Wet bulb temperature and the 2.1 fertility rate
Heat is a killer. Tens of thousands of people die each year as a consequence of heat. But the real killer is humidity plus heat.
The human body is a water-based cooling system. Evaporative cooling is the technical term for our ability to maintain body temperature. We sweat, and as that sweat evaporates it takes heat energy from our bodies.
But if there is too much moisture in the air, our sweat cannot evaporate and so we overheat. The wet bulb temperature is used to understand the heat plus humidity temperature. And more pertinent, what the lowest possible temperature is with zero evaporation.
Evaporation is reduced when the air contains more water vapour. At 100% relative humidity, the wet bulb temperature is equal to the air temperature (dry bulb temperature); at lower humidity the wet bulb temperature is lower than dry bulb temperature because of evaporative cooling.
The human body has a threshold of survivability for this, which was thought to be around 35°C wet bulb temperature. Meaning at that wet bulb temperature, a healthy young person who’s not physically active at the time, will die within 6 hours. The overheating causes cells to disintegrate; effectively we cook.
It was also thought that this wet bulb temperature had never been exceeded anywhere on earth. Recently however, in Oman, and a further 9 times since in various other places including Pakistan, the wet bulb temperature has exceeded the human survivability threshold.
Source: NASA
Then about a year ago two scientific white papers were published that pushed the wet bulb temperature threshold of survivability down. Down to somewhere between 25ºC and 28°C.
If we look at deadly heat waves that we’ve experienced in recent times — one in 2003 in Europe which killed around 30,000 people and one in 2010 in Russia that killed around 50,000 people — neither of those two heat waves had a wet bulb temperature over 28°C. So it’s very likely that 26ºC wet bulb actually is the number.
Now, some areas of the world are highly adapted to surviving such conditions. Dubai and south Florida, for example, where wealthy populations live almost permanently in air conditioning.
Many others are not so adapted and that will have consequences. Increasing occurrences of deadly wet bulb temperatures plus climate change-induced sea level rises and crop failure, will lead to mass migration.
Bangladesh is due to lose one-third of its land mass to rising sea levels. Many Pacific islands have already lost significant land to the ocean.
Source: theconversation.com
“By 2050 1.5 billion people could be displaced — mostly to the temperate north — due to climate change. By 2070 it’s 3 billion; depending on how well we manage to decarbonise and how adapted our living conditions become.” — Gaia Vince, Nomad Century
Migration to habitable zones has already started. There is a direct correlation between Central American drought and migration numbers into the USA. A dry season and crop failure in Central America, directly means more people at the USA border. A good crop year, and fewer people migrate.
Source: Olivera, Fuerte-Celis, Bolanos
Maybe though, there is opportunity here.
In order to maintain a population, on average each woman needs to have 2.1 children to replace her and her partner (the 0.1 covers infant mortality). This is known as the fertility rate. Without immigration, a population whose fertility rate is less than 2.1 will decline.
Of the countries of the Organisation for Economic Co-operation and Development (OECD), only Israel has a fertility rate over 2.1. South Korea’s is 0.8. In the U.K. we’re at about 1.5.
We need to maintain population levels because looking after old people is expensive! Sustained low fertility rates mean a rapidly ageing population, which will place a serious burden on the economy through increasing health care and welfare costs.
In Japan, adult nappies outsell infant nappies (source: Gaia Vince, Nomad Century).
The only countries that are making good with the conundrum of a decline in fertility rate and an ageing population, are doing it through immigration. Canada, Luxembourg and Australia are stand-outs, whilst recently Japan and Russia have softened their policies on immigration.
Nationalistic governments scared of immigration, like Meloni’s in Italy and Orban’s in Hungary, are encouraging young people to have babies. Incentivising them to do so. There is, however, not a single example in history where this has worked.
In a few years, countries’ economies will be under great strain due to their ageing demographic. So much so, it’s likely to become a competition to win over migrants. Climate migration could be, should be welcomed and embraced. Or will we instead put blockers in the way of people looking for somewhere their children can survive?
At 4º warming (a number that science increasingly believes is likely), the tropic-to-tropic equatorial belt will be uninhabitable. Southern Europe will be desert. Canada, Russia, Greenland and Alaska will be the main habitable zones.
2023’s average temperature was 1.48º above the pre-industrial average.
Political silence on this issue, and a general consensus that states we need fewer immigrants, means this is unlikely to be addressed.
We face monumental, planetary challenges. We need immigrants’ labour and their taxes. We also need the infrastructure to deal with climate change — new reservoirs in south-east England for increasing droughts, and more adapted environments for us to survive.
We could be coming up with these strategies and solutions today, but this subject feels miles from the political discourse of our times. The challenges require collective work. They’re agnostic to your political affiliation; we’re all facing this.
As cool and sci-fi as multiple high-rise cities in western Antarctica sounds, I’d rather we had honesty and discussion about the situation from our leaders now, than face it unprepared.
