Bringing together leading global experts

to exchange the latest scientific research, formulate rapid, large-scale policy solutions and advocacy that address the threats of lethal humidity and heat.

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.

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For every 1°C the planet warms, humidity rises by about 7 per cent as water evaporates from our oceans and waterways1.

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In a dry environment, the upper limit to human survival is reached at around 54°C2; However,
with high humidity, our bodies can go into heat stroke from as low as 31-35°C3.

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That’s because with high humidity and heat our sweat can no longer effectively cool our bodies causing our core temperature to rise.

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Without intervention, these conditions will cause your blood to thicken and organs to shut down, resulting in death within hours2.

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.

Positive power

World Leaders, we must move beyond all policies that inhibit the development and deployment of renewable technology in order to achieve the defossilisation of our economy.

At least four key policy shifts are urgently required:

  1. Complete replacement (phasing out) of fossil fuel with renewable energy
  2. Removal of barriers to green industry, including transition of fossil fuel subsidies to new green energy generation, before 2030
  3. Matching of incentives and disincentives to risk with carbon pricing
  4. Economic stimulants to encourage green growth and transformation

The direct impacts of lethal humidity with heat on human mortality will be widespread, as will the indirect effects associated with loss of labour, migration and geopolitical destabilisation.

As leaders, we must immediately address both – but never one at the expense of the other.

We can still create a world fit for our children and future generations if we act now. We call on you to endorse the Positive Power Plan.

Date: 3 December 2023

Signatories

Dr Sneha Malhotra
Chief Technology Officer, Office of Principal Scientific Advisor, Government of India

Professor Steven Sherwood
Atmospheric Sciences, Climate Change Research Centre, University of New South Wales Sydney, Australia

Professor Li Zheng
Executive Vice President, Institute of Climate Change, Tsinghua University

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

Dr Vidhya Venugopal
Professor & Country Director (NIHR GRC NCD-EC), Department of Environmental Health Engineering, Faculty of Public Health, Sri Ramachandra Institute and Higher Education, Chennai, INDIA

Emeritus Professor Trevor McDougall
AC FRS FAA, University of New South Wales Sydney Winner Prime Minister’s Prize for Science 2022

Professor Ambarish Dutta
Epidemiology Indian Institute of Public Health, Bhubaneswar Public Health Foundation of India

Professor Nerilie Abram
Climate Science, The Australian National University, Australia

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

Professor Ollie Jay
Heat and Health, Director of the Heat and Health Research Incubator, Faculty of Medicine and Health, University of Sydney, Australia

Dr Fahad Saeed
Heat and humidity effects; Regional Lead: South Asia and the Middle East, Climate Analytics

Dr Amy Booth
Clinician and Researcher, Climate Change and Global Health, Nuffield Department of Primary Care Health Sciences, University of Oxford

Professor Tim Lenton
Climate Change/Earth System Science, Global Systems Institute, University of Exeter, UK

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, University of Western Australia

Professor Ken Smith
Department of Medicine, University of Cambridge

Dr Emma Ramsay
Humid heat in urban settlements, Nanyang Technological University Singapore

Professor Stephen Luby
Department of Medicine, Stanford University

Scientia Professor Matthew England
FFA Centre for Marine Science & Innovation, University of New South Wales Australia

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
Computer Science, Department of Computer Science and Technology, University of Cambridge

Professor Callum Roberts
Marine Conservation, Centre for Ecology and Conservation, University of Exeter, UK

Associate Professor Sarah Perkins-Kirkpatrick
Climate scientist, UNSW Canberra, Australia

Professor Katrin Meissner
Climate scientist; Director, Climate Change Research Centre, University of New South Wales, Australia

Dr Tony Worby
Chief Scientist, Minderoo Foundation

Dr Shanta Barley
Chief Climate Scientist, Fortescue; Adjunct Lecturer, 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
Sustainability, Enterprise & Environment, University of Oxford

Emeritus Professor Sarah Dunlop
Minderoo Foundation and The University of Western Australia

Professor Marilyn Raphael
Geography, Institute of the Environment and Sustainability, University of California, Los Angeles, USA

Professor Jacob G. Foster
Sociology, UCLA; 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
Judge Business School & Kings College, University of Cambridge

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

Dr Sylvia Earle
Ocean Elder

Safiya Noble, Ph.D.
David O. Sears Presidential Endowed Chair of Social Sciences
Interim Director, UCLA DataX Institute

Join the Global Council

The Lethal Humidity Global Council is an assembly of dedicated leaders and global experts.

By joining, you become part of a pivotal group, shaping the global response to one of the most pressing issues of our time: lethal humidity and heat.

References

  1. NASA Jet Propulsion Laboratory (2022) Steamy Relationships: How Atmospheric Water Vapor Amplifies Earth’s Greenhouse Effect (https://climate.nasa.gov/explore/ask-nasa-climate/3143/steamy-relationships-how-atmospheric-water-vapor-amplifies-earths-greenhouse-effect/)
  2. Vanos et al. (2023) A physiological approach for assessing human survivability and liveability to heat in a changing climate. Nature Communications. (https://www.nature.com/articles/s41467-023-43121-5)
  3. Pennsylvania State University (2022) Humans can’t endure temperatures and humidities as high as previously thought (https://www.psu.edu/news/research/story/humans-cant-endure-temperatures-and-humidities-high-previously-thought/)
  • Humidity refers to “absolute humidity”, the actual amount of water vapour in the air.
  • High humidity refers to “relative humidity”, the ratio between the amount of water the air holds and the maximum amount it could hold.