University of Cambridge
Professor Matthew Connelly
Centre for the Study of Existential Risk, University of Cambridge
Professor Emily Shuckburgh OBE
Director of Cambridge Zero, University of Cambridge
Professor David Ackerley
Dean of the Rausser College of Natural Resources, UC Berkeley
Professor Somnath Baidya Roy
Professor and Head of Centre for Atmospheric Sciences and Rockefeller Foundation Climate Science and Technology Chair, Indian Institute of Technology, New Delhi
Jessica Bland
Deputy Director, Centre for the Study of Existential Risk, University of Cambridge
Associate Professor William Boos
Department of Earth and Planetary Science, University of California, Berkeley
Professor Jennifer Chayes
Dean of the College of Computing, Data Science, and Society at UC Berkeley
Professor Ulrich Ecker
Fellow of the UWA Public Policy Institute, The University of Western Australia
Dr Shaun Fitzgerald OBE FREng
Director, Centre for Climate Repair, Director of Research, Cambridge Zero
Philip Guildford
Director of Strategy and Operations at Centre for Science and Policy, University of Cambridge
Professor Mark Handcock
Distinguished Professor of Statistics, Department of Statistics and Data Science, UCLA, USA
Professor Raul Hinojosa-Ojeda
Associate Professor, Department of Chicana and Chicano Studies, UCLA, USA
Harriet Hunnable
Strategy Advisor, Cambridge Zero Policy Forum
Dr Vijay Jain
Research Fellow, Department of Civil Engineering, IIT Indore, India
Professor Ashok Jhunjhunwala
Institute Professor at Indian Institute of Technology
Professor Zhang Junliang
Shanghai Jiao Tong University
Dr Andrew King
Senior Lecturer in Climate Science, University of Melbourne
Professor Todd Lane
Deputy Dean and Professor, School of Geography, Earth and Atmospheric Sciences, University of Melbourne, Australia
Dr Erik Mackie
Cambridge Zero initiative at the University of Cambridge
Dr Tom Matthews
Senior Lecturer in Environmental Geography, King’s College London
Katie McMahan
Human Environment Geographer, University of California Santa Barbara
Dr Nicole Miranda
Senior Researcher on the Future of Cooling Programme of the Oxford Martin School
Professor Vimal Mishra
Centre for Sustainable Development, Indian Institute of Technology Gandhinagar
Professor Peter Palensky
Head of Electric Sustainable Energy department, Chair of Intelligent Electrical Power Grids, Chair of the Delft Energy Initiative and scientific director of the PowerWeb Institute
Dr Carter Powis
Smith School of Enterprise and the Environment, University of Oxford, UK
Professor Dong Qiang
Peking University
Dr Andrea Ramírez Ramírez
Professor of Low Carbon Systems and Technologies at Delft University of Technology
Bruce Riordan
Director, Berkeley Climate Change Network
Professor David Romps
Department of Earth and Planetary Science, UC Berkeley
Dr Holly Samuelson
Associate Professor of Architecture at the Harvard Graduate School of Design
Dr Sandeep Sukumaran
Associate Professor at the Centre for Atmospheric Sciences, IIT Delhi
Dr Rama Swami Bansal
Chief Scientist and Head at The Council of Scientific & Industrial Research, India
Dr V. Kelly Turner
Associate Professor of Urban Planning and Geography, UCLA
Dr Antje Weisheimer
Climate Scientist, National Cantre for Atmospheric Science, University of Oxford, and the European Centre for Medium-Range Weather Forecasts, Reading, UK
Dr Henri Werij
Dean of the Faculty of Aerospace Engineering, Delft University of Technology, Netherlands
Baroness Bryony Worthington
Member of the House of Lords of the United Kingdom, Deputy Chair of WWF-UK and Founder and Chair of Ember
Dr Satchit Balsari
Associate Professor in Emergency Medicine at Harvard Medical School and Beth Israel Deaconess Medical Center; Global Health and Population at the Harvard T.H. Chan School of Public Health, Harvard University
Professor Daniel Pauly
University Killam Professor. Sea Around Us; Institute for the Oceans and Fisheries & Department of Zoology
Dr Colin Raymond
University of California Los Angeles and NASA Jet Propulsion Lab
Dr Radley M. Horton
Professor of Climate, Columbia Climate School
Dr Peter Kalmus
Climate Scientist at NASA Jet Propulsion Lab
Dr Frances Fuller
Heat and humidity effects; Regional Lead: South Asia and the Middle East, Climate Analytics
Daniel Wordsworth
Chief Executive Officer, World Vision Australia
Dr Radhika Khosla
Associate Professor, Smith School of Enterprise and Environment; Research Director, Oxford India Centre for Sustainable Development
Professor Rashid Sumaila
Director, Fisheries Economics Research Unit and the OceanCanada Partnership, University of British Columbia
Dr Tzeporah Berman
Adjunct professor at the Faculty of Environmental Studies, York University, Toronto
Dr Justin Brashares
Professor of Ecology and Conservation, University of California Berkeley
Professor Subimal Ghosh
Institute Chair Professor, Department of Civil Engineering and Convener, Interdisciplinary Program in Climate Studies, Indian Institute of Technology Bombay
Professor Andrew Macintosh
Environmental law and policy scholar, Associate Dean at the College of Law, Australian National University
Dr Mette Morsing
Professor of Business Sustainability and Director of the Smith School of Enterprise and the Environment at University of Oxford, UK
Professor Benjamin Franta
Associate Professor of Climate Litigation, Oxford Sustainable Law Programme, University of Oxford
Dr Russell Reichelt AO
Fellow of Australian Academy of Technological Sciences and Engineering, Ocean Health and Sustainability. Queensland, Australia
Dr Marilyn Raphael
Director of UCLA Institute of the Environment and Sustainability
Dr Stephen Bygrave
Institute for Climate, Energy and Disaster Solutions, Australian National University
Professor Edward Maibach, MPH, PhD
Mason Distinguished University Professor; Director, George Mason University Center for Climate Change Communication; Fellow, American Association for the Advancement of Science; Member, National Academy of Medicine; Member, Board of Directors, Global Climate and Health Alliance