Global warming is now pushing heat into territory humans can’t tolerate

The explosive growth and success of human society over the past 10,000 years has been underpinned by a distinct range of climatic conditions. But the range of weather conditions humans can encounter on Earth — the “climate envelope” — is changing as the planet warms and conditions change. entirely new to civilization could emerge in the decades to come. Even with modern technology, this should not be taken lightly.

Being able to regulate our temperature has played a key role in allowing humans to dominate the planet. Walking on two legs, without fur, and with a sweat-based cooling system, we are well designed to beat the heat. But the hot weather is already limiting our ability to work and stay healthy. In fact, our physiology imposes limits on the level of heat and humidity we can face.

The normal temperature you see reported on the weather forecast is called the “dry bulb” temperature. Once this exceeds around 35°C, the body must rely on the evaporation of water (mainly through sweating) to dissipate the heat. “Wet-bulb” temperature is a measurement that includes the cooling effect of evaporation on a thermometer, so it is normally much lower than dry-bulb temperature. It indicates how efficiently our sweat-based cooling system can work.

Once the wet bulb temperature reaches around 35°C, the air is so hot and humid that not even sweating can lower your body temperature to a safe level. With continued exposure above this threshold, death from overheating may follow.

A limit of 35°C may seem modest, but it is not. When the UK was sweltering with a record dry temperature of 38.7°C in July 2019, the wet bulb temperature in Cambridge did not exceed 24°C. Even during the deadly heat wave of 2015 in Karachi, the wet bulb temperature remained below 30°C. In fact, outside a hammam, few people have encountered temperatures close to 35°C. He was mostly beyond the Earth’s climatic envelope that human society has developed.

But our recent search shows that the 35°C limit is getting closer, leaving an increasingly small margin of safety for the hottest and most humid places on Earth.

Heat beyond human tolerance

Modeling studies had already indicated that wet bulb temperatures could regularly cross 35°C if the world exceeds the 2°C warming limit set by the Paris climate agreement in 2015, with The Persian Gulf, South Asia and North China Plain on the front line of deadly humid heat.

Our analysis of wet bulb temperatures from 1979 to 2017 did not disagree with these warnings of what might happen. But whereas previous studies had focused on relatively large regions (at the scale of large metropolitan areas), we also looked at thousands of weather station records around the world and found that at this more local scale, Many sites closed much faster than the 35°C limit. The frequency of severe wet temperatures (above 31°C, for example) has more than doubled around the world since 1979, and in some of the hottest and wettest places on earth, such as the coast of the United Arab Emirates, wet temperatures have already exceeded 35°C. °C. The climate envelope is sinking into territory where our physiology cannot keep up.

The consequences of crossing 35°C, brief as they are, have perhaps been above all symbolic up to now, the inhabitants of the hottest places being accustomed to coping with high heat in shelter in air-conditioned spaces. But relying on artificial cooling to cope with the rising heat increase energy demand and leave many people dangerously exposed to power outages. It would also abandon the most vulnerable members of society and not help those who must venture out.

Absolute records of maximum damp heat at weather stations around the world, 1979-2017. Colin RaymondAuthor provided

The only way to avoid being swept further and more frequently into uncharted heat territory is to reduce greenhouse gas emissions to net zero. The economic downturn during the coronavirus pandemic is expected reduce emissions by 4-7% in 2020bringing them closer to where global emissions were in 2010. But concentrations of greenhouse gases continue to rise rapidly in the atmosphere. We must also adapt where possible, encouraging simple behavioral changes (such as avoiding outdoor activities during the day) and scaling up emergency response plans when extreme heat is imminent. Such measures will help buy time against the inexorable march forward of the Earth’s climatic envelope.

We hope our research will illuminate some of the challenges that may lie ahead as global temperatures rise. The emergence of unprecedented heat and humidity – beyond what our physiology can tolerate – is just part of what could be in store. An even hotter and wetter world risks generating climate extremes beyond human experience, including the potential for a host of “unknown unknowns”.

We hope that the feeling of vulnerability to the surprises left by COVID-19 will reinvigorate global commitments to achieve carbon neutrality – recognizing the value of preserving somewhat familiar conditions, rather than risking what may wait in a very harsh climate. new to come.

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Teresa H. Sadler