Skip to navigation Skip to main content

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.

2 December 2023

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.

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. Zero emissions: complete replacement (phasing out) of fossil fuel with renewable energy
  2. Removal of barriers to green industry, including fossil fuel subsidies, 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

For more information about the Lethal Humidity Global Council visit


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, Indian Institute of Technology 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

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

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

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

Dr Tom Matthews Senior Lecturer in Environmental Geography, Department of Geography, King’s College London