By late this century, more than 50 percent of humanity could be exposed to life-threatening heat and humidity. But one of climate change’s deadliest emerging risks remains poorly understood: lethal humidity.
Climate change is not only warming the planet. It is increasing humidity, because warmer air holds more moisture. While the role of climate change in amplifying extreme heat has been widely studied, only recently have the alarming implications of its combination with high humidity become clearer.
High humidity dramatically reduces the body’s ability to cool itself, significantly lowering the temperature at which severe health effects occur. The synergistic effect of extreme heat and humidity is rapidly moving large, densely populated regions of the world towards the limits of physiological survivability.
The danger extends far beyond direct exposure. My new ASPI report highlights that lethal humidity should not be understood as an isolated hazard. Its effects cascade through societies and increasingly interact with other climate-amplified risks, including floods, sea-level rise, storms, drought and infrastructure failures.
Scientists have already documented increases in simultaneous occurrences of humid heatwaves and extreme sea levels, alongside growing links between humid heat and extreme rainfall. The UN’s Intergovernmental Panel on Climate Change (IPCC), the authoritative voice of climate science, warns that climate hazards are becoming increasingly complex, with risks interacting and cascading across sectors and regions. They are, in effect, beginning to collide.
Research suggests many parts of the world are transitioning from experiencing isolated climate extremes to routinely facing compound events. What were once rare overlapping disasters may become common. Humid-heat extremes, floods, droughts and coastal inundation will increasingly occur in overlapping sequences, striking before communities and systems have recovered from earlier shocks. Recovery gaps will widen, community resilience will erode, infrastructure will weaken and governance systems will come under increasing strain.
This emerging environment will be challenging even for wealthy countries. For poorer countries, where resources are far more limited, the consequences could be devastating.
The tropics are expected to be particularly exposed. By the late 21st century, populations in some tropical regions may experience extreme humid-heat events for up to two-thirds of the year. It’s the IPCC that estimates that at least 50 percent the world’s people could be exposed to life-threatening climatic conditions due to extreme heat and humidity.
More than 3 billion people may live in tropical coastal cities facing high humid-heat stress by the end of the century – many simultaneously exposed to sea-level rise, storm surges and severe flooding. Cities such as Manila, Dhaka and Chennai are becoming hotspots where multiple climate threats overlap.
China’s exposure may be among the least recognised geopolitical risks. Under a high emissions scenario, most of China’s population could eventually be exposed to humid heat exceeding physiological limits of human survival. Analysis suggests that at 2 degrees C of warming, a point that may be reached in only a few years, a heatwave as severe as China’s worst on record, one in 2013, would occur about every second year. The implications for prosperity, stability and security deserve far greater attention.
The impacts of humid heat also ripple far beyond those directly exposed. When extreme humid heat surges demand for cooling, electricity systems can fail. When electricity fails, so too can refrigeration, water supply, sanitation systems and transport. Food security is affected and public health risks multiply. These cascading failures can amplify humanitarian crises and undermine economic and social stability.
The risks are enormous, but not insurmountable. There is strong evidence that planning and adaptation save lives. However, many existing Heat Action Plans still fail to integrate humidity, human physiology and social inequality into planning. Those omissions are dangerous.
Keeping people cool in a warming climate should become a global grand challenge, with levels of ambition and investment comparable to major international health initiatives. That challenge should extend beyond expanding access to air conditioning to include strengthening the resilience of electricity systems, accelerating affordable off-grid cooling solutions such as solar-powered technologies, and redesigning buildings and cities to reduce heat exposure.
More than 1.1 billion people are already at high risk due to lack of access to cooling, while demand for cooling is projected to more than double within the next 25 years. Ensuring access to reliable, affordable cooling may become as fundamental to human wellbeing in a warming world as access to clean water, sanitation and vaccines was in earlier eras.
But adaptation alone will not be enough if warming reaches 2 to 3 degrees C or more, as now appears increasingly likely. Even radically expanded adaptation efforts may struggle against the combined impacts of lethal humidity and other climate-amplified hazards.
Ultimately, the only durable way to prevent that future is rapid global reductions in greenhouse gas emissions. The Lethal Humidity Global Council has identified four key policy shifts urgently required to achieve this:
—Real zero emissions: complete replacement (phasing out) of fossil fuels with renewable energy.
—Removal of barriers to green industry, including fossil-fuel subsidies, before 2030.
—Matching of incentives and disincentives to risk with carbon pricing.
—Economic stimulants to encourage green growth and transformation.
The stakes are enormous, and the window for action is closing. Governments, businesses and civil society must act rapidly, decisively and purposefully to ensure lethal humidity remains a rare occurrence – rather than becoming a deadly reality for billions.